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12 RFC 5533
SHIM6 WG E. Nordmark
Internet-Draft Sun Microsystems
Expires: September 5, 2006 M. Bagnulo
UC3M
March 4, 2006
Level 3 multihoming shim protocol
draft-ietf-shim6-proto-04.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The SHIM6 protocol is a layer 3 shim for providing locator agility
below the transport protocols, so that multihoming can be provided
for IPv6 with failover and load sharing properties, without assuming
that a multihomed site will have a provider independent IPv6 address
prefix which is announced in the global IPv6 routing table. The
hosts in a site which has multiple provider allocated IPv6 address
prefixes, will use the shim6 protocol specified in this document to
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setup state with peer hosts, so that the state can later be used to
failover to a different locator pair, should the original one stop
working.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.2 Non-Goals . . . . . . . . . . . . . . . . . . . . . . . 6
1.3 Locators as Upper-layer Identifiers . . . . . . . . . . 6
1.4 IP Multicast . . . . . . . . . . . . . . . . . . . . . . 7
1.5 Renumbering Implications . . . . . . . . . . . . . . . . 8
1.6 Placement of the shim . . . . . . . . . . . . . . . . . 9
1.7 Traffic Engineering . . . . . . . . . . . . . . . . . . 11
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Notational Conventions . . . . . . . . . . . . . . . . . 15
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 16
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . 17
4.1 Context Tags . . . . . . . . . . . . . . . . . . . . . . 19
4.2 Context Forking . . . . . . . . . . . . . . . . . . . . 19
4.3 API Extensions . . . . . . . . . . . . . . . . . . . . . 20
4.4 Securing shim6 . . . . . . . . . . . . . . . . . . . . . 20
4.5 Overview of Shim Control Messages . . . . . . . . . . . 21
4.6 Extension Header Order . . . . . . . . . . . . . . . . . 22
5. Message Formats . . . . . . . . . . . . . . . . . . . . . . 24
5.1 Common shim6 Message Format . . . . . . . . . . . . . . 24
5.2 Payload Extension Header Format . . . . . . . . . . . . 24
5.3 Common Shim6 Control header . . . . . . . . . . . . . . 25
5.4 I1 Message Format . . . . . . . . . . . . . . . . . . . 27
5.5 R1 Message Format . . . . . . . . . . . . . . . . . . . 28
5.6 I2 Message Format . . . . . . . . . . . . . . . . . . . 30
5.7 R2 Message Format . . . . . . . . . . . . . . . . . . . 31
5.8 R1bis Message Format . . . . . . . . . . . . . . . . . . 33
5.9 I2bis Message Format . . . . . . . . . . . . . . . . . . 34
5.10 Update Request Message Format . . . . . . . . . . . . . 36
5.11 Update Acknowledgement Message Format . . . . . . . . . 38
5.12 Keepalive Message Format . . . . . . . . . . . . . . . . 39
5.13 Probe Message Format . . . . . . . . . . . . . . . . . . 39
5.14 Option Formats . . . . . . . . . . . . . . . . . . . . . 39
5.14.1 Responder Validator Option Format . . . . . . . . . 41
5.14.2 Locator List Option Format . . . . . . . . . . . . . 42
5.14.3 Locator Preferences Option Format . . . . . . . . . 43
5.14.4 CGA Parameter Data Structure Option Format . . . . . 45
5.14.5 CGA Signature Option Format . . . . . . . . . . . . 46
5.14.6 ULID Pair Option Format . . . . . . . . . . . . . . 47
5.14.7 Forked Instance Identifier Option Format . . . . . . 48
5.14.8 Probe Option Format . . . . . . . . . . . . . . . . 48
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5.14.9 Reachability Option Format . . . . . . . . . . . . . 48
5.14.10 Payload Reception Report Option Format . . . . . . 48
6. Conceptual Model of a Host . . . . . . . . . . . . . . . . . 49
6.1 Conceptual Data Structures . . . . . . . . . . . . . . . 49
6.2 Context States . . . . . . . . . . . . . . . . . . . . . 50
7. Establishing ULID-Pair Contexts . . . . . . . . . . . . . . 52
7.1 Uniqness of Context Tags . . . . . . . . . . . . . . . . 52
7.2 Locator Verification . . . . . . . . . . . . . . . . . . 52
7.3 Normal context establishment . . . . . . . . . . . . . . 53
7.4 Concurrent context establishment . . . . . . . . . . . . 53
7.5 Context recovery . . . . . . . . . . . . . . . . . . . . 55
7.6 Context confusion . . . . . . . . . . . . . . . . . . . 57
7.7 Sending I1 messages . . . . . . . . . . . . . . . . . . 58
7.8 Retransmitting I1 messages . . . . . . . . . . . . . . . 58
7.9 Receiving I1 messages . . . . . . . . . . . . . . . . . 59
7.9.1 Generating the R1 Validator . . . . . . . . . . . . 60
7.10 Receiving R1 messages and sending I2 messages . . . . . 61
7.11 Retransmitting I2 messages . . . . . . . . . . . . . . . 62
7.12 Receiving I2 messages . . . . . . . . . . . . . . . . . 62
7.13 Sending R2 messages . . . . . . . . . . . . . . . . . . 64
7.14 Match for Context Confusion . . . . . . . . . . . . . . 64
7.15 Receiving R2 messages . . . . . . . . . . . . . . . . . 65
7.16 Sending R1bis messages . . . . . . . . . . . . . . . . . 66
7.16.1 Generating the R1bis Validator . . . . . . . . . . . 66
7.17 Receiving R1bis messages and sending I2bis messages . . 67
7.18 Retransmitting I2bis messages . . . . . . . . . . . . . 68
7.19 Receiving I2bis messages and sending R2 messages . . . . 68
8. Handling ICMP Error Messages . . . . . . . . . . . . . . . . 70
9. Teardown of the ULID-Pair Context . . . . . . . . . . . . . 72
10. Updating the Peer . . . . . . . . . . . . . . . . . . . . 73
10.1 Sending Update Request messages . . . . . . . . . . . . 73
10.2 Retransmitting Update Request messages . . . . . . . . . 73
10.3 Newer Information While Retransmitting . . . . . . . . . 74
10.4 Receiving Update Request messages . . . . . . . . . . . 74
10.5 Receiving Update Acknowledgement messages . . . . . . . 76
11. Sending ULP Payloads . . . . . . . . . . . . . . . . . . . 77
11.1 Sending ULP Payload after a Switch . . . . . . . . . . . 77
12. Receiving Packets . . . . . . . . . . . . . . . . . . . . 79
12.1 Receiving Payload Extension Headers . . . . . . . . . . 79
12.2 Receiving Shim Control messages . . . . . . . . . . . . 79
12.3 Context Lookup . . . . . . . . . . . . . . . . . . . . . 80
13. Initial Contact . . . . . . . . . . . . . . . . . . . . . 82
14. Protocol constants . . . . . . . . . . . . . . . . . . . . 83
15. Implications Elsewhere . . . . . . . . . . . . . . . . . . 84
16. Security Considerations . . . . . . . . . . . . . . . . . 86
17. IANA Considerations . . . . . . . . . . . . . . . . . . . 88
18. Acknowledgements . . . . . . . . . . . . . . . . . . . . . 90
A. Open Issues . . . . . . . . . . . . . . . . . . . . . . . . 91
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B. Possible Protocol Extensions . . . . . . . . . . . . . . . . 92
C. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . 94
D. Simplified State Machine . . . . . . . . . . . . . . . . . . 97
D.1 Simplified State Machine diagram . . . . . . . . . . . . 102
E. Context Tag Reuse . . . . . . . . . . . . . . . . . . . . . 103
E.1 Context Recovery . . . . . . . . . . . . . . . . . . . . 103
E.2 Context Confusion . . . . . . . . . . . . . . . . . . . 103
E.3 Three Party Context Confusion . . . . . . . . . . . . . 104
F. Design Alternatives . . . . . . . . . . . . . . . . . . . . 105
F.1 Context granularity . . . . . . . . . . . . . . . . . . 105
F.2 Demultiplexing of data packets in shim6 communications . 105
F.2.1 Flow-label . . . . . . . . . . . . . . . . . . . . . 106
F.2.2 Extension Header . . . . . . . . . . . . . . . . . . 108
F.3 Context Loss Detection . . . . . . . . . . . . . . . . . 109
F.4 Securing locator sets . . . . . . . . . . . . . . . . . 111
F.5 ULID-pair context establishment exchange . . . . . . . . 114
F.6 Updating locator sets . . . . . . . . . . . . . . . . . 115
F.7 State Cleanup . . . . . . . . . . . . . . . . . . . . . 115
19. References . . . . . . . . . . . . . . . . . . . . . . . . 118
19.1 Normative References . . . . . . . . . . . . . . . . . . 118
19.2 Informative References . . . . . . . . . . . . . . . . . 118
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 120
Intellectual Property and Copyright Statements . . . . . . . 121
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1. Introduction
This document describes a layer 3 shim approach and protocol for
providing locator agility below the transport protocols, so that
multihoming can be provided for IPv6 with failover and load sharing
properties [15], without assuming that a multihomed site will have a
provider independent IPv6 address which is announced in the global
IPv6 routing table. The hosts in a site which has multiple provider
allocated IPv6 address prefixes, will use the shim6 protocol
specified in this document to setup state with peer hosts, so that
the state can later be used to failover to a different locator pair,
should the original one stop working.
We assume that redirection attacks are prevented using the mechanism
specified in HBA [7].
The reachability detection and failure detection, including how a new
working locator pair is discovered after a failure, is specified in a
separate documents [8] This document allocates message types and
option types for that sub-protocol, and leaves the specification of
the message and option formats as well as the protocol behavior to
that document.
1.1 Goals
The goals for this approach is to:
o Preserve established communications through certain classes of
failures, for example, TCP connections and application
communications using UDP.
o Have minimal impact on upper layer protocols in general and on
transport protocols in particular.
o Address the security threats in [19] through the combination of
the HBA/CGA approach specified in a separate document [7], and
techniques described in this document.
o Do not require an extra roundtrip up front to setup shim specific
state. Instead allow the upper layer traffic (e.g., TCP) to flow
as normal and defer the setup of the shim state until some number
of packets have been exchanged.
o Take advantage of multiple locators/addresses for load spreading
so that different sets of communication to a host (e.g., different
connections) might use different locators of the host. Note that
this might cause load to be spread unevenly, thus we use the term
"load spreading" instead of "load balancing". This capability
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might enable some forms of traffic engineering, but the details
for traffic engineering, including what requirements can be
satisfied, are not specified in this document, and form part of a
potential extensions to this protocol.
1.2 Non-Goals
The assumption is that the problem we are trying to solve is site
multihoming, with the ability to have the set of site locator
prefixes change over time due to site renumbering. Further, we
assume that such changes to the set of locator prefixes can be
relatively slow and managed; slow enough to allow updates to the DNS
to propagate. But it is not a goal to try to make communication
survive a renumbering event (which causes all the locators of a host
to change to a new set of locators). This proposal does not attempt
to solve the, perhaps related, problem of host mobility. However, it
might turn out that the shim6 protocol can be a useful component for
future host mobility solutions, e.g., for route optimization.
This proposal also does not try to provide a new network level or
transport level identifier namespace separated from the current IP
address namespace. Even though such a concept would be useful to
ULPs and applications, especially if the management burden for such a
name space was negligible and there was an efficient yet secure
mechanism to map from identifiers to locators, such a name space
isn't necessary (and furthermore doesn't seem to help) to solve the
multihoming problem.
1.3 Locators as Upper-layer Identifiers
This approach does not introduce a new identifier name space but
instead uses the locator that is selected in the initial contact with
the remote peer as the preserved upper-level identifier. While there
may be subsequent changes in the selected network level locators over
time in response to failures in using the original locator, the upper
level protocol stack elements will continue to use this upper level
identifier without change.
This implies that the ULID selection is performed as today's default
address selection as specified in RFC 3484 [12]. Some extensions are
needed to RFC 3484 to try different source addresses, whether or not
the shim6 protocol is used, as outlined in [13]. Underneath, and
transparently, the multihoming shim selects working locator pairs
with the initial locator pair being the ULID pair. When
communication fails the shim can test and select alternate locators.
A subsequent section discusses the issues when the selected ULID is
not initially working hence there is a need to switch locators up
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front.
Using one of the locators as the ULID has certain benefits for
applications which have long-lived session state, or performs
callbacks or referrals, because both the FQDN and the 128-bit ULID
work as handles for the applications. However, using a single 128-
bit ULID doesn't provide seamless communication when that locator is
unreachable. See [22] for further discussion of the application
implications.
There has been some discussion of using non-routable locators, such
as unique-local addresses [18], as ULIDs in a multihoming solution.
While this document doesn't specify all aspects of this, it is
believed that the approach can be extended to handle such a case.
For example, the protocol already needs to handle ULIDs that are not
initially reachable. Thus the same mechanism can handle ULIDs that
are permanently unreachable from outside their site. The issue
becomes how to make the protocol perform well when the ULID is known
a priori to be not reachable (e.g., the ULID is a ULA), for instance,
avoiding any timeout and retries in this case. In addition one would
need to understand how the ULAs would be entered in the DNS to avoid
a performance impact on existing, non-shim6 aware, IPv6 hosts
potentially trying to communicate to the (unreachable) ULA.
1.4 IP Multicast
IP Multicast requires that the IP source address field contain a
topologically correct locator for interface that is used to send the
packet, since IP multicast routing uses both the source address and
the destination group to determine where to forward the packet.
(This isn't much different than the situation with widely implemented
ingress filtering [10] for unicast.)
While in theory it would be possible to apply the shim re-mapping of
the IP address fields between ULIDs and locators, the fact that all
the multicast receivers would need to know the mapping to perform,
makes such an approach difficult in practice. Thus it makes sense to
have multicast ULPs operate directly on locators and not use the
shim. This is quite a natural fit for protocols which use RTP [14],
since RTP already has an explicit identifier in the form of the SSRC
field in the RTP headers. Thus the actual IP address fields are not
important to the application.
In summary, IP multicast will not need the shim to remap the IP
addresses.
This doesn't prevent the receiver of multicast to change its
locators, since the receiver is not explicitly identified; the
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destination address is a multicast address and not the unicast
locator of the receiver.
1.5 Renumbering Implications
As stated above, this approach does not try to make communication
survive renumbering in the general case.
When a host is renumbered, the effect is that one or more locators
become invalid, and zero or more locators are added to the host's
network interface. This means that the set of locators that is used
in the shim will change, which the shim can handle as long as not all
the original locators become invalid at the same time.
But IP addresses are also used as ULID, and making the communication
survive locators becoming invalid can potentially cause some
confusion at the upper layers. The fact that a ULID might be used
with a different locator over time open up the possibility that
communication between two ULIDs might continue to work after one or
both of those ULIDs are no longer reachable as locators, for example
due to a renumbering event. This opens up the possibility that the
ULID (or at least the prefix on which it is based) is reassigned to
another site while it is still being used (with another locator) for
existing communication.
Worst case we could end up with two separate hosts using the same
ULID while both of them are communicating with the same host.
This potential source for confusion can be avoided if we require that
any communication using a ULID must be terminated when the ULID
becomes invalid (due to the underlying prefix becoming invalid). If
that behavior is desired, it can be accomplished by explicitly
discarding the shim state when the ULID becomes invalid. The context
recovery mechanism will then make the peer aware that the context is
gone, and that the ULID is no longer present at the same locator(s).
However, terminating the communication might be overkill. Even when
an IPv6 prefix is retired and reassigned to some other site, there is
a very small probability that another host in that site picks the
same 128 bit address (whether using DHCPv6, stateless address
autoconfiguration, or picking a random interface ID [11]). Should
the identical address be used by another host, then there still
wouldn't be a problem until that host attempts to communicate with
the same peer host with which the initial user of the IPv6 address
was communicating.
The protocol as specified in this document does not perform any
action when an address becomes invalid. As we gain further
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understanding of the practical impact of renumbering this might
change in a future version of the protocol.
1.6 Placement of the shim
-----------------------
| Transport Protocols |
-----------------------
------ ------- -------------- ------------- IP endpoint
| AH | | ESP | | Frag/reass | | Dest opts | sub-layer
------ ------- -------------- -------------
---------------------
| shim6 shim layer |
---------------------
------ IP routing
| IP | sub-layer
------
Figure 1: Protocol stack
The proposal uses an multihoming shim layer within the IP layer,
i.e., below the ULPs, as shown in Figure 1, in order to provide ULP
independence. The multihoming shim layer behaves as if it is
associated with an extension header, which would be placed after any
routing-related headers in the packet (such as any hop-by-hop
options, or routing header). However, when the locator pair is the
ULID pair there is no data that needs to be carried in an extension
header, thus none is needed in that case.
Layering AH and ESP above the multihoming shim means that IPsec can
be made to be unaware of locator changes the same way that transport
protocols can be unaware. Thus the IPsec security associations
remain stable even though the locators are changing. This means that
the IP addresses specified in the selectors should be the ULIDs.
Layering the fragmentation header above the multihoming shim makes
reassembly robust in the case that there is broken multi-path routing
which results in using different paths, hence potentially different
source locators, for different fragments. Thus, effectively the
multihoming shim layer is placed between the IP endpoint sublayer,
which handles fragmentation, reassembly, and IPsec, and the IP
routing sublayer, which selects which next hop and interface to use
for sending out packets.
Applications and upper layer protocols use ULIDs which the shim6
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layer will map to/from different locators. The shim6 layer maintains
state, called ULID-pair context, per ULID pairs (that is, applies to
all ULP connections between the ULID pair) in order to perform this
mapping. The mapping is performed consistently at the sender and the
receiver, thus from the perspective of the upper layer protocols,
packets appear to be sent using ULIDs from end to end, even though
the packets travel through the network containing locators in the IP
address fields, and even though those locators might be changed by
the transmitting shim6 layer.
The context state in this approach is maintained per remote ULID i.e.
approximately per peer host, and not at any finer granularity. In
particular, it is independent of the ULPs and any ULP connections.
However, the forking capability enables shim-aware ULPs to use more
than one locator pair at a time for an single ULID pair.
---------------------------- ----------------------------
| Sender A | | Receiver B |
| | | |
| ULP | | ULP |
| | src ULID(A)=L1(A) | | ^ |
| | dst ULID(B)=L1(B) | | | src ULID(A)=L1(A) |
| v | | | dst ULID(B)=L1(B) |
| multihoming shim | | multihoming shim |
| | src L2(A) | | ^ |
| | dst L3(B) | | | src L2(A) |
| v | | | dst L3(B) |
| IP | | IP |
---------------------------- ----------------------------
| ^
------- cloud with some routers -------
Figure 2: Mapping with changed locators
The result of this consistent mapping is that there is no impact on
the ULPs. In particular, there is no impact on pseudo-header
checksums and connection identification.
Conceptually one could view this approach as if both ULIDs and
locators are being present in every packet, and with a header
compression mechanism applied that removes the need for the ULIDs to
be carried in the packets once the compression state has been
established. In order for the receiver to recreate a packet with the
correct ULIDs there is a need to include some "compression tag" in
the data packets. This serves to indicate the correct context to use
for decompression when the locator pair in the packet is insufficient
to uniquely identify the context.
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1.7 Traffic Engineering
At the time of this writing it is not clear what requirements for
traffic engineering make sense for the shim6 protocol, since the
requirements must both result in some useful behavior as well as be
implementable using a host-to-host locator agility mechanism like
shim6.
Inherent in a scalable multihoming mechanism that separates locators
from identifiers is that each host ends up with multiple locators.
This means that at least for initial contact, it is the remote peer
that needs to select which peer locator to try first. In the case of
shim6 this is performed by applying RFC 3484 address selection.
This is quite different than the common case of IPv4 multihoming
where the site has a single IP address prefix, since in that case the
peer performs no destination address selection.
Thus in "single prefix multihoming" the site, and in many cases its
upstream ISPs, can use BGP to exert some control of the ingress used
to reach the site. This capability can't easily be recreated in
"multiple prefix multihoming" such as shim6.
