wdiff draft-ietf-shim6-proto-01.txt draft-ietf-shim6-proto-02.txt

SHIM6 WG E. Nordmark
Internet-Draft Sun Microsystems
Expires: March 5, 2006 September 2005

Level 3 multihoming shim protocol
draft-ietf-shim6-proto-01.txt
draft-ietf-shim6-proto-02.txt

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Abstract

The SHIM6 working group is exploring a layer 3 shim approach for
providing locator agility below the transport protocols, so that
multihoming can be provided for IPv6 with failover and load spreading
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 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.

This document picks a particular approach to such a protocol and
tries to flush out a bunch of details, with the hope that the WG can
better understand the details in this proposal as well as discovering
and understanding alternative designs that might be better. Thus
this proposal is my no means cast in stone as the direction; quite to
the contrary it is a depth first exploration of the design space.

Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Non-Goals . . . . . . . . . . . . . . . . . . . . . . . . 5
1.3 Locators as Upper-layer Identifiers . . . . . . . . . . . 5
1.4 IP Multicast . . . . . . . . . . . . . . . . . . . . . . . 6
1.5 Renumbering Implications . . . . . . . . . . . . . . . . . 6
1.6 Placement of the shim . . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Notational Conventions . . . . . . . . . . . . . . . . . . 10 11
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . 11
4. Protocol Overview . . . . . . . . . . . . . . . . . . . . . 11 12
4.1 Context Tags . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 Securing shim6 . . . . . . . . . . . . . . . . . . . . . . 14
4.3 Overview of Shim Control Messages . . . . . . . . . . . . 14
4.4 Locator Validation . . . . . . . . . . . . . . . . . . . . 16
5. Message Formats . . . . . . . . . . . . . . . . . . . . . . 15 16
5.1 Common shim6 Message Format . . . . . . . . . . . . . . . 16
5.2 Payload Message Format . . . . . . . . . . . . . . . . . . 15
5.2 17
5.3 Common Shim6 Control header . . . . . . . . . . . . . . . 16
5.3 17
5.4 I1 Message Format . . . . . . . . . . . . . . . . . . . . 17
5.4 19
5.5 R1 Message Format . . . . . . . . . . . . . . . . . . . . 18
5.5 20
5.6 I2 Message Format . . . . . . . . . . . . . . . . . . . . 19
5.6 21
5.7 R2 Message Format . . . . . . . . . . . . . . . . . . . . 21
5.7 22
5.8 No Context Error Message Format . . . . . . . . . . . . . 22
5.8 23
5.9 Update Request Message Format . . . . . . . . . . . . . . 23
5.9 24
5.10 Update Acknowledgement Message Format . . . . . . . . . . 24
5.10 25
5.11 Reachability Probe Message Format . . . . . . . . . . . 24
5.11 26
5.12 Reachability Reply Message Format . . . . . . . . . . . 25
5.12 27
5.13 Keepalive Message Format . . . . . . . . . . . . . . . . 26
5.13 Context Locator Pair Explore 28
5.14 SHIM6 Probe Message Format . . . . . . 27
5.14 . . . . . . . . . 29
5.15 Option Formats . . . . . . . . . . . . . . . . . . . . . 28
5.14.1 29
5.15.1 Validator Option Format . . . . . . . . . . . . . . 29
5.14.2 30
5.15.2 Locator List Option Format . . . . . . . . . . . . . 30
5.14.3 31
5.15.3 Locator Preferences Option Format . . . . . . . . . 31
5.14.4 32
5.15.4 CGA Parameter Data Structure Option Format . . . . . 32
5.14.5 33
5.15.5 CGA Signature Option Format . . . . . . . . . . . . 33
5.14.6 34
5.15.6 ULID Pair Option Format . . . . . . . . . . . . . . 33
5.14.7 34
5.15.7 Packet In Error Option Format . . . . . . . . . . . 34
5.14.8 Explorer Results 35
5.15.8 SHIM6 Event Option Format . . . . . . . . . . . 34 . . 35
6. Conceptual Model of a Host . . . . . . . . . . . . . . . . . 35
6.1 Conceptual Data Structures . . . . . . . . . . . . . . . . 36
7. Establishing Host Pair Contexts . . . . . . . . . . . . . . 36
7.1 Normal context establishment . . . . . . . . . . . . . . . 37 36
7.2 Concurrent context establishment . . . . . . . . . . . . . 37
7.3 Context recovery . . . . . . . . . . . . . . . . . . . . . 38
7.4 Context confusion . . . . . . . . . . . . . . . . . . . . 39
7.5 Sending I1 messages . . . . . . . . . . . . . . . . . . . 40
7.6 Receiving I1 messages . . . . . . . . . . . . . . . . . . 40
7.7 Receiving R1 messages . . . . . . . . . . . . . . . . . . 41
7.8 Retransmitting I2 messages . . . . . . . . . . . . . . . . 41
7.9 Receiving I2 messages . . . . . . . . . . . . . . . . . . 41
7.10 Receiving R2 messages . . . . . . . . . . . . . . . . . 42
8. No Such Content Errors . . . . . . . . . . . . . . . . . . . 42
9. Handling ICMP Error Messages . . . . . . . . . . . . . . . . 42
10. Teardown of the Host Pair Context . . . . . . . . . . . . . 43
11. Updating the Locator Pairs . . . . . . . . . . . . . . . . . 43 44
12. Various Probe Mechanisms . . . . . . . . . . . . . . . . . . 43 44
13. Rehoming to a Different Locator Pair . . . . . . . . . . . . 43 44
14. Payload Packets before a Switch . . . . . . . . . Sending ULP Payloads . . . . . 43
15. Payload Packets after a Switch . . . . . . . . . . . . . . . 44
16. Open Issues . . . .
14.1 Sending ULP Payload after a Switch . . . . . . . . . . . 45
15. Receiving Packets . . . . . . . . . 45
17. Design Alternatives . . . . . . . . . . . . 45
16. Initial Contact . . . . . . . . 46
17.1 State Cleanup . . . . . . . . . . . . . . 46
17. Open Issues . . . . . . . 46
17.2 Detecting Context Loss . . . . . . . . . . . . . . . . . 46
18. Implications Elsewhere . . . . . . . . . . . . . . . . . . . 47
19. Security Considerations . . . . . . . . . . . . . . . . . . 48
20. IANA Considerations . . . . . . . . . . . . . . . . . . . . 48 49
21. Change Log Possible Protocol Extensions . . . . . . . . . . . . . . . . 49
22. Change Log . . . . . . . . . 49
22. Acknowledgements . . . . . . . . . . . . . . . . 50
23. Acknowledgements . . . . . . 49
23. References . . . . . . . . . . . . . . . . 50
A. Design Alternatives . . . . . . . . . 49
23.1 Normative References . . . . . . . . . . . 51
A.1 Context granularity . . . . . . . 49
23.2 Informative References . . . . . . . . . . . . 51
A.2 Demultiplexing of data packets in shim6 communications . . 51
A.2.1 Flow-label . . . 50
Author's Address . . . . . . . . . . . . . . . . . . . 52
A.2.2 Extension Header . . . 51
Intellectual Property and Copyright Statements . . . . . . . 52