The protocol provides a placeholder, in the form of the Locator
Preferences option, which can be used by hosts to express priority
and weight values for each locator. This is intentionally made
identical to the DNS SRV [9] specification of priority and weight, so
that DNS SRV records can be used for initial contact and the shim for
failover, and they can use the same way to describe the preferences.
The format allows adding additional notions of "metrics" over time.
But the Locator Preference option is merely a place holder when it
comes to providing traffic engineering; in order to use this in a
large site there would have to be a mechanism by which the host can
find out what preference values to use, either statically (e.g., some
new DHCPv6 option) or dynamically.
Thus traffic engineering is listed as a possible extension in
Appendix B.
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2. Terminology
This document uses the terms MUST, SHOULD, RECOMMENDED, MAY, SHOULD
NOT and MUST NOT defined in RFC 2119 [1]. The terms defined in RFC
2460 [2] are also used.
2.1 Definitions
This document introduces the following terms:
upper layer protocol (ULP)
A protocol layer immediately above IP. Examples
are transport protocols such as TCP and UDP,
control protocols such as ICMP, routing protocols
such as OSPF, and internet or lower-layer
protocols being "tunneled" over (i.e.,
encapsulated in) IP such as IPX, AppleTalk, or IP
itself.
interface A node's attachment to a link.
address An IP layer name that contains both topological
significance and acts as a unique identifier for
an interface. 128 bits. This document only uses
the "address" term in the case where it isn't
specific whether it is a locator or an
identifier.
locator An IP layer topological name for an interface or
a set of interfaces. 128 bits. The locators are
carried in the IP address fields as the packets
traverse the network.
identifier An IP layer name for an IP layer endpoint. The
transport endpoint name is a function of the
transport protocol and would typically include
the IP identifier plus a port number.
NOTE: This proposal does not specify any new form
of IP layer identifier, but still separates the
identifying and locating properties of the IP
addresses.
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upper-layer identifier (ULID)
An IP address which has been selected for
communication with a peer to be used by the upper
layer protocol. 128 bits. This is used for
pseudo-header checksum computation and connection
identification in the ULP. Different sets of
communication to a host (e.g., different
connections) might use different ULIDs in order
to enable load spreading.
Since the ULID is just one of the IP locators/
addresses of the node, there is no need for a
separate name space and allocation mechanisms.
address field The source and destination address fields in the
IPv6 header. As IPv6 is currently specified this
fields carry "addresses". If identifiers and
locators are separated these fields will contain
locators for packets on the wire.
FQDN Fully Qualified Domain Name
ULID-pair context The state that the multihoming shim maintains
between a pair of Upper-layer identifiers. The
context is identified by a context tag for each
direction of the communication, and also
identified by the pair of ULID and a Forked
Instance Identifier (see below).
Context tag Each end of the context allocates a context tag
for the context. This is used to uniquely
associate both received control packets and
payload extension headers as belonging to the
context.
Current locator pair
Each end of the context has a current locator
pair which is used to send packets to the peer.
The two ends might use different current locator
pairs though.
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Default context At the sending end, the shim uses the ULID pair
(passed down from the ULP) to find the context
for that pair. Thus, normally, a host can have
at most one context for a ULID pair. We call
this the "default context".
Context forking A mechanism which allows ULPs that are aware of
multiple locators to use separate contexts for
the same ULID pair, in order to be able use
different locator pairs for different
communication to the same ULID. Context forking
causes more than just the default context to be
created for a ULID pair.
Forked Instance Identifier (FII)
In order to handle context forking, a context is
identified by a ULID-pair and a forked context
identifier. The default context has a FII of
zero.
Initial contact We use this term to refer to the pre-shim
communication when some ULP decides to start
communicating with a peer by sending and
receiving ULP packets. Typically this would not
invoke any operations in the shim, since the shim
can defer the context establishment until some
arbitrary later point in time.
Hash Based Addresses (HBA)
A form of IPv6 address where the interface ID is
derived from a cryptographic hash of all the
prefixes assigned to the host. See [7].
Cryptographically Generated Addresses (CGA)
A form of IPv6 address where the interface ID is
derived from a cryptographic hash of the public
key. See [6].
CGA Parameter Data Structure (PDS)
The information that CGA and HBA exchanges in
order to inform the peer of how the interface ID
was computed. See [6]., [7].
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2.2 Notational Conventions
A, B, and C are hosts. X is a potentially malicious host.
FQDN(A) is the domain name for A.
Ls(A) is the locator set for A, which consists of the locators L1(A),
L2(A), ... Ln(A).
ULID(A) is an upper-layer ID for A. In this proposal, ULID(A) is
always one member of A's locator set.
CT(X) is a context tag assigned by X.
This document also makes use of internal conceptual variables to
describe protocol behavior and external variables that an
implementation must allow system administrators to change. The
specific variable names, how their values change, and how their
settings influence protocol behavior are provided to demonstrate
protocol behavior. An implementation is not required to have them in
the exact form described here, so long as its external behavior is
consistent with that described in this document. See Section 6 for a
description of the conceptual data structures.
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3. Assumptions
The design intent is to ensure that the shim6 protocol is capable of
handling path failures independently of the number of IP addresses
(locators) available to the two communicating hosts, and
independently of which host detects the failure condition.
In the case when host A and host B have an active shim6 state, with
host A having only one locator and host B having multiple locators,
it might be that host B is trying to send a packet to host A, and has
detected a failure condition with the current locator pair. As host
B has multiple locators it presumably has multiple ISPs. In this
case there are probably alternate egress paths for host B to be able
to try to reach A, but B can not vary the destination address (host A
locator) to select such alternate paths, since A has only one
locator.
This leads to the assumption that a host should be able to cause
different egress paths from its site to be used. The most reasonable
approach to accomplish this is to have the host use different source
addresses and have the source address affect the selection of the
site egress. The details of how this can be accomplished is beyond
the scope of this document, but without this capability the ability
of the shim to try different "paths" by trying different locator
pairs will have limited utility.
The above assumption applies whether or not the ISPs perform ingress
filtering.
In addition, when the site's ISPs perform ingress filtering based on
packet source addresses, shim6 assumes that packets sent with
different source and destination combinations have a reasonable
chance of making it through the relevant ISP's ingress filters. This
can be accomplished in several ways (all outside the scope of this
document), such as having the ISPs relax there ingress filters, or
selecting the egress such that it matches the IP source address
prefix.
Further discussion of this issue is captured in [20].
The shim6 approach assumes that there are no IPv6-to-IPv6 NATs on the
paths, i.e., that the two ends can exchange their own notion of their
IPv6 addresses and that those addresses will also make sense to their
peer.
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4. Protocol Overview
The shim6 protocol operates in several phases over time. The
following sequence illustrates the concepts:
o An application on host A decides to contact B using some upper-
layer protocol. This results in the ULP on A sending packets to
B. We call this the initial contact. Assuming the IP addresses
selected by Default Address Selection [12] and its extensions [13]
work, then there is no action by the shim at this point in time.
Any shim context establishment can be deferred until later.
o Some heuristic on A or B (or both) determine that it is
appropriate to pay the shim6 overhead to make this host-to-host
communication robust against locator failures. For instance, this
heuristic might be that more than 50 packets have been sent or
received, or a timer expiration while active packet exchange is in
place. This makes the shim initiate the 4-way context
establishment exchange.
As a result of this exchange, both A and B will know a list of
locators for each other.
If the context establishment exchange fails, the initiator will
then know that the other end does not support shim6, and will
continue with standard unicast behavior for the session.
o Communication continues without any change for the ULP packets.
In particular, there are no shim extension headers added to the
ULP packets, since the ULID pair is the same as the locator pair.
In addition, there might be some messages exchanged between the
shim sub-layers for (un)reachability detection.
o At some point in time something fails. Depending on the approach
to reachability detection, there might be some advice from the
ULP, or the shim (un)reachability detection might discover that
there is a problem.
At this point in time one or both ends of the communication need
to probe the different alternate locator pairs until a working
pair is found, and switch to using that locator pair.
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o Once a working alternative locator pair has been found, the shim
will rewrite the packets on transmit, and tag the packets with
shim6 Payload extension header, which contains the receiver's
context tag. The receiver will use the context tag to find the
context state which will indicate which addresses to place in the
IPv6 header before passing the packet up to the ULP. The result
is that from the perspective of the ULP the packet passes
unmodified end-to-end, even though the IP routing infrastructure
sends the packet to a different locator.
o The shim (un)reachability detection will monitor the new locator
pair as it monitored the original locator pair, so that subsequent
failures can be detected.
o In addition to failures detected based on end-to-end observations,
one endpoint might know for certain that one or more of its
locators is not working. For instance, the network interface
might have failed or gone down (at layer 2), or an IPv6 address
might have become deprecated or invalid. In such cases the host
can signal its peer that this address is no longer recommended to
try. Thus this triggers something similar to a failure handling
in that a new, working locator pair must be found.
The protocol also has the ability to express other forms of
locator preferences. A change in any preferences can be signaled
to the peer, which will made the peer record the new preferences.
A change in the preferences might optionally make the peer want to
use a different locator pair. If it makes this decision, it
follows the same locator switching procedure as after a failure
(by verifying that its peer is indeed present at the alternate
locator, etc).
o When the shim thinks that the context state is no longer used, it
can garbage collect the state; there is no coordination necessary
with the peer host before the state is removed. There is a
recovery message defined to be able to signal when there is no
context state, which can be used to detect and recover from both
premature garbage collection, as well as complete state loss
(crash and reboot) of a peer.
The exact mechanism to determine when the context state is no
longer used is implementation dependent. An implementation might
use the existence of ULP state (where known to the implementation)
as an indication that the state is still used, combined with a
timer (to handle ULP state that might not be known to the shim
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sub-layer) to determine when the state is likely to no longer be
used.
NOTE: The ULP packets in shim6 can be carried completely unmodified
as long as the ULID pair is used as the locator pair. After a switch
to a different locator pair the packets are "tagged" with a shim6
extension header, so that the receiver can always determine the
context to which they belong. This is accomplished by including an
8-octet shim6 Payload Extension header before the (extension) headers
that are processed by the IP endpoint sublayer and ULPs. If
subsequently the original ULIDs are selected as the active locator
pair then the tagging of packets with the shim6 extension header can
also be stopped.
4.1 Context Tags
A context between two hosts is actually a context between two ULIDs.
The context is identified by a pair of context tags. Each end gets
to allocate a context tag, and once the context is established, most
shim6 control messages contain the context tag that the receiver of
the message allocated. Thus at a minimum the combination of <peer
ULID, local ULID, local context tag> have to uniquely identify one
context. But since the Payload extension headers are demultiplexed
without looking at the locators in the packet, the receiver will need
to allocate context tags that are unique for all its contexts. The
context tag is a 47-bit number (the largest which can fit in an
8-octet extension header).
The mechanism for detecting a loss of context state at the peer that
is currently proposed in this document assumes that the receiver can
tell the packets that need locator rewriting, even after it has lost
all state (e.g., due to a crash followed by a reboot). This is
achieved because after a rehoming event the packets that need
receive-side rewriting, carry the Payload extension header.
4.2 Context Forking
It has been asserted that it will be important for future ULPs, in
particular, future transport protocols, to be able to control which
locator pairs are used for different communication. For instance,
host A and host B might communicate using both VoIP traffic and ftp
traffic, and those communications might benefit from using different
locator pairs. However, the fundamental shim6 mechanism uses a
single current locator pair for each context, thus a single context
can not accomplish this.
For this reason, the shim6 protocol supports the notion of context
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forking. This is a mechanism by which a ULP can specify (using some
API not yet defined) that a context for e.g., the ULID pair <A1, B2>
should be forked into two contexts. In this case the forked-off
context will be assigned a non-zero Forked Instance Identifier, while
the default context has FII zero.
The Forked Instance Identifier is a 32-bit identifier which has no
semantics in the protocol other then being part of the tuple which
identifies the context. The hosts can allocate FIIs e.g., as
sequential numbers for any given ULID pair.
No other special considerations are needed in the shim6 protocol to
handle forked contexts.
Note that forking as specified does NOT allow A to be able to tell B
that certain traffic (a 5-tuple?) should be forked for the reverse
direction. The shim6 forking mechanism as specified applies only to
the sending of ULP packets. If some ULP wants to fork for both
directions, it is up to the ULP to set this up, and then instruct the
shim at each end to transmit using the forked context.
4.3 API Extensions
Several API extensions have been discussed for shim6, but their
actual specification is out of scope for this document. The simplest
one would be to add a socket option to be able to have traffic bypass
the shim (not create any state, and not use any state created by
other traffic). This could be an IPV6_DONTSHIM socket option. Such
an option would be useful for protocols, such as DNS, where the
application has its own failover mechanism (multiple NS records in
the case of DNS) and using the shim could potentially add extra
latency with no added benefits.
Some other API extensions are discussed in Appendix B
4.4 Securing shim6
The mechanisms are secured using a combination of techniques:
o The HBA technique [7] for verifying the locators to prevent an
attacker from redirecting the packet stream to somewhere else.
o Requiring a Reachability Probe+Reply before a new locator is used
as the destination, in order to prevent 3rd party flooding
attacks.
o The first message does not create any state on the responder.
Essentially a 3-way exchange is required before the responder
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creates any state. This means that a state-based DoS attack
(trying to use up all of memory on the responder) at least
provides an IPv6 address that the attacker was using.
o The context establishment messages use nonces to prevent replay
attacks, and to prevent off-path attackers from interfering with
the establishment.
o Every control message of the shim6 protocol, past the context
establishment, carry the context tag assigned to the particular
context. This implies that an attacker needs to discover that
context tag before being able to spoof any shim6 control message.
Such discovery probably requires to be along the path in order to
be sniff the context tag value. The result is that through this
technique, the shim6 protocol is protected against off-path
attackers.
4.5 Overview of Shim Control Messages
The shim6 context establishment is accomplished using four messages;
I1, R1, I2, R2. Normally they are sent in that order from initiator
and responder, respectively. Should both ends attempt to set up
context state at the same time (for the same ULID pair), then their
I1 messages might cross in flight, and result in an immediate R2
message. [The names of these messages are borrowed from HIP [25].]
R1bis and I2bis messages are defined, which are used to recover a
context after it has been lost. A R1bis message is sent when a shim6
control or Payload extension header arrives and there is no matching
context state at the receiver. When such a message is received, it
will result in the re-creation of the shim6 context using the I2bis
and R2 messages.
The peers' lists of locators are normally exchanged as part of the
context establishment exchange. But the set of locators might be
dynamic. For this reason there is a Update Request and Update
Acknowledgement messages, and a Locator List option.
Even when the list of locators is fixed, a host might determine that
some preferences might have changed. For instance, it might
determine that there is a locally visible failure that implies that
some locator(s) are no longer usable. This uses a Locator
Preferences option in the Update Request message.
The mechanism for (un)reachability detection is called Forced
Bidirectional Communication (FBD). The FBD approach uses a Keepalive
message, which is sent when a host has received packets from the
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peer, but the ULP has not given the host an opportunity to send any
packet to the peer. The message type is reserved in this document,
but the message format and processing rules are specified in [8].
In addition, when the context is established and there is a failure
there needs to be a way to probe the set of locator pairs to
efficiently find a working pair. This document reserves an Probe
message type, with the packet format and processing rules specified
in [8].
The above probe and keepalive messages assume we have an established
ULID-pair context. However, communication might fail during the
initial contact (that is, when the application or transport protocol
is trying to setup some communication). This is handled using the
mechanisms in the ULP to try different address pairs as specified in
[12] [13]. In the future versions of the protocol, and with a richer
API between the ULP and the shim, the shim might be help optimize
discovering a working locator pair during initial contact. This is
for further study.
4.6 Extension Header Order
Since the shim is placed between the IP endpoint sub-layer and the IP
routing sub-layer in the host, the shim header will be placed before
any endpoint extension headers (fragmentation headers, destination
options header, AH, ESP), but after any routing related headers (hop-
by-hop extensions header, routing header, a destinations options
header which precedes a routing header). When tunneling is used,
whether IP-in-IP tunneling or the special form of tunneling that
Mobile IPv6 uses (with Home Address Options and Routing header type
2), there is a choice whether the shim applies inside the tunnel or
outside the tunnel, which effects the location of the shim6 header.
In most cases IP-in-IP tunnels are used as a routing technique, thus
it makes sense to apply them on the locators which means that the
sender would insert the shim6 header after any IP-in-IP
encapsulation; this is what occurs naturally when routers apply IP-
in-IP encapsulation. Thus the packets would have:
o Outer IP header
o Inner IP header
o Shim6 extension header (if needed>
o ULP
But the shim can also be used to create "shimmed tunnels" i.e., where
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an IP-in-IP tunnel uses the shim to be able to switch the tunnel
endpoint addresses between different locators. In such a case the
packets would have:
o Outer IP header
o Shim6 extension header (if needed>
o Inner IP header
o ULP
In any case, the receiver behavior is well-defined; a receiver
processes the extension headers in order. However, the precise
interaction between Mobile IPv6 and shim6 is for further study, but
it might make sense to have Mobile IPv6 operate on locators as well,
meaning that the shim would be layered on top of the MIPv6 mechanism.
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5. Message Formats
The shim6 messages are all carried using a new IP protocol number [to
be assigned by IANA]. The shim6 messages have a common header,
defined below, with some fixed fields, followed by type specific
fields.
The shim6 messages are structured as an IPv6 extension header since
the Payload extension header is used to carry the ULP packets after a
locator switch. The shim6 control messages use the same extension
header formats so that a single "protocol number" needs to be allowed
through firewalls in order for shim6 to function across the firewall.
5.1 Common shim6 Message Format
The first 17 bits of the shim6 header is common for the Payload
extension header and the control messages and looks as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: The payload which follows this header.
Hdr Ext Len: 8-bit unsigned integer. Length of the shim6 header in
8-octet units, not including the first 8 octets.
P: A single bit to distinguish Payload extension headers
from control messages.
5.2 Payload Extension Header Format
The payload extension headers is used to carry ULP packets where the
receiver must replace the content of the source and/or destination
fields in the IPv6 header before passing the packet to the ULP. Thus
this extension header is required when the locators pair that is used
is not the same as the ULID pair.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | 0 |1| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: The payload which follows this header.
Hdr Ext Len: 0 (since the header is 8 octets).
P: Set to one. A single bit to distinguish this from the
shim6 control messages.
Receiver Context Tag: 47-bit unsigned integer. Allocated by the
receiver for use to identify the context.
5.3 Common Shim6 Control header
The common part of the header has a next header and header extension
length field which is consistent with the other IPv6 extension
headers, even if the next header value is always "NO NEXT HEADER" for
the control messages; only the payload extension header use the Next
Header field.
The shim6 headers must be a multiple of 8 octets, hence the minimum
size is 8 octets.
The common shim control message header is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Header | Hdr Ext Len |0| Type |Type-specific|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Type-specific format |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
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Next Header: 8-bit selector. Normally set to NO_NXT_HDR (59).
Hdr Ext Len: 8-bit unsigned integer. Length of the shim6 header in
8-octet units, not including the first 8 octets.
P: Set to zero. A single bit to distinguish this from
the shim6 payload extension header.
Type: 7-bit unsigned integer. Identifies the actual message
from the table below. Type codes 0-63 will not
trigger R1bis messages on a missing context, while 64-
127 will trigger R1bis.
0: A single bit (set to zero) which allows shim6 and HIP
to have a common header format yet telling shim6 and
HIP messages apart.
Checksum: 16-bit unsigned integer. The checksum is the 16-bit
one's complement of the one's complement sum of the
entire shim6 header message starting with the shim6
next header field, and ending as indicated by the Hdr
Ext Len. Thus when there is a payload following the
shim6 header, the payload is NOT included in the shim6
checksum. Note that unlike protocol like ICMPv6,
there is no pseudo-header checksum part of the
checksum, in order to provide locator agility without
having to change the checksum.
Type-specific: Part of message that is different for different
message types.
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+------------+-----------------------------------------------------+
| Type Value | Message |
+------------+-----------------------------------------------------+
| 1 | I1 (first establishment message from the initiator) |
| | |
| 2 | R1 (first establishment message from the responder) |
| | |
| 3 | I2 (2nd establishment message from the initiator) |
| | |
| 4 | R2 (2nd establishment message from the responder) |
| | |
| 5 | R1bis (Reply to reference to non-existent context) |
| | |
| 6 | I2bis (Reply to a R1bis message) |
| | |
| 64 | Update Request |
| | |
| 65 | Update Acknowledgement |
| | |
| 66 | Keepalive |
| | |
| 67 | Probe Message |
+------------+-----------------------------------------------------+
Table 1
5.4 I1 Message Format
The I1 message is the first message in the context establishment
exchange.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 1 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 1
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 47-bit field. The Context Tag the initiator
has allocated for the context.
Initiator Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the responder will return in the
R1 message.
The following options are defined for this message:
ULID pair: When the IPv6 source and destination addresses in the
IPv6 header does not match the ULID pair, this option
MUST be included. An example of this is when
recovering from a lost context.
Forked Instance Identifier: When another instance of an existent
context with the same ULID pair is being created, a
Forked Instance Identifier option is included to
distinguish this new instance from the existent one.
5.5 R1 Message Format
The R1 message is the second message in the context establishment
exchange. The responder sends this in response to an I1 message,
without creating any state specific to the initiator.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 2 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 2
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Reserved2: 16-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Nonce: 32-bit unsigned integer. Copied from the I1
message.
Responder Nonce: 32-bit unsigned integer. A number picked by the
responder which the initiator will return in the I2
message.
The following options are defined for this message:
Responder Validator: Variable length option. Typically a hash
generated by the responder, which the responder uses
together with the Responder Nonce value to verify that
an I2 message is indeed sent in response to a R1
message, and that the parameters in the I2 message are
the same as those in the I1 message.
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5.6 I2 Message Format
The I2 message is the third message in the context establishment
exchange. The initiator sends this in response to a R1 message,
after checking the Initiator Nonce, etc.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 3 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 2, since the header is 24 octets when there
are no options.
Type: 3
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 47-bit field. The Context Tag the initiator
has allocated for the context.
Initiator Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the responder will return in the
R2 message.
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Responder Nonce: 32-bit unsigned integer. Copied from the R1
message.
Reserved2: 32-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt. (Needed to
make the options start on a multiple of 8 octet
boundary.)
The following options are defined for this message:
Responder Validator: Variable length option. Just a copy of the
Responder Validator option in the R1 message.
ULID pair: When the IPv6 source and destination addresses in the
IPv6 header does not match the ULID pair, this option
MUST be included. An example of this is when
recovering from a lost context.
Forked Instance Identifier: When another instance of an existent
context with the same ULID pair is being created, a
Forked Instance Identifier option is included to
distinguish this new instance from the existent one.
Locator list: Optionally sent when the initiator immediately wants
to tell the responder its list of locators. When it
is sent, the necessary HBA/CGA information for
verifying the locator list MUST also be included.
Locator Preferences: Optionally sent when the locators don't all have
equal preference.
CGA Parameter Data Structure: Included when the locator list is
included so the receiver can verify the locator list.
CGA Signature: Included when the some of the locators in the list use
CGA (and not HBA) for verification.
5.7 R2 Message Format
The R2 message is the fourth message in the context establishment
exchange. The responder sends this in response to an I2 message.
The R2 message is also used when both hosts send I1 messages at the
same time and the I1 messages cross in flight.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 4 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Responder Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 4
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Responder Context Tag: 47-bit field. The Context Tag the responder
has allocated for the context.
Initiator Nonce: 32-bit unsigned integer. Copied from the I2
message.
The following options are defined for this message:
Locator List: Optionally sent when the responder immediately wants
to tell the initiator its list of locators. When it
is sent, the necessary HBA/CGA information for
verifying the locator list MUST also be included.
Locator Preferences: Optionally sent when the locators don't all have
equal preference.
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CGA Parameter Data Structure: Included when the locator list is
included so the receiver can verify the locator list.
CGA Signature: Included when the some of the locators in the list use
CGA (and not HBA) for verification.
5.8 R1bis Message Format
Should a host receive a packet with a shim Payload extension header
or shim6 control message with type code 64-127 (such as an Update or
Probe message), and the host does not have any context state for the
received context tag, then it will generate a R1bis message.
This message allows the sender of the packet referring to the non-
existent context to re-establish the context with a reduced context
establishment exchange. Upon the reception of the R1bis message, the
receiver can proceed reestablishing the lost context by directly
sending an I2bis message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 5 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Packet Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 5
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Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Packet Context Tag: 47-bit unsigned integer. The context tag
contained in the received packet that triggered the
generation of the R1bis message.
Responder Nonce: 32-bit unsigned integer. A number picked by the
responder which the initiator will return in the I2bis
message.
The following options are defined for this message:
Responder Validator: Variable length option. Typically a hash
generated by the responder, which the responder uses
together with the Responder Nonce value to verify that
an I2bis message is indeed sent in response to a R1bis
message.
5.9 I2bis Message Format
The I2bis message is the third message in the context recovery
exchange. This is sent in response to a R1bis message, after
checking that the R1bis message refers to an existing context, etc.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 6 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Packet Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 3, since the header is 32 octets when there
are no options.
Type: 6
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Initiator Context Tag: 47-bit field. The Context Tag the initiator
has allocated for the context.
Initiator Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the responder will return in the
R2 message.
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Responder Nonce: 32-bit unsigned integer. Copied from the R1bis
message.
Reserved2: 49-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt. (Note that 17
bits are not sufficient since the options need start
on a multiple of 8 octet boundary.)
Packet Context Tag: 47-bit unsigned integer. Copied from the Packet
Context Tag contained in the received R1bis.
The following options are defined for this message:
Responder Validator: Variable length option. Just a copy of the
Responder Validator option in the R1bis message.
ULID pair: When the IPv6 source and destination addresses in the
IPv6 header does not match the ULID pair, this option
MUST be included.
Forked Instance Identifier: When another instance of an existent
context with the same ULID pair is being created, a
Forked Instance Identifier option is included to
distinguish this new instance from the existent one.
Locator list: Optionally sent when the initiator immediately wants
to tell the responder its list of locators. When it
is sent, the necessary HBA/CGA information for
verifying the locator list MUST also be included.
Locator Preferences: Optionally sent when the locators don't all have
equal preference.
CGA Parameter Data Structure: Included when the locator list is
included so the receiver can verify the locator list.
CGA Signature: Included when the some of the locators in the list use
CGA (and not HBA) for verification.
5.10 Update Request Message Format
The Update Request Message is used to update either the list of
locators, the locator preferences, and both. When the list of
locators is updated, the message also contains the option(s)
necessary for HBA/CGA to secure this. The basic sanity check that
prevents off-path attackers from generating bogus updates is the
context tag in the message.
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The update message contains options (the Locator List and the Locator
Preferences) that, when included, completely replace the previous
locator list and locator preferences, respectively. Thus there is no
mechanism to just send deltas to the locator list.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 64 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 64
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 47-bit field. The Context Tag the receiver has
allocated for the context.
Request Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the peer will return in the
acknowledgement message.
The following options are defined for this message:
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Locator List: The list of the sender's (new) locators. The locators
might be unchanged and only the preferences have
changed.
Locator Preferences: Optionally sent when the locators don't all have
equal preference.
CGA Parameter Data Structure (PDS): Included when the locator list is
included and the PDS was not included in the
I2/I2bis/R2 messages, so the receiver can verify the
locator list.
CGA Signature: Included when the some of the locators in the list use
CGA (and not HBA) for verification.
5.11 Update Acknowledgement Message Format
This message is sent in response to a Update Request message. It
implies that the Update Request has been received, and that any new
locators in the Update Request can now be used as the source locators
of packets. But it does not imply that the (new) locators have been
verified to be used as a destination, since the host might defer the
verification of a locator until it sees a need to use a locator as
the destination.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 | Hdr Ext Len |0| Type = 65 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |R| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Next Header: NO_NXT_HDR (59).
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Hdr Ext Len: At least 1, since the header is 16 octets when there
are no options.
Type: 65
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
R: 1-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 47-bit field. The Context Tag the receiver has
allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from the Update
Request message.
No options are currently defined for this message.
5.12 Keepalive Message Format
This message format is defined in [8].
The message is used to ensure that when a peer is sending ULP packets
on a context, it always receives some packets in the reverse
direction. When the ULP is sending bidirectional traffic, no extra
packets need to be inserted. But for a unidirectional ULP traffic
pattern, the shim will send back some Keepalive messages when it is
receiving ULP packets.
5.13 Probe Message Format
This message and its semantics are defined in [8].
The idea behind that mechanism is to be able to handle the case when
one locator pair works in from A to B, and another locator pair works
from B to A, but there is no locator pair which works in both
directions. The protocol mechanism is that as A is sending probe
messages to B, B will observe which locator pairs it has received
from and report that back in probe messages it is sending to A.
5.14 Option Formats
The format of the options is a snapshot of the current HIP option
format [25]. However, there is no intention to track any changes to
the HIP option format, nor is there an intent to use the same name
space for the option type values. But using the same format will
hopefully make it easier to import HIP capabilities into shim6 as
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extensions to shim6, should this turn out to be useful.
All of the TLV parameters have a length (including Type and Length
fields) which is a multiple of 8 bytes. When needed, padding MUST be
added to the end of the parameter so that the total length becomes a
multiple of 8 bytes. This rule ensures proper alignment of data. If
padding is added, the Length field MUST NOT include the padding. Any
added padding bytes MUST be zeroed by the sender, and their values
SHOULD NOT be checked by the receiver.
Consequently, the Length field indicates the length of the Contents
field (in bytes). The total length of the TLV parameter (including
Type, Length, Contents, and Padding) is related to the Length field
according to the following formula:
Total Length = 11 + Length - (Length + 3) % 8;
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type |C| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ~
~ Contents ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Type: 15-bit identifier of the type of option. The options
defined in this document are below.
C: Critical. One if this parameter is critical, and MUST
be recognized by the recipient, zero otherwise. An
implementation might view the C bit as part of the
Type field, by multiplying the type values in this
specification by two.
Length: Length of the Contents, in bytes.
Contents: Parameter specific, defined by Type.
Padding: Padding, 0-7 bytes, added if needed.
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+------+---------------------------------+
| Type | Option Name |
+------+---------------------------------+
| 1 | Responder Validator |
| | |
| 2 | Locator List |
| | |
| 3 | Locator Preferences |
| | |
| 4 | CGA Parameter Data Structure |
| | |
| 5 | CGA Signature |
| | |
| 6 | ULID Pair |
| | |
| 7 | Forked Instance Identifier |
| | |
| 10 | Probe Option |
| | |
| 11 | Reachability Option |
| | |
| 12 | Payload Reception Report Option |
+------+---------------------------------+
Table 2
5.14.1 Responder Validator Option Format
The responder can choose exactly what input uses to compute the
validator, and what one-way function (MD5, SHA1) it uses, as long as
the responder can check that the validator it receives back in the I2
or I2bis message is indeed one that:
1)- it computed,
2)- it computed for the particular context, and
3)- that it isn't a replayed I2/I2bis message.
Some suggestions on how to generate the validators are captured in
Section 7.9.1 and Section 7.16.1.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 1 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Validator ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Validator: Variable length content whose interpretation is local
to the responder.
Padding: Padding, 0-7 bytes, added if needed. See
Section 5.14.
5.14.2 Locator List Option Format
The Locator List Option is used to carry all the locators of the
sender. Note that the order of the locators is important, since the
Locator Preferences refers to the locators by using the index in the
list.
Note that we carry all the locators in this option even though some
of them can be created automatically from the CGA Parameter Data
Structure.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 2 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator List Generation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Num Locators | N Octets of Verification Method |
+-+-+-+-+-+-+-+-+ |
~ ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Locators 1 through N ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
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Locator List Generation: 32-bit unsigned integer. Indicates a
generation number which is increased by one for each
new locator list. This is used to ensure that the
index in the Locator Preferences refer to the right
version of the locator list.
Num Locators: 8-bit unsigned integer. The number of locators that
are included in the option. We call this number "N"
below.
Verification Method: N octets. The i'th octet specifies the
verification method for the i'th locator.
Padding: Padding, 0-7 bytes, added if needed so that the
Locators start on a multiple of 8 octet boundary.
NOTE that for this option there is never a need to pad
at the end, since the locators are a multiple of 8
octets in length. This internal padding is included
in the length field.
Locators: N 128-bit locators.
The defined verification methods are:
+-------+----------+
| Value | Method |
+-------+----------+
| 0 | Reserved |
| | |
| 1 | HBA |
| | |
| 2 | CGA |
| | |
| 3-255 | Reserved |
+-------+----------+
Table 3
5.14.3 Locator Preferences Option Format
The Locator Preferences option can have some flags to indicate
whether or not a locator is known to work. In addition, the sender
can include a notion of preferences. It might make sense to define
"preferences" as a combination of priority and weight the same way
that DNS SRV records has such information. The priority would
provide a way to rank the locators, and within a given priority, the
weight would provide a way to do some load sharing. See [9] for how
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SRV defines the interaction of priority and weight.
The minimum notion of preferences we need is to be able to indicate
that a locator is "dead". We can handle this using a single octet
flag for each locator.
We can extend that by carrying a larger "element" for each locator.
This document presently also defines 2-octet and 3-octet elements,
and we can add more information by having even larger elements if
need be.
The locators are not included in the preference list. Instead, the
first element refers to locator that was in the first element in the
Locator List option. The generation number carried in this option
and the Locator List option is used to verify that they refer to the
same version of the locator list.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Locator List Generation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Element Len | Element[1] | Element[2] | Element[3] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Case of Element Len = 1 is depicted.
Fields:
Locator List Generation: 32-bit unsigned integer. Indicates a
generation number for the locator list to which the
elements should apply.
Element Len: 8-bit unsigned integer. The length in octets of each
element. This specification defines the cases when
the length is 1, 2, or 3.
Element[i]: A field with a number of octets defined by the Element
Len field. Provides preferences for the i'th locator
in the Locator List option that is in use.
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Padding: Padding, 0-7 bytes, added if needed. See
Section 5.14.
When the Element length equals one, then the element consists of only
a one octet flags field. The currently defined set of flags are:
BROKEN: 0x01
TEMPORARY: 0x02
The intent of TEMPORARY is to allow the distinction between more
stable addresses and less stable addresses when shim6 is combined
with IP mobility, when we might have more stable home locators, and
less stable care-of-locators.
When the Element length equals two, then the element consists of a 1
octet flags field followed by a 1 octet priority field. The priority
has the same semantics as the priority in DNS SRV records.
When the Element length equals three, then the element consists of a
1 octet flags field followed by a 1 octet priority field, and a 1
octet weight field. The weight has the same semantics as the weight
in DNS SRV records.
This document doesn't specify the format when the Element length is
more than three, except that any such formats MUST be defined so that
the first three octets are the same as in the above case, that is, a
of a 1 octet flags field followed by a 1 octet priority field, and a
1 octet weight field.
5.14.4 CGA Parameter Data Structure Option Format
This option contains the CGA Parameter Data Structure (PDS). When
HBA is used to verify the locators, the PDS contains the HBA
multiprefix extension. When CGA is used to verify the locators, in
addition to the PDS option, the host also needs to include the
signature in the form of a CGA Signature option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ CGA Parameter Data Structure ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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Fields:
CGA Parameter Data Structure: Variable length content. Content
defined in [6] and [7].
Padding: Padding, 0-7 bytes, added if needed. See
Section 5.14.
5.14.5 CGA Signature Option Format
When CGA is used for verification of one or more of the locators in
the Locator List option, then the message in question will need to
contain this option.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 5 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ CGA Signature ~
~ +-+-+-+-+-+-+-+-+
~ | Padding |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
CGA Signature: A variable-length field containing a PKCS#1 v1.5
signature, constructed by using the sender's private
key over the following sequence of octets:
1. The 128-bit CGA Message Type tag [CGA] value for
SHIM6, 0x4A 30 5662 4858 574B 3655 416F 506A 6D48.
(The tag value has been generated randomly by the
editor of this specification.).
2. The Locator List Generation value of the
correspondent Locator List Option.
3. The subset of locators included in the
correspondent Locator List Option which
verification method is set to CGA. The locators
MUST be included in the order they are listed in
the Locator List Option.
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Padding: Padding, 0-7 bytes, added if needed. See
Section 5.14.
5.14.6 ULID Pair Option Format
I1, I2, and I2bis messages MUST contain the ULID pair; normally this
is in the IPv6 source and destination fields. In case that the ULID
for the context differ from the address pair included in the source
and destination address fields of the IPv6 packet used to carry the
I1/I2/I2bis message, the ULID pair option MUST be included in the I1/
I2/I2bis message.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 6 |0| Length = 36 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Sender ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Receiver ULID +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Reserved2: 32-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt. (Needed to
make the ULIDs start on a multiple of 8 octet
boundary.)
Sender ULID: A 128-bit IPv6 address.
Receiver ULID: A 128-bit IPv6 address.
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5.14.7 Forked Instance Identifier Option Format
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 7 |0| Length = 4 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Forked Instance Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Forked Instance Identifier: 32-bit field containing the identifier of
the particular forked instance.
5.14.8 Probe Option Format
This option is defined in [8].
5.14.9 Reachability Option Format
This option is defined in [8].
5.14.10 Payload Reception Report Option Format
This option is defined in [8].
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6. Conceptual Model of a Host
This section describes a conceptual model of one possible data
structure organization that hosts will maintain for the purposes of
shim6. The described organization is provided to facilitate the
explanation of how the shim6 protocol should behave. This document
does not mandate that implementations adhere to this model as long as
their external behavior is consistent with that described in this
document.
6.1 Conceptual Data Structures
The key conceptual data structure for the shim6 protocol is the ULID
pair context. This is a data structure which contains the following
information:
o The state of the context. See Section 6.2.
o The peer ULID; ULID(peer)
o The local ULID; ULID(local)
o The Forked Instance Identifier; FII. This is zero for the default
context i.e., when there is no forking.
o The list of peer locators, with their preferences; Ls(peer)
o The generation number for the most recently received, verified
peer locator list.
o For each peer locator, the verification method to use (from the
Locator List option).
o For each peer locator, a bit whether it has been verified using
HBA or CGA, and a bit whether the locator has been probed to
verify that the ULID is present at that location.
o The preferred peer locator - used as destination; Lp(peer)
o The set of local locators and the preferences; Ls(local)
o The generation number for the most recently sent Locator List
option.
o The preferred local locator - used as source; Lp(local)
o The context tag used to transmit control messages and payload
extension headers - allocated by the peer; CT(peer)
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o The context to expect in received control messages and payload
extension headers - allocated by the local host; CT(local)
o Timers for retransmission of the messages during context
establishment and update messages.
o Depending how an implementation determines whether a context is
still in use, there might be a need to track the last time a
packet was sent/received using the context.
o Reachability state for the locator pairs as specified in [8].
o During pair exploration, information about the probe messages that
have been sent and received as specified in [8].