1. Introduction

The SHIM6 working group, and the . . . . . . . . . 54
A.3 Context Loss Detection . . . . . . . . . . . . . . . . . . 54
A.4 Securing locator sets . . . . . . . . . . . . . . . . . . 57
A.5 Host-pair context establishment exchange . . . . . . . . . 59
A.6 Updating locator sets . . . . . . . . . . . . . . . . . . 60
A.7 State Cleanup . . . . . . . . . . . . . . . . . . . . . . 61
24. References . . . . . . . . . . . . . . . . . . . . . . . . . 61
24.1 Normative References . . . . . . . . . . . . . . . . . . 61
24.2 Informative References . . . . . . . . . . . . . . . . . 62
Author's Address . . . . . . . . . . . . . . . . . . . . . . 63
Intellectual Property and Copyright Statements . . . . . . . 64

1. Introduction

The SHIM6 working group, and the MULTI6 WG that preceded it, is
exploring a layer 3 shim approach for providing locator agility below
the transport protocols, so that multihoming can be provided for IPv6
with failover and load spreading properties [13], [14], 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.

This document takes the outlines contained in [21] [22] and [20] [21] and
expands to an actual proposed protocol.

We assume that redirection attacks are prevented using the mechanism
specified in HBA [5]. [6].

The WG mailing list is discussing the scheme used for reachability
detection [6]. [7]. The schemes that are being discussed are Context
Unreachability Detection (CUD) or Force Bidirectional communication
Detection (FBD). This document doesn't discuss the tradeoffs between
the two, but it does suggest a set of keepalive and probe messages
that are sufficient to handle both. Once the WG has decided which
approach to take, we can remove the unneeded messages.

There is a related but slightly separate issue of how the hosts can
find which of the locator pairs is working after a failure. This is
discussed in [7]. We don't yet know how these details will be done,
but this draft specifies a separate "Explore" message as a
placeholder for such a protocol mechanism.

1.1 Goals

The goals [8].

NOTE that the direction taken in the latest version of [8] is to use
FBD and some new SHIM6 message types. Some of that work has been
reflected in this document, but there are other edits that remain.

1.1 Goals

The goals for this approach is to:
o Preserve established communications through failures, for example,
TCP connections and application communications using UDP.
o Have no impact on upper layer protocols in general and on
transport protocols in particular.
o Address the security threats in [16] [17] through a separate document
[5],
[6], and techniques described in this document.
o No extra roundtrip for setup; deferred setup.
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.

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, perhaps related, problems such as host multihoming or host
mobility.

This proposal also does not try to provide an IP identifier. Even
though such a concept would be useful to ULPs and applications,
especially if the management burden for such a name space was zero
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

Central to this approach is to not introduce a new identifier name
space but instead use one of the locators as the upper-layer ID,
while allowing the locators used in the address fields to change over
time in response to failures of using the original locator.

This implies that the ULID selection is performed as today's default
address selection as specified in [11]. [12]. 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
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 [17] [18] for further discussion of the application
implications.

There has been some discussion of using non-routable locators, such
as unique-local addresses [15], [16], 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 not
reachable, 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 [9] [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 [12], [13],
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.

1.5 Renumbering Implications

As stated above, this approach does not try to make communication
survive renumbering. However, 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).

However, this might be an overkill. Even when an IPv6 prefix is
retired and reassigned to some other site, there is still 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 [10]). [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 host with which the initial user of the IPv6 address was
communicating.

1.6 Placement of the shim

-----------------------
| Transport Protocols |
-----------------------

---------------------
| 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 ordered
immediately placed after any hop-by-hop options
routing-related headers in the packet. 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. The MOBIKE WG
is looking at ways to have IPsec security associations survive even
though the IP addresses changes, which is a different approach.

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 on a host selects which default router next hop and interface to use
etc.
for sending out packets.