6.2 Context States
The states that are used to describe the shim6 protocol are as
follows:
+---------------------+---------------------------------------------+
| State | Explanation |
+---------------------+---------------------------------------------+
| IDLE | State machine start |
| | |
| I1-SENT | Initiating context establishment exchange |
| | |
| I2-SENT | Waiting to complete context establishment |
| | exchange |
| | |
| I2BIS-SENT | Potential context loss detected |
| | |
| | |
| ESTABLISHED | SHIM context established |
| | |
| E-FAILED | Context establishment exchange failed |
| | |
| NO-SUPPORT | ICMP payload type unknown (type 4, code 1) |
| | received indicating that shim6 is not |
| | supported |
+---------------------+---------------------------------------------+
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In addition, in each of the aforementioned states, the following
state information is stored:
+---------------------+---------------------------------------------+
| State | Information |
+---------------------+---------------------------------------------+
| IDLE | None |
| | |
| I1-SENT | ULID(peer), ULID(local), [FII], CT(local), |
| | INIT nonce, Lp(local), Lp(peer), Ls(local) |
| | |
| I2-SENT | ULID(peer), ULID(local), [FII], CT(local), |
| | INIT nonce, RESP nonce, Lp(local), Lp(peer),|
| | Ls(local) |
| | |
| ESTABLISHED | ULID(peer), ULID(local), [FII], CT(local), |
| | CT(peer), Lp(local), Lp(peer), Ls(local) |
| | Ls(peer), INIT nonce?(to receive late R2) |
| | |
| I2BIS-SENT | ULID(peer), ULID(local), [FII], CT(local), |
| | CT(peer), Lp(local), Lp(peer), Ls(local) |
| | Ls(peer), CT(R1bis) |
| | |
| E-FAILED | ULID(peer), ULID(local) |
| | |
| NO-SUPPORT | ULID(peer), ULID(local) |
+---------------------+---------------------------------------------+
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7. Establishing ULID-Pair Contexts
ULID-pair contexts are established using a 4-way exchange, which
allows the responder to avoid creating state on the first packet. As
part of this exchange each end allocates a context tag, and it shares
this context tag and its set of locators with the peer.
In some cases the 4-way exchange is not necessary, for instance when
both ends try to setup the context at the same time, or when
recovering from a context that has been garbage collected or lost at
one of the hosts.
7.1 Uniqness of Context Tags
As part of establishing a new context, each host has to assign a
unique context tag. Since the Payload Extension headers are
demultiplexed based solely on the context tag value (without using
the locators), the context tag MUST be unique for each context.
In addition, in order to minimize the reuse of context tags, the host
SHOULD randomly cycle through the 2^47 context tag values,(e.g.
following the guidelines described in [17]).
7.2 Locator Verification
The peer's locators might need to be verified during context
establishment as well as when handling locator updates in Section 10.
There are two separate aspects of locator verification. One is to
verify that the locator is tied to the ULID, i.e., that the host
which "owns" the ULID is also the one that is claiming the locator
"ownership". The shim6 protocol uses the HBA or CGA techniques for
doing this verification. The other is to verify that the host is
indeed reachable at the claimed locator. Such verification is needed
both to make sure communication can proceed, but also to prevent 3rd
party flooding attacks [19]. These different verifications happen at
different times, since the first might need to be performed before
packets can be received by the peer with the source locator in
question, but the latter verification is only needed before packets
are sent to the locator.
Before a host can use a locator (different than the ULID) as the
source locator, it must know that the peer will accept packets with
that source locator as being part of this context. Thus the HBA/CGA
verification SHOULD be performed by the host before the host
acknowledges the new locator, by sending an Update Acknowledgement
message, or an R2 message.
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Before a host can use a locator (different than the ULID) as the
destination locator it MUST perform the HBA/CGA verification if this
was not performed before upon the reception of the locator set. In
addition, it MUST verify that the ULID is indeed present at that
locator. This verification is performed by doing a return-
routability test as part of the Probe sub-protocol [8].
If the verification method in the Locator List option is not
supported by the host, or if the verification method is not
consistent with the CGA Parameter Data Structure (e.g., the Parameter
Data Structure doesn't contain the multiprefix extension, and the
verification method says to use HBA), then the host MUST ignore the
Locator List and the message in which it is contained, and the host
SHOULD generates an ICMP parameter problem (type 4, code 0), with the
Pointer referencing the octet in the Verification method that was
found inconsistent.
7.3 Normal context establishment
The normal context establishment consists of a 4 message exchange in
the order of I1, R1, I2, R2 as can be seen in Figure 24.
Initiator Responder
IDLE IDLE
------------- I1 -------------->
I1-SENT
<------------ R1 ---------------
IDLE
------------- I2 -------------->
I2-SENT
<------------ R2 ---------------
ESTABLISHED ESTABLISHED
Figure 24: Normal context establishment
7.4 Concurrent context establishment
When both ends try to initiate a context for the same ULID pair, then
we might end up with crossing I1 messages. Alternatively, since no
state is created when receiving the I1, a host might send a I1 after
having sent a R1 message.
Since a host remembers that it has sent an I1, it can respond to an
I1 from the peer (for the same ULID-pair), with a R2, resulting in
the message exchange shown in Figure 25. Such behavior is needed for
other reasons such as to correctly respond to retransmitted I1
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messages, which occur when the R2 message has been lost.
Host A Host B
IDLE IDLE
-\
I1-SENT---\
---\ /---
--- I1 ---\ /--- I1-SENT
---\
/--- I1 ---/ ---\
/--- -->
<---
-\
I1-SENT---\
---\ /---
--- R2 ---\ /--- I1-SENT
---\
/--- R2 ---/ ---\
/--- -->
<--- ESTABLISHED
ESTABLISHED
Figure 25: Crossing I1 messages
If a host has received an I1 and sent an R1, it has no state to
remember this. Thus if the ULP on the host sends down packets, this
might trigger the host to send an I1 message itself. Thus while one
end is sending an I1 the other is sending an I2 as can be seen in
Figure 26.
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Host A Host B
IDLE IDLE
-\
---\
I1-SENT ---\
--- I1 ---\
---\
---\
-->
/---
/--- IDLE
---
/--- R1--/
/---
<---
-\
I2-SENT---\
---\ /---
--- I2---\ /--- I1-SENT
---\
/--- I1 ---/ ---\
/--- -->
<--- ESTABLISHED
-\
I2-SENT---\
---\ /---
--- R2 ---\ /---
---\
/--- R2 ---/ ---\
/--- -->
<--- ESTABLISHED
ESTABLISHED
Figure 26: Crossing I2 and I1
7.5 Context recovery
Due to garbage collection, we can end up with one end having and
using the context state, and the other end not having any state. We
need to be able to recover this state at the end that has lost it,
before we can use it.
This need can arise in the following cases:
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o The communication is working using the ULID pair as the locator
pair, but a problem arises, and the end that has retained the
context state decides to probe alternate locator pairs.
o The communication is working using a locator pair that is not the
ULID pair, hence the ULP packets sent from a peer that has
retained the context state use the shim6 Payload extension header.
o The host that retained the state sends a control message (e.g. an
Update Request message).
In all the cases the result is that the peer without state receives a
shim message for which it has to context for the context tag.
In all of those cases we can recover the context by having the node
which doesn't have a context state, send back an R1bis message, and
have then complete the recovery with a I2bis and R2 message as can be
seen in Figure 27.
Host A Host B
Context for
CT(peer)=X Discards context for
CT(local)=X
ESTABLISHED IDLE
---- payload, probe, etc. -----> No context state
for CT(local)=X
<------------ R1bis ------------
IDLE
------------- I2bis ----------->
I2BIS_SENT
<------------ R2 ---------------
ESTABLISHED ESTABLISHED
Figure 27: Context loss at receiver
If one end has garbage collected or lost the context state, it might
try to create a new context state (for the same ULID pair), by
sending an I1 message. The peer (that still has the context state)
will reply with an R1 message and the full 4-way exchange will be
performed again in this case as can be seen in Figure 28.
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Host A Host B
Context for
CT(peer)=X Discards context for
ULIDs A1, B1 CT(local)=X
ESTABLISHED IDLE
Finds <------------ I1 --------------- Tries to setup
existing for ULIDs A1, B1
context,
but CT(peer) I1-SENT
doesn't match
------------- R1 --------------->
Left old context
in ESTABLISHED
<------------ I2 ---------------
Recreate context
with new CT(peer) I2-SENT
and Ls(peer).
ESTABLISHED
------------- R2 -------------->
ESTABLISHED ESTABLISHED
Figure 28: Context loss at sender
7.6 Context confusion
Since each end might garbage collect the context state we can have
the case when one end has retained the context state and tries to use
it, while the other end has lost the state. We discussed this in the
previous section on recovery. But for the same reasons, when one
host retains context tag X as CT(peer) for ULID pair <A1, B1>, the
other end might end up allocating that context tag as CT(local) for
another ULID pair, e.g., <A3, B1> between the same hosts. In this
case we can not use the recovery mechanisms since there needs to be
separate context tags for the two ULID pairs.
This type of "confusion" can be observed in two cases (assuming it is
A that has retained the state and B has dropped it):
o B decides to create a context for ULID pair <A3, B1>, and
allocates X as its context tag for this, and sends an I1 to A.
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o A decides to create a context for ULID pair <A3, B1>, and starts
the exchange by sending I1 to B. When B receives the I2 message,
it allocates X as the context tag for this context.
In both cases, A can detect that B has allocated X for ULID pair <A3,
B1> even though that A still X as CT(peer) for ULID pair <A1, B1>.
Thus A can detect that B must have lost the context for <A1, B1>.
The confusion can be detected when I2/I2bis/R2 is received since we
require that those messages MUST include a sufficiently large set of
locators in a Locator List option that the peer can determine whether
or not two contexts have the same host as the peer by comparing if
there is any common locators in Ls(peer).
The requirement is that the old context which used the context tag
MUST be removed; it can no longer be used to send packets. Thus A
would forcibly remove the context state for <A1, B1, X>, so that it
can accept the new context for <A3, B1, X>. An implementation MAY
re-create a context to replace the one that was removed; in this case
for <A1, B1>. The normal I1, R1, I2, R2 establishment exchange would
then pick unique context tags for that replacement context. This re-
creation is OPTIONAL, but might be useful when there is ULP
communication which is using the ULID pair whose context was removed.
Note that an I1 message with a duplicate context tag should not cause
the removal of the old context state; this operation needs to be
deferred until the reception of the I2 message.
7.7 Sending I1 messages
When the shim layer decides to setup a context for a ULID pair, it
starts by allocating and initializing the context state for its end.
As part of this it assigns a random context tag to the context that
is not being used as CT(local) by any other context . In the case
that a new API is used and the ULP requests a forked context, the
Forked Instance Identifier value will be set to a non-zero value.
Otherwise, the FII value is zero. Then the initiator can send an I1
message and set the context state to I1-SENT. The I1 message MUST
include the ULID pair; normally in the IPv6 source and destination
fields. But if the ULID pair for the context is not used as locator
pair for the I1 message, then a ULID option MUST be included in the
I1 message. In addition, if a Forked Instance Identifier value is
non-zero, the I1 message MUST include a Context Instance Identifier
option containing the correspondent value.
7.8 Retransmitting I1 messages
If the host does not receive an I2 or R2 message in response to the
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I1 message after I1_TIMEOUT time, then it needs to retransmit the I1
message. The retransmissions should use a retransmission timer with
binary exponential backoff to avoid creating congestion issues for
the network when lots of hosts perform I1 retransmissions. Also, the
actual timeout value should be randomized between 0.5 and 1.5 of the
nominal value to avoid self-synchronization.
If, after I1_RETRIES_MAX retransmissions, there is no response, then
most likely the peer does not implement the shim6 protocol, or there
could be a firewall that blocks the protocol. In this case it makes
sense for the host to remember to not try again to establish a
context with that ULID. However, any such negative caching should
retained for at most NO_R1_HOLDDOWN_TIME, to be able to later setup a
context should the problem have been that the host was not reachable
at all when the shim tried to establish the context.
If the host receives an ICMP error with "payload type unknown" (type
4, code 1) and the included packet is the I1 message it just sent,
then this is a more reliable indication that the peer ULID does not
implement shim6. Again, in this case, the host should remember to
not try again to establish a context with that ULID. Such negative
caching should retained for at most ICMP_HOLDDOWN_TIME, which should
be significantly longer than the previous case.
7.9 Receiving I1 messages
A host MUST silently discard any received I1 messages that do not
satisfy all of the following validity checks in addition to those
specified in Section 12.2:
o The Hdr Ext Len field is at least 1, i.e., the length is at least
16 octets.
Upon the reception of an I1 message, the host extracts the ULID pair
and the Forked Instance Identifier from the message. If there is no
ULID-pair option, then the ULID pair is taken from the source and
destination fields in the IPv6 header. If there is no FII option in
the message, then the FII value is taken to be zero.
Next the host looks for an existing context which matches the ULID
pair and the FII.
If no state is found (i.e., the state is IDLE), then the host replies
with a R1 message as specified below.
If such a context exists in ESTABLISHED state, the host verifies that
the locator of the Initiator is included in Ls(peer) (This check is
unnecessary if there is no ULID-pair option in the I1 message).
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If the state exists in ESTABLISHED state and the locators do not fall
in the locator sets, then the host replies with a R1 message as
specified below. This completes the I1 processing, with the context
state being unchanged.
If the state exists in ESTABLISHED state and the locators do fall in
the sets, then the host compares CT(peer) for the context with the CT
contained in the I1 message.
o If the context tags match, then this probably means that the R2
message was lost and this I1 is a retransmission. In this case,
the host replies with a R2 message containing the information
available for the existent context.
o If the context tags do not match, then it probably means that the
Initiator has lost the context information for this context and it
is trying to establish a new one for the same ULID-pair. In this
case, the host replies with a R1 message as specified below. This
completes the I1 processing, with the context state being
unchanged.
If the state exists in other state (I1-SENT, I2-SENT, I2BIS-SENT), we
are in the situation of Concurrent context establishment described
in Section 7.4. In this case, the host leaves CT(peer) unchanged,
and replies with a R2 message. This completes the I1 processing,
with the context state being unchanged.
When the host needs to send a R1 message in response to the I1
message, it copies the Initiator Nonce from the I1 message to the R1
message, generates a Responder Nonce and calculates a Responder
Validator option as suggested in the following section. No state is
created on the host in this case.
When the host needs to send a R2 message in response to the I1
message, it copies the Initiator Nonce from the I1 message to the R2
message, and otherwise follows the normal rules for forming an R2
message (see Section 7.13).
7.9.1 Generating the R1 Validator
One way for the responder to properly generate validators is to
maintain a single secret (S) and a running counter for the Responder
Nonce.
In the case the validator is generated to be included in a R1
message, for each I1 message. The responder can increase the
counter, use the counter value as the responder nonce, and use the
following information as input to the one-way function:
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o The the secret S
o That Responder Nonce
o The Initiator Context Tag from the I1 message
o The ULIDs from the I1 message
o The locators from the I1 message (strictly only needed if they are
different from the ULIDs)
o The forked instance identifier if such option was included in the
I1 message
and then the output of the hash function is used as the validator
octet string.
7.10 Receiving R1 messages and sending I2 messages
A host MUST silently discard any received R1 messages that do not
satisfy all of the following validity checks in addition to those
specified in Section 12.2:
o The Hdr Ext Len field is at least 1, i.e., the length is at least
16 octets.
Upon the reception of an R1 message, the host extracts the Initiator
Nonce and the Locator Pair from the message (the latter from the
source and destination fields in the IPv6 header). Next the host
looks for an existing context which matches the Initiator Nonce and
where the locators are contained in Ls(peer) and Ls(local),
respectively. If no such context is found, then the R1 message is
silently discarded.
If such a context is found, then the host looks at the state:
o If the state is I1-SENT, then it sends an I2 message as specified
below.
o In any other state (I2-SENT, I2BIS-SENT, ESTABLISHED) then the
host has already sent an I2 message then this is probably a reply
to a retransmitted I1 message, so this R1 message MUST be silently
discarded.
When the host sends an I2 message, then it includes the Responder
Validator option that was in the R1 message. The I2 message MUST
include the ULID pair; normally in the IPv6 source and destination
fields. If a ULID-pair option was included in the I1 message then it
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MUST be included in the I2 message as well. In addition, if the
Forked Instance Identifier value for this context is non-zero, the I2
message MUST contain a Forked Instance Identifier Option carrying
this value. Besides, the I2 message contains an Initiator Nonce.
This is not required to be the same than the one included in the
previous I1 message.
The I2 message also includes the Initiator's locator list and the CGA
parameter data structure. If CGA (and not HBA) is used to verify the
locator list, then Initiator also signs the key parts of the message
and includes a CGA signature option containing the signature.
When the I2 message has been sent, the state is set to I2-SENT.
7.11 Retransmitting I2 messages
If the initiator does not receive an R2 message after I2_TIMEOUT time
after sending an I2 message it MAY retransmit the I2 message, using
binary exponential backoff and randomized timers. The Responder
Validator option might have a limited lifetime, that is, the peer
might reject Responder Validator options that are older than
VALIDATOR_MIN_LIFETIME to avoid replay attacks. Thus the initiator
SHOULD fall back to retransmitting the I1 message when there is no R2
received after retransmitting the I2 message I2_RETRIES_MAX times.
7.12 Receiving I2 messages
A host MUST silently discard any received I2 messages that do not
satisfy all of the following validity checks in addition to those
specified in Section 12.2:
o The Hdr Ext Len field is at least 2, i.e., the length is at least
24 octets.
Upon the reception of an I2 message, the host extracts the ULID pair
and the Forked Instance identifier from the message. If there is no
ULID-pair option, then the ULID pair is taken from the source and
destination fields in the IPv6 header. If there is no FII option in
the message, then the FII value is taken to be zero.
Next the host verifies that the Responder Nonce is a recent one, and
that the Responder Validator option matches the validator the host
would have computed for the ULID, locators, responder nonce, and FII.
If a CGA Parameter Data Structure (PDS) is included in the message,
then the host MUST verify if the actual PDS contained in the message
corresponds to the ULID(peer).
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If any of the above verifications fails, then the host silently
discard the message and it has completed the I2 processing.
If all the above verifications are successful, then the host proceeds
to look for a context state for the Initiator. The host looks for a
context with the extracted ULID pair and FII. If none exist then
state of the (non-existing) context is viewed as being IDLE, thus the
actions depend on the state as follows:
o If the state is IDLE (i.e., the context does not exist) the host
allocates a context tag (CT(local)), creates the context state for
the context, and sets its state to ESTABLISHED. It records
CT(peer), and the peer's locator set as well as its own locator
set in the context. It SHOULD perform the HBA/CGA verification of
the peer's locator set at this point in time, as specified in
Section 7.2. Then the host sends an R2 message back as specified
below.
o If the state is I1-SENT, then the host verifies if the source
locator is included in Ls(peer) or, it is included in the Locator
List contained in the the I2 message and the HBA/CGA verification
for this specific locator is successful
* If this is not the case, then the message is silently discarded
and the context state remains unchanged.
* If this is the case, then the host updates the context
information (CT(peer), Ls(peer)) with the data contained in the
I2 message and the host MUST send a R2 message back as
specified below. Note that before updating Ls(peer)
information, the host SHOULD perform the HBA/CGA validation of
the peer's locator set at this point in time as specified in
Section 7.2. The host moves to ESTABLISHED state.
o If the state is ESTABLISHED, I2-SENT, or I2BIS-SENT, then the host
verifies if the source locator is included in Ls(peer) or, it is
included in the Locator List contained in the the I2 message and
the HBA/CGA verification for this specific locator is successful
* If this is not the case, then the message is silently discarded
and the context state remains unchanged.
* If this is the case, then the host updates the context
information (CT(peer), Ls(peer)) with the data contained in the
I2 message and the host MUST send a R2 message back as
specified in Section 7.13. Note that before updating Ls(peer)
information, the host SHOULD perform the HBA/CGA validation of
the peer's locator set at this point in time as specified in
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Section 7.2. The context state remains unchanged.
7.13 Sending R2 messages
Before the host sends the R2 message it MUST look for a possible
context confusion i.e. where it would end up with multiple contexts
using the same CT(peer) for the same peer host. See Section 7.14.
When the host needs to send an R2 message, the host forms the message
using its locators and its context tag, copies the Initiator Nonce
from the triggering message (I2, I2bis, or I1), and includes the
necessary options so that the peer can verify the locators. In
particular, the R2 message includes the Responder's locator list and
the PDS option. If CGA (and not HBA) is used to verify the locator
list, then the Responder also signs the key parts of the message and
includes a CGA Signature option containing the signature.
R2 messages are never retransmitted. If the R2 message is lost, then
the initiator will retransmit either the I2/I2bis or I1 message.
Either retransmission will cause the responder to find the context
state and respond with an R2 message.
7.14 Match for Context Confusion
When the host receives an I2, I2bis, or R2 it MUST look for a
possible context confusion i.e. where it would end up with multiple
contexts using the same CT(peer) for the same peer host. This can
happen when it has received the above messages since they create a
new context with a new CT(peer). Same issue applies when CT(peer) is
updated for an existing context.
The host takes CT(peer) for the newly created or updated context, and
looks for other contexts which:
o Are in state ESTABLISHED or I2BIS-SENT.
o Have the same CT(peer).
o Where Ls(peer) has at least one locator in common with the newly
created or updated context.
If such a context is found, then the host checks if the ULID pair or
the Forked Instance Identifier different than the ones in the newly
created or updated context:
o If either or both are different, then the peer is reusing the
context tag for the creation of a context with different ULID pair
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or FII, which is an indication that the peer has lost the original
context. In this case, we are in the Context confusion situation,
and the host MUST NOT use the old context to send any packets. It
MAY just discard the old context (after all, the peer has
discarded it), or it MAY attempt to re-establish the old context
by sending a new I1 message and moving its state to I1-SENT. In
any case, once that this situation is detected, the host MUST NOT
keep two contexts with overlapping Ls(peer) locator sets and the
same context tag in ESTABLISHED state, since this would result in
demultiplexing problems on the peer.
o If both are the same, then this context is actually the context
that is created or updated, hence there is no confusion.
7.15 Receiving R2 messages
A host MUST silently discard any received R2 messages that do not
satisfy all of the following validity checks in addition to those
specified in Section 12.2:
o The Hdr Ext Len field is at least 1, i.e., the length is at least
16 octets.
Upon the reception of an R2 message, the host extracts the Initiator
Nonce and the Locator Pair from the message (the latter from the
source and destination fields in the IPv6 header). Next the host
looks for an existing context which matches the Initiator Nonce and
where the locators are Lp(peer) and Lp(local), respectively. Based
on the state:
o If no such context is found, i.e., the state is IDLE, then the
message is silently dropped.
o If state is I1-SENT, I2-SENT, or I2BIS-SENT then the host performs
the following actions: If a CGA Parameter Data Structure (PDS) is
included in the message, then the host MUST verify that the actual
PDS contained in the message corresponds to the ULID(peer) as
specified in Section 7.2. If the verification fails, then the
message is silently dropped. If the verification succeeds, then
the host records the information from the R2 message in the
context state; it records the peer's locator set and CT(peer).
The host SHOULD perform the HBA/CGA verification of the peer's
locator set at this point in time, as specified in Section 7.2.
The host sets its state to ESTABLISHED.
o If the state is ESTABLISHED, the R2 message is silently ignored,
(since this is likely to be a reply to a retransmitted I2
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message).
Before the host completes the R2 processing it MUST look for a
possible context confusion i.e. where it would end up with multiple
contexts using the same CT(peer) for the same peer host. See
Section 7.14.
7.16 Sending R1bis messages
Upon the receipt of a shim6 payload extension header where there is
no current SHIM6 context at the receiver, the receiver is to respond
with an R1bis message in order to enable a fast re-establishment of
the lost SHIM6 context.
Also a host is to respond with a R1bis upon receipt of any control
messages that has a message type in the range 64-127 (i.e., excluding
the context setup messages such as I1, R1, R1bis, I2, I2bis, R2 and
future extensions), where the control message refers to a non
existent context.
We assume that all the incoming packets that trigger the generation
of an R1bis message contain a locator pair (in the address fields of
the IPv6 header) and a Context Tag.
Upon reception of any of the packets described above, the host will
reply with an R1bis including the following information:
o The Responder Nonce is a number picked by the responder which the
initiator will return in the I2bis message.
o Packet Context Tag is the context tag contained in the received
packet that triggered the generation of the R1bis message.
o The Responder Validator option is included, with a validator that
is computed as suggested in the next section.
7.16.1 Generating the R1bis Validator
One way for the responder to properly generate validators is to
maintain a single secret (S) and a running counter for the Responder
Nonce.
In the case the validator is generated to be included in a R1bis
message, for each received payload extension header or control
message, the responder can increase the counter, use the counter
value as the responder nonce, and use the following information as
input to the one-way function:
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o The the secret S
o That Responder Nonce
o The Receiver Context tag included in the received packet
o The locators from the received packet
and then the output of the hash function is used as the validator
octet string.
7.17 Receiving R1bis messages and sending I2bis messages
A host MUST silently discard any received R1bis messages that do not
satisfy all of the following validity checks in addition to those
specified in Section 12.2:
o The Hdr Ext Len field is at least 1, i.e., the length is at least
16 octets.
Upon the reception of an R1bis message, the host extracts the Packet
Context Tag and the Locator Pair from the message (the latter from
the source and destination fields in the IPv6 header). Next the host
looks for an existing context where the Packet Context Tag matches
CT(peer) and where the locators match Lp(peer) and Lp(local),
respectively.
o If no such context is not found, i.e., the state is IDLE, then the
R1bis message is silently discarded.
o If the state is I1-SENT, I2-SENT, or I2BIS-SENT, then the R1bis
message is silently discarded.
o If the state is ESTABLISHED, then we are in the case where the
peer has lost the context and the goal is to try to re-establish
it. For that, the host leaves CT(peer) unchanged in the context
state, transitions to I2BIS-SENT state, and sends a I2bis message,
including the computed Responder Validator option, the Packet
Context Tag, and the Responder Nonce received in the R1bis
message. This I2bis message is sent using the locator pair
included in the R1bis message. In the case that this locator pair
differs from the ULID pair defined for this context, then an ULID
option MUST be included in the I2bis message. In addition, if the
Forked Instance Identifier for this context is non-zero, then a
Forked Instance Identifier option carrying the instance identifier
value for this context MUST be included in the I2bis message.
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7.18 Retransmitting I2bis messages
If the initiator does not receive an R2 message after I2bis_TIMEOUT
time after sending an I2bis message it MAY retransmit the I2bis
message, using binary exponential backoff and randomized timers. The
Responder Validator option might have a limited lifetime, that is,
the peer might reject Responder Validator options that are older than
VALIDATOR_MIN_LIFETIME to avoid replay attacks. Thus the initiator
SHOULD fall back to retransmitting the I1 message when there is no R2
received after retransmitting the I2bis message I2bis_RETRIES_MAX
times.
7.19 Receiving I2bis messages and sending R2 messages
A host MUST silently discard any received I2bis messages that do not
satisfy all of the following validity checks in addition to those
specified in Section 12.2:
o The Hdr Ext Len field is at least 3, i.e., the length is at least
32 octets.
Upon the reception of an I2bis message, the host extracts the ULID
pair and the Forked Instance identifier from the message. If there
is no ULID-pair option, then the ULID pair is taken from the source
and destination fields in the IPv6 header. If there is no FII option
in the message, then the FII value is taken to be zero.
Next the host verifies that the Responder Nonce is a recent one, and
that the Responder Validator option matches the validator the host
would have computed for the ULID, locators, responder nonce, and FII
as part of sending an R1bis message.
If a CGA Parameter Data Structure (PDS) is included in the message,
then the host MUST verify if the actual PDS contained in the message
corresponds to the ULID(peer).
If any of the above verifications fails, then the host silently
discard the message and it has completed the I2bis processing.
If both verifications are successful, then the host proceeds to look
for a context state for the Initiator. The host looks for a context
with the extracted ULID pair and FII. If none exist then state of
the (non-existing) context is viewed as being IDLE, thus the actions
depend on the state as follows:
o If the state is IDLE (i.e., the context does not exist) the host
allocates a context tag (CT(local)), creates the context state for
the context, and sets its state to ESTABLISHED. The host SHOULD
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NOT use the Packet Context Tag in the I2bis message for CT(local);
instead it should pick a new random context tag just as when it
processes an I2 message. It records CT(peer), and the peer's
locator set as well as its own locator set in the context. It
SHOULD perform the HBA/CGA verification of the peer's locator set
at this point in time as specified in Section 7.2. Then the host
sends an R2 message back as specified in Section 7.13.
o If the state is I1-SENT, then the host verifies if the source
locator is included in Ls(peer) or, it is included in the Locator
List contained in the the I2 message and the HBA/CGA verification
for this specific locator is successful
* If this is not the case, then the message is silently
discarded. The the context state remains unchanged.
* If this is the case, then the host updates the context
information (CT(peer), Ls(peer)) with the data contained in the
I2 message and the host MUST send a R2 message back as
specified below. Note that before updating Ls(peer)
information, the host SHOULD perform the HBA/CGA validation of
the peer's locator set at this point in time as specified in
Section 7.2. The host moves to ESTABLISHED state.
o If the state is ESTABLISHED, I2-SENT, or I2BIS-SENT, then the host
verifies if the source locator is included in Ls(peer) or, it is
included in the Locator List contained in the the I2 message and
the HBA/CGA verification for this specific locator is successful
* If this is not the case, then the message is silently
discarded. The the context state remains unchanged.
* If this is the case, then the host updates the context
information (CT(peer), Ls(peer)) with the data contained in the
I2 message and the host MUST send a R2 message back as
specified in Section 7.13. Note that before updating Ls(peer)
information, the host SHOULD perform the HBA/CGA validation of
the peer's locator set at this point in time as specified in
Section 7.2. The context state remains unchanged.
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8. Handling ICMP Error Messages
The routers in the path as well as the destination might generate
various ICMP error messages, such as host unreachable, packet too
big, and payload type unknown. It is critical that these packets
make it back up to the ULPs so that they can take appropriate action.
This is an implementation issue in the sense that the mechanism is
completely local to the host itself. But the issue of how ICMP
errors are correctly dispatched to the ULP on the host are important,
hence this section specifies the issue.
+--------------+
| IPv6 Header |
| |
+--------------+
| ICMPv6 |
| Header |
- - +--------------+ - -
| IPv6 Header |
| src, dst as | Can be dispatched
IPv6 | sent by ULP | unmodified to ULP
| on host | ICMP error handler
Packet +--------------+
| ULP |
in | Header |
+--------------+
Error | |
~ Data ~
| |
- - +--------------+ - -
Figure 29: ICMP error handling without payload extension header
When the ULP packets are sent without the payload extension header,
that is, while the initial locators=ULIDs are working, this
introduces no new concerns; an implementation's existing mechanism
for delivering these errors to the ULP will work. See Figure 29.
But when the shim on the transmitting side inserts the payload
extension header and replaces the ULIDs in the IP address fields with
some other locators, then an ICMP error coming back will have a
"packet in error" which is not a packet that the ULP sent. Thus the
implementation will have to apply the reverse mapping to the "packet
in error" before passing the ICMP error up to the ULP. See
Figure 30.
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+--------------+
| IPv6 Header |
| |
+--------------+
| ICMPv6 |
| Header |
- - +--------------+ - -
| IPv6 Header |
| src, dst as | Needs to be
IPv6 | modified by | transformed to
| shim on host | have ULIDs
+--------------+ in src, dst fields,
Packet | SHIM6 ext. | and SHIM6 ext.
| Header | header removed
in +--------------+ before it can be
| Transport | dispatched to the ULP
Error | Header | ICMP error handler.
+--------------+
| |
~ Data ~
| |
- - +--------------+ - -
Figure 30: ICMP error handling with payload extension header
Note that this mapping is different than when receiving packets from
the peer with a payload extension headers, because in that case the
packets contain CT(local). But the ICMP errors have a "packet in
error" with an payload extension header containing CT(peer). This is
because they were intended to be received by the peer. In any case,
since the <Source Locator, Destination Locator, CT(peer)> has to be
unique when received by the peer, the local host should also only be
able to find one context that matches this tuple.
If the ICMP error is a Packet Too Big, the reported MTU must be
adjusted to be 8 octets less, since the shim will add 8 octets when
sending packets.
After the "packet in error" has had the original ULIDs inserted, then
this payload extension header can be removed. The result is a
"packet in error" that is passed to the ULP which looks as if the
shim did not exist.
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9. Teardown of the ULID-Pair Context
Each host can unilaterally decide when to tear down a ULID-pair
context. It is RECOMMENDED that hosts do not tear down the context
when they know that there is some upper layer protocol that might use
the context. For example, an implementation might know this if there
is an open socket which is connected to the ULID(peer). However,
there might be cases when the knowledge is not readily available to
the shim layer, for instance for UDP applications which do not
connect their sockets, or any application which retains some higher
level state across (TCP) connections and UDP packets.
Thus it is RECOMMENDED that implementations minimize premature
teardown by observing the amount of traffic that is sent and received
using the context, and only after it appears quiescent, tear down the
state. A reasonable approach would be not to tear down a context
until at least 5 minutes have passed since the last message was sent
or received using the context.
Since there is no explicit, coordinated removal of the context state,
there are potential issues around context tag reuse. One end might
remove the state, and potentially reuse that context tag for some
other communication, and the peer might later try to use the old
context (which it didn't remove). The protocol has mechanisms to
recover from this, which work whether the state removal was total and
accidental (e.g., crash and reboot of the host), or just a garbage
collection of shim state that didn't seem to be used. However, the
host should try to minimize the reuse of context tags by trying to
randomly cycle through the 2^47 context tag values. (See Appendix E
for a summary how the recovery works in the different cases.)
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10. Updating the Peer
The Update Request and Acknowledgement are used both to update the
list of locators (only possible when CGA is used to verify the
locator(s)), as well as updating the preferences associated with each
locator.
10.1 Sending Update Request messages
When a host has a change in the locator set, then it can communicate
this to the peer by sending an Update Request. When a host has a
change in the preferences for its locator set, it can also
communicate this to the peer. The Update Request message can include
just a Locator List option, to convey the new set of locators (which
requires a CGA signature option as well), just a Locator Preferences
option, or both a new Locator List and new Locator Preferences.
Should the host send a new Locator List, the host picks a new random
local generation number, records this in the context, and puts it in
the Locator List option. Any Locator Preference option, whether send
in the same Update Request or in some future Update Request, will use
that generation number to make sure the preferences get applied to
the correct version of the locator list.
The host picks a random Request Nonce for each update, and keeps the
same nonce for any retransmissions of the Update Request. The nonce
is used to match the acknowledgement with the request.
10.2 Retransmitting Update Request messages
If the host does not receive an Update Acknowledgement R2 message in
response to the Update Request message after UPDATE_TIMEOUT time,
then it needs to retransmit the Update Request message. The
retransmissions should use a retransmission timer with binary
exponential backoff to avoid creating congestion issues for the
network when lots of hosts perform Update Request retransmissions.
Also, the actual timeout value should be randomized between 0.5 and
1.5 of the nominal value to avoid self-synchronization.
Should there be no response, the retransmissions continue forever.
The binary exponential backoff stops at MAX_UPDATE_TIMEOUT. But the
only way the retransmissions would stop when there is no
acknowledgement, is when the shim, through the Probe protocol or some
other mechanism, decides to discard the context state due to lack of
ULP usage in combination with no responses to the Probes.
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10.3 Newer Information While Retransmitting
There can be at most one outstanding Update Request message at any
time. Thus until e.g. an update with a new Locator List has been
acknowledged, any even newer Locator List or new Locator Preferences
can not just be sent. However, when there is newer information and
the older information has not yet been acknowledged, the host can
instead of waiting for an acknowledgement, abandon the previous
update and construct a new Update Request (with a new Request Nonce)
which includes the new information as well as the information that
hadn't yet been acknowledged.
For example, if the original locator list was just (A1, A2), and if
an Update Request with the Locator List (A1, A3) is outstanding, and
the host determines that it should both add A4 to the locator list,
and mark A1 as BROKEN, then it would need to:
o Pick a new random Request Nonce for the new Update Request.
o Pick a new random Generation number for the new locator list.
o Form the new locator list - (A1, A3, A4)
o Form a Locator Preference option which uses the new generation
number and has the BROKEN flag for the first locator.
o Send the Update Request and start a retransmission timer.
Any Update Acknowledgement which doesn't match the current request
nonce, for instance an acknowledgement for the abandoned Update
Request, will be silently ignored.
10.4 Receiving Update Request messages
A host MUST silently discard any received Update Request messages
that do not satisfy all of the following validity checks in addition
to those specified in Section 12.2:
o The Hdr Ext Len field is at least 1, i.e., the length is at least
16 octets.
Upon the reception of an Update Request message, the host extracts
the Context Tag from the message. It then looks for a context which
has a CT(local) that matches the context tag. If no such context is
found, it sends a R1bis message as specified in Section 7.16.
Since context tags can be reused, the host MUST verify that the IPv6
source address field is part of Ls(peer) and that the IPv6
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destination address field is part of Ls(local). If this is not the
case, the sender of the Update Request has a stale context which
happens to match the CT(local) for this context. In this case the
host MUST send a R1bis message, and otherwise ignore the Update
Request message.
If a CGA Parameter Data Structure (PDS) is included in the message,
then the host MUST verify if the actual PDS contained in the packet
corresponds to the ULID(peer). If this verification fails, the
message is silently discarded.
Then, depending on the state of the context:
o If ESTABLISHED: Proceed to process message.
o If I1-SENT, discard the message and stay in I1-SENT.
o If I2-SENT, then send R2 and proceed to process the message.
o If I2BIS-SENT, then send R2 and proceed to process the message.
The verification issues for the locators carried in the Locator
Update message are specified in Section 7.2. If the locator list can
not be verified, this procedure might send an ICMP Parameter Problem
error. In any case, if it can not be verified, there is no further
processing of the Update Request.
Once any Locator List option in the Update Request has been verified,
the peer generation number in the context is updated to be the one in
the Locator List option.
If the Update message contains a Locator Preference option, then the
Generation number in the preference option is compared with the peer
generation number in the context. If they do not match, then the
host generates an ICMP parameter problem (type 4, code 0) with the
Pointer field referring to the first octet in the Generation number
in the Locator Preference option. In addition, if the number of
elements in the Locator Preference option does not match the number
of locators in Ls(peer), then an ICMP parameter problem is sent with
the Pointer referring to the first octet of the Length field in the
Locator Preference option. In both cases of failures, no further
processing is performed for the Locator Update message.
If the generation number matches, the locator preferences are
recorded in the context.
Once the Locator List option (if present) has been verified and any
new locator list or locator preferences have been recorded, the host
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sends an Update Acknowledgement message, copying the nonce from the
request, and using the CT(peer) in as the Receiver Context Tag.
Any new locators, or more likely new locator preferences, might
result in the host wanting to select a different locator pair for the
context. For instance, if the Locator Preferences lists the current
Lp(peer) as BROKEN. The host uses the Probe message in [8] to verify
that the new locator is reachable before changing Lp(peer).
10.5 Receiving Update Acknowledgement messages
A host MUST silently discard any received Update Acknowledgement
messages that do not satisfy all of the following validity checks in
addition to those specified in Section 12.2:
o The Hdr Ext Len field is at least 1, i.e., the length is at least
16 octets.
Upon the reception of an Update Acknowledgement message, the host
extracts the Context Tag and the Request Nonce from the message. It
then looks for a context which has a CT(local) that matches the
context tag. If no such context is found, it sends a R1bis message
as specified in Section 7.16.
Since context tags can be reused, the host MUST verify that the IPv6
source address field is part of Ls(peer) and that the IPv6
destination address field is part of Ls(local). If this is not the
case, the sender of the Update Acknowledgement has a stale context
which happens to match the CT(local) for this context. In this case
the host MUST send a R1bis message, and otherwise ignore the Update
Acknowledgement message.
Then, depending on the state of the context:
o If ESTABLISHED: Proceed to process message.
o If I1-SENT, discard the message and stay in I1-SENT.
o If I2-SENT, then send R2 and proceed to process the message.
o If I2BIS-SENT, then send R2 and proceed to process the message.
If the Request Nonce doesn't match the Nonce for the last sent Update
Request for the context, then the Update Acknowledgement is silently
ignored. If the nonce matches, then the update has been completed
and the Update retransmit timer can be reset.