Applications and upper layer protocols use ULIDs which the shim6
layer will map to/from different locators. The shim6 layer maintains
state, called host-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, but with a header
compression mechanism applied that removes the need for the ULIDs
once the state has been established. In order for the receiver to
recreate a packet with the correct ULIDs there might be a need to
include some "compression tag" in the data packets. This would serve
to indicate the correct context to use for decompression when the
locator pair in the packet is insufficient to uniquely identify the
context.

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 (taken from [21]): [22]):
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 a 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 (stack
name in [23]). [24]). 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.

upper-layer identifier (ULID)
An IP locator 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

Host-pair context The state that the multihoming shim maintains for
a particular peer. maintains.
The context is for a ULID pair, as and is identified
by a context tag for each
direction. direction of the
communication.

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 packets with the shim6 Payload extension
header 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 be peer.
The two ends might use different current locator
pairs though.

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.

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.

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.

3. Assumptions

The general approach of a level3 shim as well as this specific
proposal makes the following assumptions:
o When there is ingress filtering in the ISPs, that the use of all
<source, destination> locator pairs will cause the packets to exit
using different ISPs so that all exit ISPs can be tried. Since
there might be only one destination locator, when the peer
supports shim6 but is not multihomed, this implies that the
selection of the exit ISP should be related to the source address
in the packets.
o Even without ingress filtering, there is the assumption that if
the host tries all <source, destination> locator pairs, that it
has done a good enough job of trying to find a working path to the
peer. Since we want the protocol to provide benefits even if the
peer has a single locator, this seems to imply that the choice of
source locator needs to somehow affect the exit path from the
site.

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 [11] [12] 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 might make
sense to make this communication robust against locator failures.
For instance, this heuristic might be that more than 50 packets
have been sent or received. 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
revert to standard unicast behavior for the session.
o Communication continues without any change for the ULP packets.
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 advise 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 and explore the different alternate locator pairs until a
working pair is found, and rehome to using that pair.
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 message as an extension header, which contains the
receiver's context tag. The receiver will use the <Source
Locator, Destination Locator, 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 be 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 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 Working Group has discussed whether or not hosts can express
other forms of locator preferences. If this is the case, a change
in the preferences can be signaled to the peer, which might make
the peer choose to try a different locator pair. Thus, this can
also be treated similarly to a failure.
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 an error
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 ULP packets in shim6 are 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 "shim payload" extension header before the (extension)
headers that are processed by the IP endpoint sublayer and ULPs.

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, the
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 context tag> MUST uniquely identify one
context.

In addition, the non-shim6 messages, which we call payload packets,
will not contain the ULIDs after a failure. This introduces the
requirement that the <peer locator, local locator, local context tag>
MUST uniquely identify the context. Since the peer's set of locators
might be dynamic the simplest form of unique allocation of the local
context tag is to pick a number that is unique on the host. Hosts
which serve multiple ULIDs using disjoint sets of locators can
maintain the context tag allocation per such disjoint set.

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 Message extension header.

Even though we do not overload the flow label field to carry the
context tag, any protocol (such as RSVP or NSIS) which signals
information about flows from the host stack to devices in the path,
need to be made aware of the locator agility introduced by a layer 3
shim, so that the signaling can be performed for the locator pairs
that are currently being used.

TBD: add forking - multiple contexts between ULID pairs, default
context, etc etc. Need to explain that context forking assumes an API
from the ULP.

TBD: add that shim can be disabled for some ULP traffic if we define
an API for this purpose.

4.2 Securing shim6

The mechanisms are secured using a combination of techniques:
o The HBA technique [5] [6] for validating 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.

4.3 Overview of Shim Control Messages

The shim 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 [22].] [23].]

There is a No Context error message defined, when a control or
payload packet arrives and there is no matching context state at the
receiver. When such a message is received, it will result in the
destruction of the shim context and a re-establishment.

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 Locator List Update message and
acknowledgement.

Even though 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. Currently this mechanism has a
separate message pair (Rehome Request and acknowledgement), but
perhaps this can be encoded using the Locator List Update message
pair with a preference option and no change to the list of locators.

At least two approaches (CUD and FBD) have been discussed for the
shim (un)reachability detection [6]. [7]. This document attempt to define
messages for both cases; once the WG has picked an approach we can
delete any unneeded messages.

The CUD approach uses a probe message and acknowledgement, which can
be suppressed e.g. using positive advise from the ULP. This message
pair also seems needed to verify that the host is indeed present at a
new locator before the data stream is redirected to that locator, in
order to prevent 3rd party DoS attacks.

The FBD approach uses a keepalive message, which is sent when a host
has received packets from the peer, but the ULP has not given the
host an opportunity to send any payload packet to the peer.

The above probe and keepalive messages assume we have an established
host-pair context. However, communication might fail during the
initial context (that is, when the application or transport protocol
is trying to setup some communication). If we want the shim to be
able to optimize discovering a working locator pair in that case, we
need a mechanism to test the reachability of locators independent of
some context. We define a locator pair test message and
acknowledgement for this purpose, even though it isn't yet clear
whether we need such a thing.

Finally, when the context is established and there is a failure there
needs to be a way to probe and explore the set of locator pairs to
efficiently find a working pair. We define an explore message as a
place holder for some mechanism in this space [7].

5. Message Formats

The shim6 messages are all carried using [8].

4.4 Locator Validation

Before a new IP protocol number TBD
[to be assigned by IANA]. The shim6 messages have host can use a common header,
defined below, with some fixed fields, followed by type specific
fields.