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11. Sending ULP Payloads
When there is no context state for the ULID pair on the sender, there
is no effect on how ULP packets are sent. If the host is using some
heuristic for determining when to perform a deferred context
establishment, then the host might need to do some accounting (count
the number of packets sent and received) even before there is a ULID-
pair context.
If the context is not in ESTABLISHED or I2BIS-SENT state, then it
there is also no effect on how the ULP packets are sent. Only in the
ESTABLISHED and I2BIS-SENT states does the host have CT(peer) and
Ls(peer) set.
If there is a ULID-pair context for the ULID pair, then the sender
needs to verify whether context uses the ULIDs as locators, that is,
whether Lp(peer) == ULID(peer) and Lp(local) == ULID(local).
If this is the case, then packets can be sent unmodified by the shim.
If it is not the case, then the logic in Section 11.1 will need to be
used.
There will also be some maintenance activity relating to
(un)reachability detection, whether packets are sent with the
original locators or not. The details of this is out of scope for
this document and is specified in [8].
11.1 Sending ULP Payload after a Switch
When sending packets, if there is a ULID-pair context for the ULID
pair, and the ULID pair is no longer used as the locator pair, then
the sender needs to transform the packet. Apart from replacing the
IPv6 source and destination fields with a locator pair, an 8-octet
header is added so that the receiver can find the context and inverse
the transformation.
If there has been a failure causing a switch, and later the context
switches back to sending things using the ULID pair as the locator
pair, then there is no longer a need to do any packet transformation
by the sender, hence there is no need to include the 8-octet
extension header.
First, the IP address fields are replaced. The IPv6 source address
field is set to Lp(local) and the destination address field is set to
Lp(peer). NOTE that this MUST NOT cause any recalculation of the ULP
checksums, since the ULP checksums are carried end-to-end and the ULP
pseudo-header contains the ULIDs which are preserved end-to-end.
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The sender skips any "routing sub-layer extension headers" that the
ULP might have included, thus it skips any hop-by-hop extension
header, any routing header, and any destination options header that
is followed by a routing header. After any such headers the shim6
extension header will be added. This might be before a Fragment
header, a Destination Options header, an ESP or AH header, or a ULP
header.
The inserted shim6 Payload extension header includes the peer's
context tag. It takes on the next header value from the preceding
extension header, since that extension header will have a next header
value of SHIM6.
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12. Receiving Packets
As in normal IPv6 receive side packet processing the receiver parses
the (extension) headers in order. Should it find a shim6 extension
header it will look at the "P" field in that header. If this bit is
zero, then the packet must be passed to the shim6 payload handling
for rewriting. Otherwise, the packet is passed to the shim6 control
handling.
12.1 Receiving Payload Extension Headers
The receiver extracts the context tag from the payload extension
header, and uses this to find a ULID-pair context. If no context is
found, the receiver SHOULD generate a R1bis message (see
Section 7.16).
Then, depending on the state of the context:
o If ESTABLISHED: Proceed to process message.
o If I1-SENT, discard the message and stay in I1-SENT.
o If I2-SENT, then send R2 and proceed to process the message.
o If I2BIS-SENT, then send R2 and proceed to process the message.
With the context in hand, the receiver can now replace the IP address
fields with the ULIDs kept in the context. Finally, the Payload
extension header is removed from the packet (so that the ULP doesn't
get confused by it), and the next header value in the preceding
header is set to be the actual protocol number for the payload. Then
the packet can be passed to the protocol identified by the next
header value (which might be some function associated with the IP
endpoint sublayer, or a ULP).
If the host is using some heuristic for determining when to perform a
deferred context establishment, then the host might need to do some
accounting (count the number of packets sent and received) for
packets that does not have a shim6 extension header and for which
there is no context. But the need for this depends on what
heuristics the implementation has chosen.
12.2 Receiving Shim Control messages
A shim control message has the checksum field verified. The Shim
header length field is also verified against the length of the IPv6
packet to make sure that the shim message doesn't claim to end past
the end of the IPv6 packet. Finally, it checks that the neither the
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IPv6 destination field nor the IPv6 source field is a multicast
address. If any of those checks fail, the packet is silently
dropped.
The message is then dispatched based on the shim message type. Each
message type is then processed as described elsewhere in this
document. If the packet contains a shim message type which is
unknown to the receiver, then an ICMPv6 Parameter Problem error is
generated and sent back. The pointer field in the Parameter Problem
is set to point at the first octet of the shim message type. The
error is rate limited just like other ICMP errors [5].
All the control messages can contain any options with C=0. If there
is any option in the message with C=1 that isn't known to the host,
then the host MUST send an ICMPv6 Parameter Problem, with the Pointer
field referencing the first octet of the Option Type.
12.3 Context Lookup
We assume that each shim context has its own state machine. We
assume that a dispatcher delivers incoming packets to the state
machine that it belongs to. Here we describe the rules used for the
dispatcher to deliver packets to the correct shim context state
machine.
There is one state machine per context identified that is
conceptually identified by ULID pair and Forked Instance Identifier
(which is zero by default), or identified by CT(local). However, the
detailed lookup rules are more complex, especially during context
establishment.
Clearly, if the required context is not established, it will be in
IDLE state.
During context establishment, the context is identified as follows:
o I1 packets: Deliver to the context associated with the ULID pair
and the Forked Instance Identifier.
o I2 packets: Deliver to the context associated with the ULID pair
and the Forked Instance Identifier.
o R1 packets: Deliver to the context with the locator pair included
in the packet and the Initiator nonce included in the packet (R1
does not contain ULID pair nor the CT(local)). If no context
exist with this locator pair and Initiator nonce, then silently
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discard.
o R2 packets: Deliver to the context with the locator pair included
in the packet and the Initiator nonce included in the packet (R2
does not contain ULID pair nor the CT(local)). If no context
exists with this locator pair and INIT nonce, then silently
discard.
o R1bis packet: deliver to the context that has the locator pair and
the CT(peer) equal to the Packet Context Tag included in the R1bis
packet.
o I2bis packets: Deliver to the context associated with the ULID
pair and the Forked Instance Identifier.
o Payload extension headers: Deliver to the context with CT(local)
equal to the Receiver Context Tag included in the packet.
o Other control messages (Update, Keepalive, Probe): Deliver to the
context with CT(local) equal to the Receiver Context Tag included
in the packet. Verify that the IPv6 source address field is part
of Ls(peer) and that the IPv6 destination address field is part of
Ls(local). If not, send a R1bis message.
o ICMP errors which contain a shim6 payload extension header or
other shim control packet in the "packet in error": Use the
"packet in error" for dispatching as follows. Deliver to the
context with CT(peer) equal to the Receiver Context Tag, Lp(local)
being the IPv6 source address, and Lp(peer) being the IPv6
destination address.
In addition, the shim on the sending side needs to be able to find
the context state when a ULP packet is passed down from the ULP. In
that case the lookup key is the pair of ULIDs and FII=0. If we have
a ULP API that allows the ULP to do context forking, then presumably
the ULP would pass down the Forked Instance Identifier.
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13. Initial Contact
The initial contact is some non-shim communication between two ULIDs,
as described in Section 2. At that point in time there is no
activity in the shim.
Whether the shim ends up being used or not (e.g., the peer might not
support shim6) it is highly desirable that the initial contact can be
established even if there is a failure for one or more IP addresses.
The approach taken is to rely on the applications and the transport
protocols to retry with different source and destination addresses,
consistent with what is already specified in Default Address
Selection [12], and some fixes to that specification [13] to make it
try different source addresses and not only different destination
addresses.
The implementation of such an approach can potentially result in long
timeouts. For instance, a naive implementation at the socket API
which uses getaddrinfo() to retrieve all destination addresses and
then tries to bind() and connect() to try all source and destination
address combinations waiting for TCP to time out for each combination
before trying the next one.
However, if implementations encapsulate this in some new connect-by-
name() API, and use non-blocking connect calls, it is possible to
cycle through the available combinations in a more rapid manner until
a working source and destination pair is found. Thus the issues in
this domain are issues of implementations and the current socket API,
and not issues of protocol specification. In all honesty, while
providing an easy to use connect-by-name() API for TCP and other
connection-oriented transports is easy; providing a similar
capability at the API for UDP is hard due to the protocol itself not
providing any "success" feedback. But even the UDP issue is one of
APIs and implementation.
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14. Protocol constants
The protocol uses the following constants:
I1_RETRIES_MAX
I1_TIMEOUT = 4 seconds
NO_R1_HOLDDOWN_TIME = 1 min
ICMP_HOLDDOWN_TIME = 10 min
I2_TIMEOUT = 4 seconds
I2_RETRIES_MAX = 2
I2bis_TIMEOUT = 4 seconds
I2bis_RETRIES_MAX = 2
VALIDATOR_MIN_LIFETIME = 30 seconds
UPDATE_TIMEOUT = 4 seconds
The retransmit timers (I1_TIMEOUT, I2_TIMEOUT, UPDATE_TIMEOUT) are
subject to binary exponential backoff, as well as randomization
across a range of 0.5 and 1.5 times the nominal (backed off) value.
This removes any risk of synchronization between lots of hosts
performing independent shim operations at the same time.
The randomization is applied after the binary exponential backoff.
Thus the first retransmission would happen based on a uniformly
distributed random number in the range [0.5*4, 1.5*4] seconds, the
second retransmission [0.5*8, 1.5*8] seconds after the first one,
etc.
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15. Implications Elsewhere
The general shim6 approach, as well as the specifics of this proposed
solution, has implications elsewhere. The key implications are:
o Applications that perform referrals, or callbacks using IP
addresses as the 'identifiers' can still function in limited ways,
as described in [22]. But in order for such applications to be
able to take advantage of the multiple locators for redundancy,
the applications need to be modified to either use fully qualified
domain names as the 'identifiers', or they need to pass all the
locators as the 'identifiers' i.e., the 'identifier' from the
applications perspective becomes a set of IP addresses instead of
a single IP address.
o Firewalls that today pass limited traffic, e.g., outbound TCP
connections, would presumably block the shim6 protocol. This
means that even when shim6 capable hosts are communicating, the I1
messages would be dropped, hence the hosts would not discover that
their peer is shim6 capable. This is in fact a feature, since if
the hosts managed to establish a ULID-pair context, then the
firewall would probably drop the "different" packets that are sent
after a failure (those using the shim6 payload extension header
with a TCP packet inside it). Thus stateful firewalls that are
modified to pass shim6 messages should also be modified to pass
the payload extension header, so that the shim can use the
alternate locators to recover from failures. This presumably
implies that the firewall needs to track the set of locators in
use by looking at the shim6 control exchanges. Such firewalls
might even want to verify the locators using the HBA/CGA
verification themselves, which they can do without modifying any
of the shim6 packets they pass through.
o Signaling protocols for QoS or other things that involve having
devices in the network path look at IP addresses and port numbers,
or IP addresses and Flow Labels, need to be invoked on the hosts
when the locator pair changes due to a failure. At that point in
time those protocols need to inform the devices that a new pair of
IP addresses will be used for the flow. Note that this is the
case even though this protocol, unlike some earlier proposals,
does not overload the flow label as a context tag; the in-path
devices need to know about the use of the new locators even though
the flow label stays the same.
o MTU implications. The path MTU mechanisms we use are robust
against different packets taking different paths through the
Internet, by computing a minimum over the recently observed path
MTUs. When shim6 fails over from using one locator pair to
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another pair, this means that packets might travel over a
different path through the Internet, hence the path MTU might be
quite different. Perhaps such a path change would be a good hint
to the path MTU mechanism to try a larger MTU?
The fact that the shim will add an 8 octet payload extension
header to the ULP packets after a locator switch, can also affect
the usable path MTU for the ULPs. In this case the MTU change is
local to the sending host, thus conveying the change to the ULPs
is an implementation matter.
o The precise interaction between Mobile IPv6 and shim6 is for
further study, but it might make sense to have Mobile IPv6 operate
on locators, meaning that the shim would be layered on top of the
MIPv6 mechanism.
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16. Security Considerations
This document satisfies the concerns specified in [19] as follows:
o The HBA technique [7] for verifying the locators to prevent an
attacker from redirecting the packet stream to somewhere else.
o Requiring a Reachability Probe+Reply before a new locator is used
as the destination, in order to prevent 3rd party flooding
attacks.
o The first message does not create any state on the responder.
Essentially a 3-way exchange is required before the responder
creates any state. This means that a state-based DoS attack
(trying to use up all of memory on the responder) at least
provides an IPv6 address that the attacker was using.
o The context establishment messages use nonces to prevent replay
attacks, and to prevent off-path attackers from interfering with
the establishment.
o Every control message of the shim6 protocol, past the context
establishment, carry the context tag assigned to the particular
context. This implies that an attacker needs to discover that
context tag before being able to spoof any shim6 control message.
Such discovery probably requires to be along the path in order to
be sniff the context tag value. The result is that through this
technique, the shim6 protocol is protected against off-path
attackers.
Some of the residual threats in this proposal are:
o An attacker which arrives late on the path (after the context has
been established) can use the R1bis message to cause one peer to
recreate the context, and at that point in time the attacker can
observe all of the exchange. But this doesn't seem to open any
new doors for the attacker since such an attacker can observe the
context tags that are being used, and once known it can use those
to send bogus messages.
o An attacker which is present on the path so that it can find out
the context tags, can generate a R1bis message after it has moved
off the path. For this packet to be effective it needs to have a
source locator which belongs to the context, thus there can not be
"too much" ingress filtering between the attackers new location
and the communicating peers. But this doesn't seem to be that
severe, because once the R1bis causes the context to be re-
established, a new pair of context tags will be used, which will
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not be known to the attacker. If this is still a concern, we
could require a 2-way handshake "did you really loose the state?"
in response to the error message.
o It might be possible for an attacker to try random 47-bit context
tags and see if they can cause disruption for communication
between two hosts. If a 47-bit tag, which is the largest that
fits in an 8-octet extension header, isn't sufficient, one could
use an even larger tag in the shim6 control messages, and use the
low-order 47 bits in the payload extension header.
o When the payload extension header is used, an attacker that can
guess the 47-bit random context tag, can inject packets into the
context with any source locator. Thus if there is ingress
filtering between the attacker, this could potentially allow to
bypass the ingress filtering. However, in addition to guessing
the 47-bit context tag, the attacker also needs to find a context
where, after the receiver's replacement of the locators with the
ULIDs, the the ULP checksum is correct. But even this wouldn't be
sufficient with ULPs like TCP, since the TCP port numbers and
sequence numbers must match an existing connection. Thus, even
though the issues for off-path attackers injecting packets are
different than today with ingress filtering, it is still very hard
for an off-path attacker to guess. If IPsec is applied then the
issue goes away completely.
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17. IANA Considerations
IANA is directed to allocate a new IP Protocol Number value for the
SHIM6 Protocol.
IANA is directed to record a CGA message type for the SHIM6 Protocol
in the [CGA] namespace registry with the value 0x4A30 5662 4858 574B
3655 416F 506A 6D48.
IANA is directed to establish a SHIM6 Parameter Registry with two
components: SHIM6 Type registrations and SHIM6 Options registrations.
The initial contents of the SHIM6 Type registry are as follows:
+------------+-----------------------------------------------------+
| Type Value | Message |
+------------+-----------------------------------------------------+
| 0 | RESERVED |
| | |
| 1 | I1 (first establishment message from the initiator) |
| | |
| 2 | R1 (first establishment message from the responder) |
| | |
| 3 | I2 (2nd establishment message from the initiator) |
| | |
| 4 | R2 (2nd establishment message from the responder) |
| | |
| 5 | R1bis (Reply to reference to non-existent context) |
| | |
| 6 | I2bis (Reply to a R1bis message) |
| | |
| 7-59 | Can be allocated using Standards Action |
| | |
| 60-63 | For Experimental use |
| | |
| 64 | Update Request |
| | |
| 65 | Update Acknowledgement |
| | |
| 66 | Keepalive |
| | |
| 67 | Probe Message |
| | |
| 68-123 | Can be allocated using Standards Action |
| | |
| 124-127 | For Experimental use |
+------------+-----------------------------------------------------+
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The initial contents of the SHIM6 Options registry are as follows:
+--------------+----------------------------------+
| Type | Option Name |
+--------------+----------------------------------+
| 0 | RESERVED |
| | |
| 1 | Responder Validator |
| | |
| 2 | Locator List |
| | |
| 3 | Locator Preferences |
| | |
| 4 | CGA Parameter Data Structure |
| | |
| 5 | CGA Signature |
| | |
| 6 | ULID Pair |
| | |
| 7 | Forked Instance Identifier |
| | |
| 8-9 | Allocated using Standards action |
| | |
| 10 | Probe Option |
| | |
| 11 | Reachability Option |
| | |
| 12 | Payload Reception Report Option |
| | |
| 13-16383 | Allocated using Standards action |
| | |
| 16384-32767 | For Experimental use |
+--------------+----------------------------------+
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18. Acknowledgements
Over the years many people active in the multi6 and shim6 WGs have
contributed ideas a suggestions that are reflected in this
specification. Special thanks to the careful comments from Geoff
Houston and Shinta Sugimoto on earlier versions of this draft.
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Appendix A. Open Issues
The following known open issues in this protocol specification are:
o NONE.
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Appendix B. Possible Protocol Extensions
During the development of this protocol, several issues have been
brought up as important one to address, but are ones that do not need
to be in the base protocol itself but can instead be done as
extensions to the protocol. The key ones are:
o As stated in the assumptions in Section 3, the in order for the
shim6 protocol to be able to recover from a wide range of
failures, for instance when one of the communicating hosts is
singly-homed, and cope with a site's ISPs that do ingress
filtering based on the source IPv6 address, there is a need for
the host to be able to influence the egress selection from its
site. Further discussion of this issue is captured in [20].
o Is there need for keeping the list of locators private between the
two communicating endpoints? We can potentially accomplish that
when using CGA but not with HBA, but it comes at the cost of doing
some public key encryption and decryption operations as part of
the context establishment. The suggestion is to leave this for a
future extension to the protocol.
o Defining some form of end-to-end "compression" mechanism that
removes the need for including the Shim6 Payload extension header
when the locator pair is not the ULID pair.
o Supporting the dynamic setting of locator preferences on a site-
wide basis, and use the Locator Preference option in the shim6
protocol to convey these preferences to remote communicating
hosts. This could mirror the DNS SRV record's notion of priority
and weight.
o Potentially recommend that more application protocols use DNS SRV
records to allow a site some influence on load spreading for the
initial contact (before the shim6 context establishment) as well
as for traffic which does not use the shim.
o Specifying APIs for the ULPs to be aware of the locators the shim
is using, and be able to influence the choice of locators
(controlling preferences as well as triggering a locator pair
switch). This includes providing APIs the ULPs can use to fork a
shim context.
o Whether it is feasible to relax the suggestions for when context
state is removed, so that one can end up with an asymmetric
distribution of the context state and still get (most of) the shim
benefits. For example, the busy server would go through the
context setup but would quickly remove the context state after
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this (in order to save memory) but the not-so-busy client would
retain the context state. The context recovery mechanism
presented in Section 7.5 would then be recreate the state should
the client send either a shim control message (e.g., probe message
because it sees a problem), or a ULP packet in an payload
extension header (because it had earlier failed over to an
alternative locator pair, but had been silent for a while). This
seems to provide the benefits of the shim as long as the client
can detect the failure. If the client doesn't send anything, and
it is the server that tries to send, then it will not be able to
recover because the shim on the server has no context state, hence
doesn't know any alternate locator pairs.
o Study whether a host explicitly fail communication when a ULID
becomes invalid (based on RFC 2462 lifetimes or DHCPv6), or should
we let the communication continue using the invalidated ULID (it
can certainly work since other locators will be used).
o Study what it would take to make the shim6 control protocol not
rely at all on a stable source locator in the packets. This can
probably be accomplished by having all the shim control messages
include the ULID-pair option.
o If each host might have lots of locators, then the currently
requirement to include essentially all of them in the I2 and R2
messages might be constraining. If this is the case we can look
into using the CGA Parameter Data Structure for the comparison,
instead of the prefix sets, to be able to detect context
confusion. This would place some constraint on a (logical) only
using e.g., one CGA public key, and would require some carefully
crafted rules on how two PDSs are compared for "being the same
host". But if we don't expect more than a handful locators per
host, then we don't need this added complexity.
o ULP specified timers for the reachability detection mechanism
(which can be useful particularly when there are forked contexts).
o Pre-verify some "backup" locator pair, so that the failover time
can be shorter.
o Study how shim6 and Mobile IPv6 might interact. There existing an
initial draft on this topic [21].
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Appendix C. Change Log
The following changes have been made since draft-ietf-shim6-proto-03:
o Editorial clarifications based on comments from Geoff, Shinta,
Jari.
o Added "no IPv6 NATs as an explicit assumption.
o Moving some things out of the Introduction and Overview sections
to remove all SHOULDs and MUSTs from there.
o Added requirement that any Locator Preference options which use an
element length greater than 3 octets have the already defined
first 3 octets of flags, priority and weight.
o Fixed security hole where a single message (I1) could cause
CT(peer) to be updated. Now a three-way handshake is required
before CT(peer) is updated for an existing context.
The following changes have been made since draft-ietf-shim6-proto-02:
o Replaced the Context Error message with the R1bis message.
o Removed the Packet In Error option, since it was only used in the
Context Error message.
o Introduced a I2bis message which is sent in response to an I1bis
message, since the responders processing is quite in this case
than in the regular R1 case.
o Moved the packet formats for the Keepalive and Probe message types
and Event option to [8]. Only the message type values and option
type value are specified for those in this document.
o Removed the unused message types.
o Added a state machine description as an appendix.
o Filled in all the TBDs - except the IANA assignment of the
protocol number.
o Specified how context recovery and forked contexts work together.
This required the introduction of a Forked Instance option to be
able to tell which of possibly forked instances is being
recovered.
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o Renamed the "host-pair context" to be "ULID-pair context".
o Picked some initial retransmit timers for I1 and I2; 4 seconds.
o Added timer values as protocol constants. The retransmit timers
use binary exponential backoff and randomization (between .5 and
1.5 of the nominal value).
o Require that the R1/R1bis verifiers be usable for some minimum
time so that the initiator knows for how long time it can safely
retransmit I2 before it needs to go back to sending I1 again.
Picked 30 seconds.
o Split the message type codes into 0-63, which will not generate
R1bis messages, and 64-127 which will generate R1bis messages.
This allows extensibility of the protocol with new message types
while being able to control when R1bis is generated.
o Expanded the context tag from 32 to 47 bits.
o Specified that enough locators need to be included in I2 and R2
messages. Specified that the HBA/CGA verification must be
performed when the locator set is received.
o Specified that ICMP parameter problem errors are sent in certain
error cases, for instance when the verification method is unknown
to the receiver, or there is an unknown message type or option
type.
o Renamed "payload message" to be "payload extension header".
o Many editorial clarifications suggested by Geoff Huston.
o Modified the dispatching of payload extension header to only
compare CT(local) i.e., not compare the source and destination
IPv6 address fields.
The following changes have been made since draft-ietf-shim6-proto-00:
o Removed the use of the flow label and the overloading of the IP
protocol numbers. Instead, when the locator pair is not the ULID
pair, the ULP payloads will be carried with an 8 octet extension
header. The belief is that it is possible to remove these extra
bytes by defining future shim6 extensions that exchange more
information between the hosts, without having to overload the flow
label or the IP protocol numbers.
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o Grew the context tag from 20 bits to 32 bits, with the possibility
to grow it to 47 bits. This implies changes to the message
formats.
o Almost by accident, the new shim6 message format is very close to
the HIP message format.
o Adopted the HIP format for the options, since this makes it easier
to describe variable length options. The original, ND-style,
option format requires internal padding in the options to make
them 8 octet length in total, while the HIP format handles that
using the option length field.
o Removed some of the control messages, and renamed the other ones.
o Added a "generation" number to the Locator List option, so that
the peers can ensure that the preferences refer to the right
"version" of the Locator List.
o In order for FBD and exploration to work when there the use of the
context is forked, that is different ULP messages are sent over
different locator pairs, things are a lot easier if there is only
one current locator pair used for each context. Thus the forking
of the context is now causing a new context to be established for
the same ULID; the new context having a new context tag. The
original context is referred to as the "default" context for the
ULID pair.
o Added more background material and textual descriptions.
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Appendix D. Simplified State Machine
The states are defined in Section 6.2. The intent is that the
stylized description below be consistent with the textual description
in the specification, but should they conflict, the textual
description is normative.
The following table describes the possible actions in state IDLE and
their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1 | Send R1 and stay in IDLE |
| | |
| Heuristics trigger | Send I1 and move to I1-SENT |
| a new context | |
| establishment | |
| | |
| Receive I2, verify | If successful, send R2 and move to |
| validator and | ESTABLISHED |
| RESP nonce | |
| | If fail, stay in IDLE |
| | |
| Receive I2bis, | If successful, send R2 and move to |
| verify validator | ESTABLISHED |
| and RESP nonce | |
| | If fail, stay in IDLE |
| | |
| R1, R1bis, R2 | N/A (This context lacks the required info |
| | for the dispatcher to deliver them) |
| | |
| Receive payload | Send R1bis and stay in IDLE |
| extension header | |
| or other control | |
| packet | |
+---------------------+---------------------------------------------+
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The following table describes the possible actions in state I1-SENT
and their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive R1, verify | If successful, send I2 and move to I2-SENT |
| INIT nonce | |
| | If fail, discard and stay in I1-SENT |
| | |
| Receive I1 | Send R2 and stay in I1-SENT |
| | |
| Receive R2, verify | If successful, move to ESTABLISHED |
| INIT nonce | |
| | If fail, discard and stay in I1-SENT |
| | |
| Receive I2, verify | If successful, send R2 and move to |
| validator and RESP | ESTABLISHED |
| nonce | |
| | If fail, discard and stay in I1-SENT |
| | |
| Receive I2bis, | If successful, send R2 and move to |
| verify validator | ESTABLISHED |
| and RESP nonce | |
| | If fail, discard and stay in I1-SENT |
| | |
| Timeout, increment | If counter =< I1_RETRIES_MAX, send I1 and |
| timeout counter | stay in I1-SENT |
| | |
| | If counter > I1_RETRIES_MAX, go to E-FAILED |
| | |
| Receive ICMP payload| Move to E-FAILED |
| unknown error | |
| | |
| R1bis | N/A (Dispatcher doesn't deliver since |
| | CT(peer) is not set) |
| | |
| Receive Payload or | Discard and stay in I1-SENT |
| extension header | |
| or other control | |
| packet | |
+---------------------+---------------------------------------------+
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The following table describes the possible actions in state I2-SENT
and their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive R2, verify | If successful move to ESTABLISHED |
| INIT nonce | |
| | If fail, stay in I2-SENT |
| | |
| Receive I1 | Send R2 and stay in I2-SENT |
| | |
| Receive I2 | Send R2 and stay in I2-SENT |
| verify validator | |
| and RESP nonce | |
| | |
| Receive I2bis | Send R2 and stay in I2-SENT |
| verify validator | |
| and RESP nonce | |
| | |
| Receive R1 | Discard and stay in I2-SENT |
| | |
| Timeout, increment | If counter =< I2_RETRIES_MAX, send I2 and |
| timeout counter | stay in I2-SENT |
| | |
| | If counter > I2_RETRIES_MAX, send I1 and go |
| | to I1-SENT |
| | |
| R1bis | N/A (Dispatcher doesn't deliver since |
| | CT(peer) is not set) |
| | |
| Receive payload or | Accept and send I2 (probably R2 was sent |
| extension header | by peer and lost) |
| other control | |
| packet | |
+---------------------+---------------------------------------------+
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The following table describes the possible actions in state I2BIS-
SENT and their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive R2, verify | If successful move to ESTABLISHED |
| INIT nonce | |
| | If fail, stay in I2BIS-SENT |
| | |
| Receive I1 | Send R2 and stay in I2BIS-SENT |
| | |
| Receive I2 | Send R2 and stay in I2BIS-SENT |
| verify validator | |
| and RESP nonce | |
| | |
| Receive I2bis | Send R2 and stay in I2BIS-SENT |
| verify validator | |
| and RESP nonce | |
| | |
| Receive R1 | Discard and stay in I2BIS-SENT |
| | |
| Timeout, increment | If counter =< I2_RETRIES_MAX, send I2bis |
| timeout counter | and stay in I2BIS-SENT |
| | |
| | If counter > I2_RETRIES_MAX, send I1 and |
| | go to I1-SENT |
| | |
| R1bis | N/A (Dispatcher doesn't deliver since |
| | CT(peer) is not set) |
| | |
| Receive payload or | Accept and send I2bis (probably R2 was |
| extension header | sent by peer and lost) |
| other control | |
| packet | |
+---------------------+---------------------------------------------+
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The following table describes the possible actions in state
ESTABLISHED and their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Receive I1, compare | If no match, send R1 and stay in ESTABLISHED|
| CT(peer) with | |
| received CT | If match, send R2 and stay in ESTABLISHED |
| | |
| | |
| Receive I2, verify | If successful, then send R2 and stay in |
| validator and RESP | ESTABLISHED |
| nonce | |
| | Otherwise, discard and stay in ESTABLISHED |
| | |
| Receive I2bis, | If successful, then send R2 and stay in |
| verify validator | ESTABLISHED |
| and RESP nonce | |
| | Otherwise, discard and stay in ESTABLISHED |
| | |
| Receive R2 | Discard and stay in ESTABLISHED |
| | |
| Receive R1 | Discard and stay in ESTABLISHED |
| | |
| Receive R1bis | Send I2bis and move to I2BIS-SENT |
| | |
| | |
| Receive payload or | Process and stay in ESTABLISHED |
| extension header | |
| other control | |
| packet | |
| | |
| Implementation | Discard state and go to IDLE |
| specific heuristic | |
| (E.g., No open ULP | |
| sockets and idle | |
| for some time ) | |
+---------------------+---------------------------------------------+
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The following table describes the possible actions in state E-FAILED
and their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Wait for | Go to IDLE |
| NO_R1_HOLDDOWN_TIME | |
| | |
| Any packet | Process as in IDLE |
+---------------------+---------------------------------------------+
The following table describes the possible actions in state NO-
SUPPORT and their respective triggers:
+---------------------+---------------------------------------------+
| Trigger | Action |
+---------------------+---------------------------------------------+
| Wait for | Go to IDLE |
| ICMP_HOLDDOWN_TIME | |
| | |
| Any packet | Process as in IDLE |
+---------------------+---------------------------------------------+
Appendix D.1 Simplified State Machine diagram
For the time being, a pdf version of the state machine diagram can be
found at: http://www.it.uc3m.es/marcelo/state_machine.pdf
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Appendix E. Context Tag Reuse
The shim6 protocol doesn't have a mechanism for coordinated state
removal between the peers, because such state removal doesn't seem to
help given that a host can crash and reboot at any time. A result of
this is that the protocol needs to be robust against a context tag
being reused for some other context. This section summarizes the
different cases in which a tag can be reused, and how the recovery
works.
The different cases are exemplified by the following case. Assume
host A and B were communicating using a context with the ULID pair
<A1, B2>, and that B had assigned context tag X to this context. We
assume that B uses only the context tag to demultiplex the received
payload extension headers, since this is the more general case.
Further we assume that B removes this context state, while A retains
it. B might then at a later time assign CT(local)=X to some other
context, and we have several cases:
o The context tag is reassigned to a context for the same ULID pair
<A1, B2>. We've called this "Context Recovery" in this document.
o The context tag is reassigned to a context for a different ULID
pair between the same to hosts, e.g., <A3, B3>. We've called this
"Context Confusion" in this document.
o The context tag is reassigned to a context between B and other
host C, for instance for the ULID pair <C3, B2>. That is a form
of three party context confusion.
Appendix E.1 Context Recovery
This case is relatively simple, and is discussed in Section 7.5. The
observation is that since the ULID pair is the same, when either A or
B tries to establish the new context, A can keep the old context
while B re-creates the context with the same context tag CT(B) = X.
Appendix E.2 Context Confusion
This cases is a bit more complex, and is discussed in Section 7.6.
When the new context is created, whether A or B initiates it, host A
can detect when it receives B's locator set (in the I2, or R2
message), that it ends up with two contexts to the same peer host
(overlapping Ls(peer) locator sets) that have the same context tag
CT(peer) = X. At this point in time host A can clear up any
possibility of causing confusion by not using the old context to send
any more packets. It either just discards the old context (it might
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not be used by any ULP traffic, since B had discarded it), or it
recreates a different context for the old ULID pair (<A1, B2>), for
which B will assign a unique CT(B) as part of the normal context
establishment mechanism.
Appendix E.3 Three Party Context Confusion
The third case does not have a place where the old state on A can be
verified, since the new context is established between B and C. Thus
when B receives payload extension headers with X as the context tag,
it will find the context for <C3, B2>, hence rewrite the packets to
have C3 in the source address field and B2 in the destination address
field before passing them up to the ULP. This rewriting is correct
when the packets are in fact sent by host C, but if host A ever
happens to send a packet using the old context, then the ULP on A
sends a packet with ULIDs <A1, B2> and the packet arrives at the ULP
on B with ULIDs <C3, B2>.
This is clearly an error, and the packet will most likely be rejected
by the ULP on B due to a bad pseudo-header checksum. Even if the
checksum is ok (probability 2^-16), the ULP isn't likely to have a
connection for those ULIDs and port numbers. And if the ULP is
connection-less, processing the packet is most likely harmless; such
a ULP must be able to copy with random packets being sent by random
peers in any case.
This broken state, where packets sent from A to B using the old
context on host A might persist for some time, but it will not remain
for very long. The unreachability detection on host A will kick in,
because it does not see any return traffic (payload or Keepalive
messages) for the context. This will result in host A sending Probe
messages to host B to find a working locator pair. The effect of
this is that host B will notice that it does not have a context for
the ULID pair <A1, B2> and CT(B) = X, which will make host B send an
R1bis packet to re-establish that context. The re-established
context, just like in the previous section, will get a unique CT(B)
assigned by host B, thus there will no longer be any confusion.
In summary, there are cases where a context tag might be reused while
some peer retains the state, but the protocol can recover from it.
The probability of these events is low given the 47 bit context tag
size. However, it is important that these recovery mechanisms be
tested. Thus during development and testing it is recommended that
implementations not use the full 47 bit space, but instead keep e.g.
the top 40 bits as zero, only leaving the host with 128 unique
context tags. This will help test the recovery mechanisms.
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Appendix F. Design Alternatives
This document has picked a certain set of design choices in order to
try to work out a bunch of the details, and stimulate discussion.
But as has been discussed on the mailing list, there are other
choices that make sense. This appendix tries to enumerate some
alternatives.
Appendix F.1 Context granularity
Over the years various suggestions have been made whether the shim
should, even if it operates at the IP layer, be aware of ULP
connections and sessions, and as a result be able to make separate
shim contexts for separate ULP connections and sessions. A few
different options have been discussed:
o Each ULP connection maps to its own shim context.
o The shim is unaware of the ULP notion of connections and just
operates on a host-to-host (IP address) granularity.
o Hybrids where the shim is aware of some ULPs (such as TCP) and
handles other ULPs on a host-to-host basis.
Having shim state for every ULP connection potentially means higher
overhead since the state setup overhead might become significant;
there is utility in being able to amortize this over multiple
connections.
But being completely unaware of the ULP connections might hamper ULPs
that want different communication to use different locator pairs, for
instance for quality or cost reasons.
The protocol has a shim which operates with host-level granularity
(strictly speaking, with ULID-pair granularity, to be able to
amortize the context establishment over multiple ULP connections.
This is combined with the ability for shim-aware ULPs to request
context forking so that different ULP traffic can use different
locator pairs.
Appendix F.2 Demultiplexing of data packets in shim6 communications
Once a ULID-pair context is established between two hosts, packets
may carry locators that differ from the ULIDs presented to the ULPs
using the established context. One of main functions of the SHIM6
layer is to perform the mapping between the locators used to forward
packets through the network and the ULIDs presented to the ULP. In
order to perform that translation for incoming packets, the SHIM6
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layer needs to first identify which of the incoming packets need to
be translated and then perform the mapping between locators and ULIDs
using the associated context. Such operation is called
demultiplexing. It should be noted that because any address can be
used both as a locator and as a ULID, additional information other
than the addresses carried in packets, need to be taken into account
for this operation.
For example, if a host has address A1 and A2 and starts communicating
with a peer with addresses B1 and B2, then some communication
(connections) might use the pair <A1, B1> as ULID and others might
use e.g., <A2, B2>. Initially there are no failures so these address
pairs are used as locators i.e. in the IP address fields in the
packets on the wire. But when there is a failure the shim6 layer on
A might decide to send packets that used <A1, B1> as ULIDs using <A2,
B2> as the locators. In this case B needs to be able to rewrite the
IP address field for some packets and not others, but the packets all
have the same locator pair.
In order to accomplish the demultiplexing operation successfully,
data packets carry a context tag that allows the receiver of the
packet to determine the shim context to be used to perform the
operation.
Two mechanisms for carrying the context tag information have been
considered in depth during the shim protocol design. Those carrying
the context tag in the flow label field of the IPv6 header and the
usage of a new extension header to carry the context tag. In this
appendix we will describe the pros and cons of each approach and
justify the selected option.
Appendix F.2.1 Flow-label
A possible approach is to carry the context tag in the Flow Label
field of the IPv6 header. This means that when a shim6 context is
established, a Flow Label value is associated with this context (and
perhaps a separate flow label for each direction).
The simplest approach that does this is to have the triple <Flow
Label, Source Locator, Destination Locator> identify the context at
the receiver.
The problem with this approach is that because the locator sets are
dynamic, it is not possible at any given moment to be sure that two
contexts for which the same context tag is allocated will have
disjoint locator sets during the lifetime of the contexts.
Suppose that Node A has addresses IPA1, IPA2, IPA3 and IPA4 and that
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Host B has addresses IPB1 and IPB2.
Suppose that two different contexts are established between HostA and
HostB.
Context #1 is using IPA1 and IPB1 as ULIDs. The locator set
associated to IPA1 is IPA1 and IPA2 while the locator set associated
to IPB1 is just IPB1.
Context #2 uses IPA3 and IPB2 as ULIDs. The locator set associated
to IPA3 is IPA3 and IPA4 and the locator set associated to IPB2 is
just IPB2.
Because the locator sets of the Context #1 and Context #2 are
disjoint, hosts could think that the same context tag value can be
assigned to both of them. The problem arrives when later on IPA3 is
added as a valid locator for IPA1 and IPB2 is added as a valid
locator for IPB1 in Context #1. In this case, the triple <Flow
Label, Source Locator, Destination Locator> would not identify a
unique context anymore and correct demultiplexing is no longer
possible.
A possible approach to overcome this limitation is simply not to
repeat the Flow Label values for any communication established in a
host. This basically means that each time a new communication that
is using different ULIDs is established, a new Flow Label value is
assigned to it. By this mean, each communication that is using
different ULIDs can be differentiated because it has a different Flow
Label value.
The problem with such approach is that it requires that the receiver
of the communication allocates the Flow Label value used for incoming
packets, in order to assign them uniquely. For this, a shim
negotiation of the Flow Label value to use in the communication is
needed before exchanging data packets. This poses problems with non-
shim capable hosts, since they would not be able to negotiate an
acceptable value for the Flow Label. This limitation can be lifted
by marking the packets that belong to shim sessions from those that
do not. These marking would require at least a bit in the IPv6
header that is not currently available, so more creative options
would be required, for instance using new Next Header values to
indicate that the packet belongs to a shim6 enabled communication and
that the Flow Label carries context information as proposed in the
now expired NOID draft. . However, even if this is done, this
approach is incompatible with the deferred establishment capability
of the shim protocol, which is a preferred function, since it
suppresses the delay due to the shim context establishment prior to
initiation of the communication and it also allows nodes to define at
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which stage of the communication they decide, based on their own
policies, that a given communication requires to be protected by the
shim.