5.1 Payload Message Format

The payload message is used to carry ULP packets where the receiver
must replace locator (different than the content of ULID) as the
source and or destination fields in
the IPv6 header before passing the packet to the ULP. Thus this
extension header is included when the locators pair locator, it must know that is used is
not the same peer will accept packets with
that source locator as being part of this context. The peer might
wish to do some verification of the ULID pair.

Since the shim is placed between the IP endpoint sub-layer and the IP
routing sub-layer in the locator before accepting it as a
source address. This document does not require any such
verification. But if it is done by a host, the shim header will in all cases such
verification need to be placed finished 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, the host acknowledges the new
locator, by sending an Update Acknowledgement message, R2 an message.

Before a destinations options
header which precedes host can use a routing header). When tunneling is used,
whether IP-in-IP tunneling or locator (different than the special form ULID) as the
destination locator it must perform the full verification of tunneling that
Mobile IPv6 uses (with Home Address Options and Routing header type
2), there is a choice whether the shim applies inside
locator. This includes both verifying it using HBA/CGA, and
verifying that the tunnel or
outside ULID is indeed reachable at the tunnel, which effects locator. The
latter in order to prevent 3rd party flooding attacks.

5. Message Formats

The shim6 messages are all carried using a new IP protocol number TBD
[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 location of Payload Message is used to carry the ULP packets after a locator
switch. The shim6 header.

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| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: The payload which follows this header.
Hdr Ext Len: 0 (since control messages use the same extension header is 8 octets).
Reserved: Reserved for future use. Zero on transmit. MUST
formats so that a single "protocol number" needs to be
ignored on receipt.
Receiver Context Tag: 32-bit unsigned integer. Allocated by the
receiver allowed
through firewalls in order for use shim6 to identify the context (together
with function across the source and destination locators).

5.2 firewall.

5.1 Common Shim6 Control header shim6 Message Format

The common part first 17 bits of the shim6 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" common for the control messages; only Payload
Message and the payload control messages use the Next Header
field.

The shim6 headers must be a multiple of 8 octets, hence the minimum
size is 8 octets.

The common message header is and looks 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |P|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: 8-bit selector. Normally set to NO_NXT_HDR (59).
Indicates the next header value for the shim6 The payload
messages. 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.
Type: 7-bit unsigned integer. Identifies the actual message
from the table below.
0:

P: A single bit (set to zero) which allows shim6 and HIP to have a common header format yet telling shim6 and
HIP distinguish Payload messages apart.
Checksum: 16-bit unsigned integer. from
control messages.

5.2 Payload Message Format

The checksum payload message is used to carry ULP packets where the 16-bit
one's complement of receiver
must replace the one's complement sum content of the
entire shim6 header message starting with source and or destination fields in
the shim6
next IPv6 header field, and ending as indicated by before passing the Hdr
Ext Len. packet to the ULP. Thus this
extension header is included when there the locators pair that is a payload following used is
not the
shim6 header, same as the payload ULID pair.

Since the shim is NOT included placed between the IP endpoint sub-layer and the IP
routing sub-layer in the shim6
checksum.

+------------+-----------------------------------------------------+
| Type Value | Message |
+------------+-----------------------------------------------------+
| 1 | I1 (first establishment message from host, the initiator) |
| 2 | R1 (first establishment message from 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 responder) |
| 3 | I2 (2nd establishment message from special form of tunneling that
Mobile IPv6 uses (with Home Address Options and Routing header type
2), there is a choice whether the initiator) |
| 4 | R2 (2nd establishment message from shim applies inside the responder) |
| 5 | No Context Error |
| 6 | Update Request |
| 7 | Update Acknowledgement |
| 8 | Reachability Probe |
| 9 | Reachability Reply |
| 10 | Keepalive |
| 11 | Context Locator Pair Explore |
+------------+-----------------------------------------------------+

Table 1

5.3 I1 Message Format

The I1 message is tunnel or
outside the first message in tunnel, which effects the context establishment
exchange. location of the shim6 header.

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 Next Header | Reserved2 0 |1| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 1
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Reserved2: 16-bit field. 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.
Reserved: Reserved for future use. Zero on transmit. MUST be
ignored on receipt.
Initiator
Receiver Context Tag: 32-bit field. The Context Tag the initiator
has allocated for the context.
Initiator Nonce: 32-bit unsigned integer. A random number picked Allocated by the initiator which
receiver for use to identify the responder will return in context (together
with the
R1 message. source and destination locators).

5.3 Common Shim6 Control header

The following options are allowed in common part of the message:
ULID pair: TBD Do we need to carry header has a next header and header extension
length field which is consistent with the ULIDs, or assume they are other IPv6 extension
headers, even if the same as next header value is always "NO NEXT HEADER" for
the address fields in control messages; only the IPv6 header?
Depends on how we handle failures during initial
contact.

5.4 R1 Message Format

The R1 message is the second message in payload messages use the context establishment
exchange. Next Header
field.

The responder sends this in response to an I1 message,
without creating any state specific to shim6 headers must be a multiple of 8 octets, hence the initiator. minimum
size is 8 octets.

The common 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 59 Next Header | Hdr Ext Len |0| Type = 2 | Reserved1 |0| |Type specific|0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
+ Options +
| Type specific format |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: 8-bit selector. Normally set to NO_NXT_HDR (59).
Type: 2
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Reserved2: 16-bit field. Reserved
Indicates the next header value 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 shim6 payload
messages.
Hdr Ext Len: 8-bit unsigned integer. A number picked by the
responder which the initiator will return in Length of the I2
message.

The following options are allowed shim6 header in
8-octet units, not including the message:
Responder Validator: Variable length option. Typically a hash
generated by the responder, which the responder uses
together with the Responder Nonce value first 8 octets.
P: Set to verify that
an I2 message is indeed sent in response zero. A single bit to a R1
message, and that distinguish this from
the parameters in shim6 payload messages.
Type: 7-bit unsigned integer. Identifies the I2 actual message are
the same as those in
from the I1 message.

5.5 I2 Message Format table below.
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.

Table 1

5.4 I1 Message Format

The I1 message is the third first 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 1 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 3 1
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 Context Tag: 32-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.
Responder Nonce: 32-bit unsigned integer. Copied from the
R1 message.

The following options are allowed in the message:
Responder Validator: Variable length option. Just a copy of the
Validator option in the R1 message.
ULID pair: TBD Do we need to carry the ULIDs, or assume they are
the same as the address fields in the IPv6 header?
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
validating the locator list MUST
Depends on how we handle failures during initial
contact. We also need it to 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 able to reestablish
the locator list.
CGA Signature: Included host-pair context after a failure when one end has
lost the some of the locators in the list use
CGA (and not HBA) for validation.

5.6 R2 context state.

5.5 R1 Message Format

The R2 R1 message is the fourth second 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 message,
without creating any state specific to the I1 messages cross in flight. initiator.

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 2 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Context Tag Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 4 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.
Responder Context Tag: 32-bit field. The Context Tag the responder
has allocated for the context.

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 allowed in the message:
Locator List: Optionally sent when
Responder Validator: Variable length option. Typically a hash
generated by the responder, which the responder immediately wants
to tell uses
together with the initiator its list of locators. When it Responder Nonce value to verify that
an I2 message is sent, the necessary HBA/CGA information for
validating the locator list MUST also be included.

Locator Preferences: Optionally indeed sent when in response to a R1
message, and that the locators don't all have
equal preference.
CGA Parameter Data Structure: Included when parameters in the locator list is
included and the PDS was not included in the context
establishment messages, so the receiver can verify the
locator list.
CGA Signature: Included when the some of I2 message are
the locators same as those in the list use
CGA (and not HBA) for validation.

5.7 No Context Error I1 message.

5.6 I2 Message Format

Should a host receive a packet with a shim Payload

The I2 message or shim6
control message, such a a locator update, and the host does not have
any context state for the locators (in is the IPv6 source and
destination fields) and third message in the context tag, then it will generate a No
Context Error. establishment
exchange. The error includes the packet that was received,
subject initiator sends this in response to a R1 message,
after checking the packet not exceeding 1280 octets. 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 = 5 3 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Responder Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 5 3
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 Context Tag: 32-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.
Responder Nonce: 32-bit unsigned integer. Copied from the R1
message.

The following options are allowed in the message:
Packet in Error:
Responder Validator: Variable length option containing option. Just a copy of the IPv6 packet
that was
Validator option in error, starting with the IPv6 header, and
normally containing R1 message.
ULID pair: TBD Do we need to carry the full packet. If ULIDs, or assume they are
the resulting
No Context Error message would exceed 1280 octets, same as the
Packet In Error option will not include the full
packet in error address fields in order to limit the error IPv6 header? We
also need it to 1280
octets.

5.8 Update Request Message Format

The Update Request Message is used be able to update either reestablish the host-pair
context after a failure when one end has lost the
context state.
Locator list: Optionally sent when the initiator immediately wants
to tell the responder its list or
locators, of locators. When it
is sent, the necessary HBA/CGA information for
validating the locator preferences, and both. When 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 validation.

5.7 R2 Message Format

The R2 message is updated, the fourth message also contains in the option(s)
necessary for HBA/CGA context establishment
exchange. The responder sends this in response to secure this. an I2 message.
The basic sanity check that
prevents off-path attackers from generating bogus updates R2 message is also used when both hosts send I1 messages at the
context tag in
same time and the message. I1 messages cross in flight.

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 4 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Responder Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Initiator Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 6 4
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.
Receiver
Responder Context Tag: 32-bit field. The Context Tag the receiver responder
has allocated for the context.
Request
Initiator Nonce: 32-bit unsigned integer. A random number picked by
the initiator which the peer will return in Copied from the
acknowledgement I2
message.

The following options are allowed in the message:
Locator List: The list of the senders (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. responder immediately wants
to tell the initiator its list of locators. When it
is sent, the necessary HBA/CGA information for
validating 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 validation.

5.9 Update Acknowledgement

5.8 No Context Error Message Format

This

Should a host receive a packet with a shim Payload message is sent in response to or shim6
control message, such a Update Request message. It
implies that the Update Request has been received, a locator update, and that any new
locators in the Update Request can now be used as the source locators
of packets. But it host does not imply that have
any context state for the (new) locators have been
verified (in the IPv6 source and
destination fields) and the context tag, then it will be used as generate a destination, since No
Context Error. The error includes the host might
defer packet that was received,
subject to the verification of a locator until it is used as a
destination. packet not exceeding 1280 octets.

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 = 7 5 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

Next Header: NO_NXT_HDR (59).
Type: 7 5
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.
Receiver Context Tag: 32-bit field.

The Context Tag following options are allowed in the receiver has
allocated for message:
Packet in Error: Variable length option containing the context.
Request Nonce: 32-bit unsigned integer. Copied from IPv6 packet
that was in error, starting with the Update
Request message. IPv6 header, and
normally containing the full packet. If the resulting
No options are currently defined for this message.

5.10 Reachability Probe Context Error message would exceed 1280 octets, the
Packet In Error option will not include the full
packet in error in order to limit the error to 1280
octets.