In order to cope with the identified limitations, an alternative
approach that does not constraints the flow label values used by
communications that are using ULIDs equal to the locators (i.e. no
shim translation) is to only require that different flow label values
are assigned to different shim contexts. In such approach
communications start with unmodified flow label usage (could be zero,
or as suggested in [16]). The packets sent after a failure when a
different locator pair is used would use a completely different flow
label, and this flow label could be allocated by the receiver as part
of the shim context establishment. Since it is allocated during the
context establishment, the receiver of the "failed over" packets can
pick a flow label of its choosing (that is unique in the sense that
no other context is using it as a context tag), without any
performance impact, and respecting that for each locator pair, the
flow label value used for a given locator pair doesn't change due to
the operation of the multihoming shim.
In this approach, the constraint is that Flow Label values being used
as context identifiers cannot be used by other communications that
use non-disjoint locator sets. This means that once that a given
Flow Label value has been assigned to a shim context that has a
certain locator sets associated, the same value cannot be used for
other communications that use an address pair that is contained in
the locator sets of the context. This is a constraint in the
potential Flow Label allocation strategies.
A possible workaround to this constraint is to mark shim packets that
require translation, in order to differentiate them from regular IPv6
packets, using the artificial Next Header values described above. In
this case, the Flow Label values constrained are only those of the
packets that are being translated by the shim. This last approach
would be the preferred approach if the context tag is to be carried
in the Flow Label field. This is not only because it imposes the
minimum constraints to the Flow Label allocation strategies, limiting
the restrictions only to those packets that need to be translated by
the shim, but also because Context Loss detection mechanisms greatly
benefit from the fact that shim data packets are identified as such,
allowing the receiving end to identify if a shim context associated
to a received packet is suppose to exist, as it will be discussed in
the Context Loss detection appendix below.
Appendix F.2.2 Extension Header
Another approach, which is the one selected for this protocol, is to
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carry the context tag in a new Extension Header. These context tags
are allocated by the receiving end during the shim6 protocol initial
negotiation, implying that each context will have two context tags,
one for each direction. Data packets will be demultiplexed using the
context tag carried in the Extension Header. This seems a clean
approach since it does not overload existing fields. However, it
introduces additional overhead in the packet due to the additional
header. The additional overhead introduced is 8 octets. However, it
should be noted that the context tag is only required when a locator
other than the one used as ULID is contained in the packet. Packets
where both the source and destination address fields contain the
ULIDs do not require a context tag, since no rewriting is necessary
at the receiver. This approach would reduce the overhead, because
the additional header is only required after a failure. On the other
hand, this approach would cause changes in the available MTU for some
packets, since packets that include the Extension Header will have an
MTU 8 octets shorter. However, path changes through the network can
result in different MTU in any case, thus having a locator change,
which implies a path change, affect the MTU doesn't introduce any new
issues.
Appendix F.3 Context Loss Detection
In this appendix we will present different approaches considered to
detect context loss and potential context recovery strategies. The
scenario being considered is the following: Node A and Node B are
communicating using IPA1 and IPB1. Sometime later, a shim context is
established between them, with IPA1 and IPB1 as ULIDs and
IPA1,...,IPAn and IPB1,...,IPBm as locator set respectively.
It may happen, that later on, one of the hosts, e.g. Host A looses
the shim context. The reason for this can be that Host A has a more
aggressive garbage collection policy than HostB or that an error
occurred in the shim layer at host A resulting in the loss of the
context state.
The mechanisms considered in this appendix are aimed to deal with
this problem. There are essentially two tasks that need to be
performed in order to cope with this problem: first, the context loss
must be detected and second the context needs to be recovered/
reestablished.
Mechanisms for detecting context. loss
These mechanisms basically consist in that each end of the context
periodically sends a packet containing context-specific information
to the other end. Upon reception of such packets, the receiver
verifies that the required context exists. In case that the context
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does not exist, it sends a packet notifying the problem to the
sender.
An obvious alternative for this would be to create a specific context
keepalive exchange, which consists in periodically sending packets
with this purpose. This option was considered and discarded because
it seemed an overkill to define a new packet exchange to deal with
this issue.
An alternative is to piggyback the context loss detection function in
other existent packet exchanges. In particular, both shim control
and data packets can be used for this.
Shim control packets can be trivially used for this, because they
carry context specific information, so that when a node receives one
of such packets, it will verify if the context exists. However, shim
control frequency may not be adequate for context loss detection
since control packet exchanges can be very limited for a session in
certain scenarios.
Data packets, on the other hand, are expected to be exchanged with a
higher frequency but they do not necessarily carry context specific
information. In particular, packets flowing before a locator change
(i.e. packet carrying the ULIDs in the address fields) do not need
context information since they do not need any shim processing.
Packets that carry locators that differ from the ULIDs carry context
information.
However, we need to make a distinction here between the different
approaches considered to carry the context tag, in particular between
those approaches where packets are explicitly marked as shim packets
and those approaches where packets are not marked as such. For
instance, in the case where the context tag is carried in the Flow
Label and packets are not marked as shim packets (i.e. no new Next
Header values are defined for shim), a receiver that has lost the
associated context is not able to detect that the packet is
associated with a missing context. The result is that the packet
will be passed unchanged to the upper layer protocol, which in turn
will probably silently discard it due to a checksum error. The
resulting behavior is that the context loss is undetected. This is
one additional reason to discard an approach that carries the context
tag in the Flow Label field and does not explicitly mark the shim
packets as such. On the other hand, approaches that mark shim data
packets (like the Extension Header or the Flow Label with new Next
Header values approaches) allow the receiver to detect if the context
associated to the received packet is missing. In this case, data
packets also perform the function of a context loss detection
exchange. However, it must be noted that only those packets that
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carry a locator that differs form the ULID are marked. This
basically means that context loss will be detected after an outage
has occurred i.e. alternative locators are being used.
Summarizing, the proposed context loss detection mechanisms uses shim
control packets and payload extension headers to detect context loss.
Shim control packets detect context loss during the whole lifetime of
the context, but the expected frequency in some cases is very low.
On the other hand, payload extension headers have a higher expected
frequency in general, but they only detect context loss after an
outage. This behavior implies that it will be common that context
loss is detected after a failure i.e. once that it is actually
needed. Because of that, a mechanism for recovering from context
loss is required if this approach is used.
Overall, the mechanism for detecting lost context would work as
follows: the end that still has the context available sends a message
referring to the context. Upon the reception of such message, the
end that has lost the context identifies the situation and notifies
the context loss event to the other end by sending a packet
containing the lost context information extracted from the received
packet.
One option is to simply send an error message containing the received
packets (or at least as much of the received packet that the MTU
allows to fit in). One of the goals of this notification is to allow
the other end that still retains context state, to reestablish the
lost context. The mechanism to reestablish the loss context consists
in performing the 4-way initial handshake. This is a time consuming
exchange and at this point time may be critical since we are
reestablishing a context that is currently needed (because context
loss detection may occur after a failure). So, another option, which
is the one used in this protocol, is to replace the error message by
a modified R1 message, so that the time required to perform the
context establishment exchange can be reduced. Upon the reception of
this modified R1 message, the end that still has the context state
can finish the context establishment exchange and restore the lost
context.
Appendix F.4 Securing locator sets
The adoption of a protocol like SHIM that allows the binding of a
given ULID with a set of locators opens the doors for different types
of redirection attacks as described in [19]. The goal in terms of
security for the design of the shim protocol is not to introduce any
new vulnerability in the Internet architecture. It is a non-goal to
provide additional protection than the currently available in the
single-homed IPv6 Internet.
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Multiple security mechanisms were considered to protect the shim
protocol. In this appendix we will present some of them.
The simplest option to protect the shim protocol was to use cookies
i.e. a randomly generated bit string that is negotiated during the
context establishment phase and then it is included in following
signaling messages. By this mean, it would be possible to verify
that the party that was involved in the initial handshake is the same
party that is introducing new locators. Moreover, before using a new
locator, an exchange is performed through the new locator, verifying
that the party located at the new locator knows the cookie i.e. that
it is the same party that performed the initial handshake.
While this security mechanisms does indeed provide a fair amount of
protection, it does leave the door open for the so-called time
shifted attacks. In these attacks, an attacker that once was on the
path, it discovers the cookie by sniffing any signaling message.
After that, the attacker can leave the path and still perform a
redirection attack, since as he is in possession of the cookie, he
can introduce a new locator in the locator set and he can also
successfully perform the reachability exchange if he is able to
receive packets at the new locator. The difference with the current
single-homed IPv6 situation is that in the current situation the
attacker needs to be on-path during the whole lifetime of the attack,
while in this new situation where only cookie protection if provided,
an attacker that once was on the path can perform attacks after he
has left the on-path location.
Moreover, because the cookie is included in signaling messages, the
attacker can discover the cookie by sniffing any of them, making the
protocol vulnerable during the whole lifetime of the shim context. A
possible approach to increase the security was to use a shared secret
i.e. a bit string that is negotiated during the initial handshake but
that is used as a key to protect following messages. With this
technique, the attacker must be present on the path sniffing packets
during the initial handshake, since it is the only moment where the
shared secret is exchanged. While this improves the security, it is
still vulnerable to time shifted attacks, even though it imposes that
the attacker must be on path at a very specific moment (the
establishment phase) to actually be able to launch the attack. While
this seems to substantially improve the situation, it should be noted
that, depending on protocol details, an attacker may be able to force
the recreation of the initial handshake (for instance by blocking
messages and making the parties think that the context has been
lost), so the resulting situation may not differ that much from the
cookie based approach.
Another option that was discussed during the design of the protocol
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was the possibility of using IPsec for protecting the shim protocol.
Now, the problem under consideration in this scenario is how to
securely bind an address that is being used as ULID with a locator
set that can be used to exchange packets. The mechanism provided by
IPsec to securely bind the address used with the cryptographic keys
is the usage of digital certificates. This implies that an IPsec
based solution would require that the generation of digital
certificates that bind the key and the ULID by a common third trusted
party for both parties involved in the communication. Considering
that the scope of application of the shim protocol is global, this
would imply a global public key infrastructure. The major issues
with this approach are the deployment difficulties associated with a
global PKI.
Finally two different technologies were selected to protect the shim
protocol: HBA [7] and CGA [6]. These two approaches provide a
similar level of protection but they provide different functionality
with a different computational cost.
The HBA mechanism relies on the capability of generating all the
addresses of a multihomed host as an unalterable set of intrinsically
bound IPv6 addresses, known as an HBA set. In this approach,
addresses incorporate a cryptographic one-way hash of the prefix-set
available into the interface identifier part. The result is that the
binding between all the available addresses is encoded within the
addresses themselves, providing hijacking protection. Any peer using
the shim protocol node can efficiently verify that the alternative
addresses proposed for continuing the communication are bound to the
initial address through a simple hash calculation. A limitation of
the HBA technique is that once generated the address set is fixed and
cannot be changed without also changing all the addresses of the HBA
set. In other words, the HBA technique does not support dynamic
addition of address to a previously generated HBA set. An advantage
of this approach is that it requires only hash operations to verify a
locator set, imposing very low computational cost to the protocol.
In a CGA based approach the address used as ULID is a CGA that
contains a hash of a public key in its interface identifier. The
result is a secure binding between the ULID and the associated key
pair. This allows each peer to use the corresponding private key to
sign the shim messages that convey locator set information. The
trust chain in this case is the following: the ULID used for the
communication is securely bound to the key pair because it contains
the hash of the public key, and the locator set is bound to the
public key through the signature. The CGA approach then supports
dynamic addition of new locators in the locator set, since in order
to do that, the node only needs to sign the new locator with the
private key associated with the CGA used as ULID. A limitation of
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this approach is that it imposes systematic usage of public key
cryptography with its associate computational cost.
Any of these two mechanisms HBA and CGA provide time-shifted attack
protection, since the ULID is securely bound to a locator set that
can only be defined by the owner of the ULID.
So, the design decision adopted was that both mechanisms HBA and CGA
are supported, so that when only stable address sets are required,
the nodes can benefit from the low computational cost offered by HBA
while when dynamic locator sets are required, this can be achieved
through CGAs with an additional cost. Moreover, because HBAs are
defined as a CGA extension, the addresses available in a node can
simultaneously be CGAs and HBAs, allowing the usage of the HBA and
CGA functionality when needed without requiring a change in the
addresses used.
Appendix F.5 ULID-pair context establishment exchange
Two options were considered for the ULID-pair context establishment
exchange: a 2-way handshake and a 4-way handshake.
A key goal for the design of this exchange was that protection
against DoS attacks. The attack under consideration was basically a
situation where an attacker launches a great amount of ULID-pair
establishment request packets, exhausting victim's resources, similar
to TCP SYN flooding attacks.
A 4 way-handshake exchange protects against these attacks because the
receiver does not creates any state associate to a given context
until the reception of the second packet which contains a prior
contact proof in the form of a token. At this point the receiver can
verify that at least the address used by the initiator is at some
extent valid, since the initiator is able to receive packets at this
address. In the worse case, the responder can track down the
attacker using this address. The drawback of this approach is that
it imposes a 4 packet exchange for any context establishment. This
would be a great deal if the shim context needed to be established up
front, before the communication can proceed. However, thanks to
deferred context establishment capability of the shim protocol, this
limitation has a reduced impact in the performance of the protocol.
(It may however have a greater impact in the situation of context
recover as discussed earlier, but in this case, it is possible to
perform optimizations to reduce the number of packets as described
above)
The other option considered was a 2-way handshake with the
possibility to fall back to a 4-way handshake in case of attack. In
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this approach, the ULID-pair establishment exchange normally consists
in a 2-packet exchange and it does not verify that the initiator has
performed a prior contact before creating context state. In case
that a DoS attack is detected, the responder falls back to a 4-way
handshake similar to the one described previously in order to prevent
the detected attack to proceed. The main difficulty with this attack
is how to detect that a responder is currently under attack. It
should be noted, that because this is 2-way exchange, it is not
possible to use the number of half open sessions (as in TCP) to
detect an ongoing attack and different heuristics need to be
considered.
The design decision taken was that considering the current impact of
DoS attacks and the low impact of the 4-way exchange in the shim
protocol thanks to the deferred context establishment capability, a
4-way exchange would be adopted for the base protocol.
Appendix F.6 Updating locator sets
There are two possible approaches to the addition and removal of
locators: atomic and differential approaches. The atomic approach
essentially send the complete locators set each time that a variation
in the locator set occurs. The differential approach send the
differences between the existing locator set and the new one. The
atomic approach imposes additional overhead, since all the locator
set has to be exchanged each time while the differential approach
requires re-synchronization of both ends through changes i.e. that
both ends have the same idea about what the current locator set is.
Because of the difficulties imposed by the synchronization
requirement, the atomic approach was selected.
Appendix F.7 State Cleanup
There are essentially two approaches for discarding an existing state
about locators, keys and identifiers of a correspondent node: a
coordinated approach and an unilateral approach.
In the unilateral approach, each node discards the information about
the other node without coordination with the other node based on some
local timers and heuristics. No packet exchange is required for
this. In this case, it would be possible that one of the nodes has
discarded the state while the other node still hasn't. In this case,
a No-Context error message may be required to inform about the
situation and possibly a recovery mechanism is also needed.
A coordinated approach would use an explicit CLOSE mechanism, akin to
the one specified in HIP [25]. If an explicit CLOSE handshake and
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associated timer is used, then there would no longer be a need for
the No Context Error message due to a peer having garbage collected
its end of the context. However, there is still potentially a need
to have a No Context Error message in the case of a complete state
loss of the peer (also known as a crash followed by a reboot). Only
if we assume that the reboot takes at least the CLOSE timer, or that
it is ok to not provide complete service until CLOSE timer minutes
after the crash, can we completely do away with the No Context Error
message.
In addition, other aspect that is relevant for this design choice is
the context confusion issue. In particular, using an unilateral
approach to discard context state clearly opens the possibility of
context confusion, where one of the ends unilaterally discards the
context state, while the peer does not. In this case, the end that
has discarded the state can re-use the context tag value used for the
discarded state for a another context, creating a potential context
confusion situation. In order to illustrate the cases where problems
would arise consider the following scenario:
o Hosts A and B establish context 1 using CTA and CTB as context
tags.
o Later on, A discards context 1 and the context tag value CTA
becomes available for reuse.
o However, B still keeps context 1.
This would become a context confusion situation in the following two
cases:
o A new context 2 is established between A and B with a different
ULID pair (or Forked Instance Identifier), and A uses CTA as
context tag, If the locator sets used for both contexts are not
disjoint, we are in a context confusion situation.
o A new context is established between A and C and A uses CTA as
context tag value for this new context. Later on, B sends Payload
extension header and/or control messages containing CTA, which
could be interpreted by A as belonging to context 2 (if no proper
care is taken). Again we are in a context confusion situation.
One could think that using a coordinated approach would eliminate
these context confusion situations, making the protocol much simpler.
However, this is not the case, because even in the case of a
coordinated approach using a CLOSE/CLOSE ACK exchange, there is still
the possibility of a host rebooting without having the time to
perform the CLOSE exchange. So, it is true that the coordinated
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approach eliminates the possibility of a context confusion situation
because premature garbage collection, but it does not prevents the
same situations due to a crash and reboot of one of the involved
hosts. The result is that even if we went for a coordinated
approach, we would still need to deal with context confusion and
provide the means to detect and recover from this situations.
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19. References
19.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[3] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[4] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[5] Conta, A. and S. Deering, "Internet Control Message Protocol
(ICMPv6) for the Internet Protocol Version 6 (IPv6)
Specification", RFC 2463, December 1998.
[6] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.
[7] Bagnulo, M., "Hash Based Addresses (HBA)",
draft-ietf-shim6-hba-01 (work in progress), October 2005.
[8] Arkko, J. and I. Beijnum, "Failure Detection and Locator Pair
Exploration Protocol for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-03 (work in progress),
December 2005.
19.2 Informative References
[9] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[10] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[11] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.
[12] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.
[13] Bagnulo, M., "Updating RFC 3484 for multihoming support",
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draft-bagnulo-ipv6-rfc3484-update-00 (work in progress),
December 2005.
[14] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[15] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582, August 2003.
[16] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6
Flow Label Specification", RFC 3697, March 2004.
[17] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[18] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.
[19] Nordmark, E. and T. Li, "Threats Relating to IPv6 Multihoming
Solutions", RFC 4218, October 2005.
[20] Huitema, C., "Ingress filtering compatibility for IPv6
multihomed sites", draft-huitema-shim6-ingress-filtering-00
(work in progress), September 2005.
[21] Bagnulo, M. and E. Nordmark, "SHIM - MIPv6 Interaction",
draft-bagnulo-shim6-mip-00 (work in progress), July 2005.
[22] Nordmark, E., "Shim6 Application Referral Issues",
draft-ietf-shim6-app-refer-00 (work in progress), July 2005.
[23] Abley, J., "Shim6 Applicability Statement",
draft-ietf-shim6-applicability-00 (work in progress),
July 2005.
[24] Huston, G., "Architectural Commentary on Site Multi-homing
using a Level 3 Shim", draft-ietf-shim6-arch-00 (work in
progress), July 2005.
[25] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-05
(work in progress), March 2006.
[26] Eronen, P., "IKEv2 Mobility and Multihoming Protocol (MOBIKE)",
draft-ietf-mobike-protocol-08 (work in progress),
February 2006.
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Authors' Addresses
Erik Nordmark
Sun Microsystems
17 Network Circle
Menlo Park, CA 94025
USA
Phone: +1 650 786 2921
Email: erik.nordmark@sun.com
Marcelo Bagnulo
Universidad Carlos III de Madrid
Av. Universidad 30
Leganes, Madrid 28911
SPAIN
Phone: +34 91 6248814
Email: marcelo@it.uc3m.es
URI: http://www.it.uc3m.es
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