5.9 Update Request Message Format

The Reachability Probe message Update Request Message is used to prevent 3rd party DoS
attacks, and can also be used to verify whether a context is
reachable at a given locator should that be needed for update either the general
reachability detection mechanism (e.g., if we pick list or
locators, the CUD mechanism
where one end sends probes and expects a reply).

Before a host uses a locator for preferences, and both. When the peer that list of
locators is different than updated, the
ULID, it needs message also contains the option(s)
necessary for HBA/CGA to verify secure this. The basic sanity check that the peer
prevents off-path attackers from generating bogus updates is indeed present at that
locator by sending a Context Verify and receiving an acknowledgement.
This message includes the ULID pair as well as the
context tag, so
that tag in the peer can indeed verify that it has that ULID message.

The update message contains options (the Locator List and that the
context tag Locator
Preferences) that, when included, completely replace the previous
locator list and locator preferences, respectively. Thus there is correct. no
mechanisms 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 = 8 6 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

Next Header: NO_NXT_HDR (59).
Type: 8 6
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.
Receiver Context Tag: 32-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 responder peer will return in the
acknowledgement message.

The following options are allowed in the message:
ULID pair:
Locator List: The ULID pair that is being probed.

5.11 Reachability Reply Message Format

This is sent in response to a Reachability Probe message. Although,
if the receiver list of the Reachability Probe does not have a matching
context it will send senders (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 Signature: Included when the some of the locators in the list use
CGA (and not HBA) for validation.

5.10 Update Acknowledgement Message Format

This message is sent in response to a No Context Error 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 = 9 7 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

Next Header: NO_NXT_HDR (59).
Type: 9 7
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.
Receiver Context Tag: 32-bit field. The Context Tag the receiver has
allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from the request Update
Request message.

The following

No options are allowed in the message:
ULID pair: The ULID pair that is being probed. Copied from the
Probe currently defined for this message.

5.12 Keepalive

5.11 Reachability Probe Message Format

TBD: Given [8] we do not need this message any more.

The keepalive Reachability Probe message would is used to prevent 3rd party DoS
attacks, and can also be used to verify whether a context is
reachable at a given locator should that be needed for the general
reachability detection mechanism (e.g., if we decide to do pick the Force
Bidirectional communication as CUD mechanism
where one end sends probes and expects a way to get verification reply).

Before a host uses a locator for the peer that is different than the
locator pair continues
ULID, it needs to work. If we are not going to do FBD we
probably will not need this message. verify that the peer is indeed present at that
locator by sending a Context Verify and receiving an acknowledgement.
This message includes the ULID pair as well as the context tag, so
that the peer can indeed verify that it has that ULID and that the
context tag is correct.

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 = 10 8 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:

Next Header: NO_NXT_HDR (59).
Type: 10 8
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.
Receiver Context Tag: 32-bit field. The Context Tag the receiver has
allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from A random number picked by
the Reachability
Probe initiator which the responder will return in the
acknowledgement message.

The following options are currently defined for this message.

5.13 Context Locator Pair Explore Message Format

This is a placeholder for the protocol mechanism outlined allowed in [7]. the message:
ULID pair: The idea behind ULID pair 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 being probed.

5.12 Reachability Reply Message Format

TBD: Given [8] we do not need this message any more.

This is no locator pair which works sent in both
directions. The protocol mechanism is that as A is sending explore
messages response to B, B will observe which locator pairs it has received
from and report that back in explore messages a Reachability Probe message. Although,
if the receiver of the Reachability Probe does not have a matching
context it is sending to A. will send a No Context Error 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 = 11 9 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number | Checksum | Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 11 9
Reserved1: 7-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Sequence Number:

Reserved2: 16-bit unsigned integer. Used to determine which
messages have been received by the peer. field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Receiver Context Tag: 32-bit field. The Context Tag the receiver has
allocated for the context.
Request Nonce: 32-bit unsigned integer. Copied from the request
message.

The following options are allowed in the message:
Explorer Results: Indication of what Explorer messages the sender has
recently received
ULID pair: The ULID pair that is being probed. Copied from the peer.

5.14 Option Formats

All of the TLV parameters have a length (including Type and Length
fields) which is a multiple of 8 bytes. When needed, padding MUST
Probe message.

5.13 Keepalive Message Format

TBD: Given [8] we do not need this message any more.

The keepalive message would be
added used if we decide to do the end of the parameter so that the total length becomes Force
Bidirectional communication as 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 way to get verification that the Length field
according
locator pair continues to the following formula:

Total Length = 11 + Length - (Length + 3) % 8; work. If we are not going to do FBD we
probably will not need this 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 |C| Length = 10 | Reserved1 |0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum |
/ Contents /
/ +-+-+-+-+-+-+-+-+ Reserved2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Padding Receiver Context Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Type: 15-bit identifier of the type of option. The options
defined in this document
| Request Nonce |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Options +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Next Header: NO_NXT_HDR (59).
Type: 10
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.
Receiver Context Tag: 32-bit field. The Context Tag the receiver has
allocated for the context.

Request Nonce: 32-bit unsigned integer. Copied from the Reachability
Probe message.

No options are below.
C: Critical. One if currently defined for this parameter message.

5.14 SHIM6 Probe Message Format

This message and its semantics are defined in [8]. The idea behind
that mechanism is critical, 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.

5.15 Option Formats

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 recognized by
added to the recipient, zero otherwise. An
implementation might view end of the C bit as part 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.

+------------------------------+------+
| Option Name | Type |
+------------------------------+------+
| Validator | 1 |
| Locator List | 2 |
| Locator Preferences | 3 |
| CGA Parameter Data Structure | 4 |
| CGA Signature | 5 |
| ULID Pair | 6 |
| Packet In Error | 7 |
| SHIM6 Event Option | 8 |
+------------------------------+------+

Table 2

5.15.1 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 verify that the validator it receives back in the
I2 message is indeed one that 1) it computed, 2) it computed for the
particular context, and 3) that it isn't a replayed I2 message.

One way for the responder to do this is to maintain a single secret
(S) and a running counter for the Responder Nonce. For each I1
message, the responder can then increase the counter, use the counter
value as the responder nonce, and use the following information as
input to the one-way function:
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)
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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Validator: Variable length content whose interpretation is local
to the responder.

5.15.2 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.

TBD: We can get a simpler format if we split this into two options:
one with the locators and one with just the verification methods.

Fields:
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.
Locators: N 128-bit locators.

The defined verification methods are:

+-------+----------+
| Value | Method |
+-------+----------+
| 0 | Reserved |
| 1 | HBA |
| 2 | CGA |
| 3-255 | Reserved |
+-------+----------+

Table 3

5.15.3 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
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] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 draft 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.

When the Element length equals one, then the element consists of only
a flags field. The set of flags is TBD: Assume there will be two
initially: BROKEN and TEMPORARY. The intent of the latter 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, the 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, the 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.

5.15.4 CGA Parameter Data Structure Option Format

This option contains the CGA parameter data structure (hereafter
called the PDS). When HBA is used to validate the locators, the PDS
contains the HBA multiprefix extension. When CGA is used to validate
the locators, in addition to the CGA PDS, the signature will need to
be included as 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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
CGA Parameter Data Structure: Variable length content. Content
defined in [5] and [6].

5.15.5 CGA Signature Option Format

When CGA is used for validation of one or more of the locators in the
PDS, 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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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 validation
method is set to CGA. The locators MUST be
included in the order they are listed in the
Locator List Option.

5.15.6 ULID Pair Option Format

It isn't clear whether we need this option. It depends whether we
want to be able to setup a context for a ULID pair when that ULID
pair can't be used to communicate. Thus the IPv6 addresses in the
context establishment would not be the ULIDs.

Fields:
Reserved: 48-bit field. Reserved for future use. Zero on
transmit. MUST be ignored on receipt.
Sender ULID: A 128-bit IPv6 address.
Receiver ULID: A 128-bit IPv6 address.

5.15.7 Packet In Error 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ IPv6 header, shim6/TCP/UDP header, etc ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Packet: A variable length field which contains the packet in
error starting with the IPv6 header.

5.15.8 SHIM6 Event Option Format

This option is defined in [8].

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 host
pair context. This is a data structure which contains the following
information:
o The peer ULID; ULID(peer)
o The local ULID; ULID(local)
o The list of peer locators, with their preferences; Ls(peer)
o For each peer locator, the validation method to use (from the
Locator List option).
o For each peer locator, a bit whether it has been validated 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 preferred local locator - used as source; Lp(local)
o The context tag used to transmit control messages and ULP packets
- allocated by the peer; CT(peer)
o The context to expect in received control messages and extension
headers - allocated by the local host; CT(local)
o Reachability state for the locator pairs.
o During pair exploration, information about the probe messages that
have been sent and received.

The receiver finds the context by looking it up using <Source
Locator, Destination Locator, CT(local)>, where the context tag is in
the shim header. The sender 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.

7. Establishing Host Pair Contexts

Host 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 Normal context establishment

The normal context establishment consists of a 4 message exchange in
the order of I1, R1, I2, R2.

Initiator Responder

------------- I1 -------------->

<------------ R1 ---------------

------------- I2 -------------->

<------------ R2 ---------------

Figure 24

7.2 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, or since the 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), with a R2.

Initiator Responder

-\
---\
---\ /---
--- I1 ---\ /---
---\
/--- I1 ---/ ---\
/--- -->
<---

-\
---\
---\ /---
--- R2 ---\ /---
---\
/--- R2 ---/ ---\
/--- -->
<---

Figure 25

If a host has received an I1 and sent an R1, then a ULP can trigger
it to send an I1 message itself, since it doesn't retain any state
when receiving the I1 message. Thus while one end is sending an I1
the other is sending an I2.

Initiator Responder

-\
---\
---\
--- I1 ---\
---\
---\
-->

/---
/---
---
/--- R1--/
/---
<---

-\
---\
---\ /---
--- I2---\ /---
---\
/--- I1 ---/ ---\
/--- -->
<---

-\
---\
---\ /---
--- R2 ---\ /---
---\
/--- R2 ---/ ---\
/--- -->
<---

Figure 26

7.3 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 two cases:

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 and explore 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 shim payload header.
In both cases the result is that the peer without state receives a
shim message for which it has to context for the <source locator,
destination locator, context tag>.

In both of those case we can recover the context by having the node
which doesn't have a context state, send back an R1bis [TBD] message,
and have this complete a recover with a I2 and R2 message.

If one end has garbage collected or lost the context state, it might
try to create the context state (for the same ULID pair), by sending
an I1 message. The peer can simply reply with an R2 message in this
case.

7.4 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 for ULID pair <A1, B1>, the other end
might end up allocating that context tag 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.
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 solution to this issue is TBD. The know possibilities are:
o Have A forcibly destroy the context for <A1, B1>, so that it can
accept the new context for <A3, B1>.

o Have A accept the context for <A3, B1>, forget about the old
context, but initiate a new (replacement) context for <A1, B1> by
sending an I1 message. That I1 through R2 exchange will make B
allocate a new context tag for <A1, B1>.
o Avoid the problem by changing the context tag allocation so that A
and B allocates half of the bits (16 each) of the context tags, so
that even if one end looses state, the peer can make sure that the
context tags for each context are unique.

7.5 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 its context tag to the context. Then it
can send an I1 message.

If the host does not receive an I2 or R2 message in response to the
I1 message, 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 this.

If, after several 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 host pair
context with that ULID. However, any such negative caching should
retained for a limit time; a few minutes would be appropriate, to
allow things to recover should the host not be reachable at all when
the shim tries to establish the context.

If the host receives an ICMP error with "payload type unknown" and
the included packet is the I1 packet it just sent, then this is a
more reliable indication that the peer ULID does not implement shim6.

7.6 Receiving I1 messages

If the host looks up a context for the ULID pair and the peer's (not
its) context tag. If it finds such a context, the it needs to verify
that the locators in the message are in fact part of the locator sets
that are recorded in the existing context state. If this is not the
case, then the I1 message MUST be silently ignored. (This can only
happen when there is an ULID pair option in the I1 message.) If the
locators are ok, then the host can respond with an R2 message as if
it had received an I2 message and not an I1 message.

If there is no existing context state, then the host forms a verifier
and sends this back to the peer in an I2 message. No state is
created on the host in this case.

7.7 Receiving R1 messages

When the host receives an R1 message, it verifies that the nonce
matches what it sent in the I1 message, and that it has context state
for the ULID pair. It then sends an I2 message, which includes the
verifier option that was in the R1 message. The I2 message also
includes A's locator list and the CGA parameter data structure. If
CGA (and not HBA) is used to verify the locator list, then A also
signs the key parts of the message and includes a CGA signature
option containing the signature.

The host may receive an R1[bis] TBD message that was not sent in
response to an I1 message but instead sent as a result of context
recovery. The difference between an R1bis and an R1 message is that
the former use the context tag of the responder. TBD how there are
handled and whether they are identical to an R1.

7.8 Retransmitting I2 messages

If the initiator does not receive an R2 message after sending an I2
message it MAY retransmit the I2 message. But since the verifier
option might have a limited lifetime, that is, the peer might reject
verifier options that are too old to avoid replay attacks, the
initiator SHOULD fall back to retransmitting the I1 message when
there is no response to one or a few I2 messages.

7.9 Receiving I2 messages

The responder checks that the nonce and the verifier option is
consistent with what it might have sent in a recent R1 message (by
verifying the hash it computed.) If this is ok, then the host checks
if it already has context state for the ULID pair and the CT(peer).
If it has such state, the I2 message was probably a retransmission.
In this case the host sends an R2 message.

If there is no context state, the responder allocates a context tag
(CT(local)) and creates the context state for the context. It
records the peer's locator set as well as its own locator set in the
context. It MAY verify the peers locator set at this point in time,
but the requirement is that a locator MUST be verified before the
host starts sending packets to that locator, thus the host MAY defer
the verification until later.

The host forms an R2 message with its locators and its context tag,
and includes the necessary options so that the peer can verify the
locators.

R2 messages are never retransmitted. If the R2 message is lost, then
the initiator will retransmit either the I2 or I1 message. Either
retransmission will cause the responder to find the context state and
respond with an R2 message.

7.10 Receiving R2 messages

The initiator can receive an R2 message in response to either an I1
or an I2 message, but the handling of the R2 is the same in both
cases. The host first verifies that the nonce is the same as the one
it sent (in the I1 or I2 message). If it doesn't match, the R2
message is silently dropped.

Then the host records the information from the R2 message in the
context state. It records the peer's locator set in the context. It
MAY verify the peers locator set at this point in time, but the
requirement is that a locator MUST be verified before the host starts
sending packets to that locator, thus the host MAY defer the
verification until later.

8. No Such Content Errors

TBD

The Interim Meeting discussed ways to recover the context state at
one end when the other end sees a failure (and starts sending Probe
messages). The discussed approach is to use a R1 (or R1bis) message
in response to a message with an unknown context, which would cause
the context to be recreated.

The idea is that on receipt of a SHIM6 payload packet where there is
no current SHIM6 context at the receiver, the receiver is to respond
with an R1bis packet in order to re-establish SHIM6 context. The
R1bis packet differs from the R1 packet in that an R1 packet echoes
the I1 fields, while this R1bis offers state back to the sender. One
key difference is that the I1 packet contains the initiator's context
tag, while the payload message header contains the receivers context
tag. Either way the next control packet is an I2 in response. The
senders previous context state is to be flushed in receipt of the
Type field, by multiplying R2
packet following the type values in R1bis, I2 exchange.

The details of this
specification by two.
Length: Length type of exchange needs to be worked out, but the Contents, in bytes.
Contents: Parameter specific, defined by Type.
Padding: Padding, 0-7 bytes, added if needed.

+------------------------------+------+
| Option Name | Type |
+------------------------------+------+
| Validator | 1 |
| Locator List | 2 |
| Locator Preferences | 3 |
| CGA Parameter Data Structure | 4 |
| CGA Signature | 5 |
| ULID Pair | 6 |
| Packet In
likely result is that we will not need a separate "No context" error
message.

9. Handling ICMP Error | 7 |
| Explorer Results | 8 |
+------------------------------+------+

Table 2

5.14.1 Validator Option Format Messages

The responder can choose exactly what input uses to compute routers in the
validator, and what one-way function (MD5, SHA1) it uses, path as long well as the responder can verify destination might generate
various ICMP error messages, such as host unreachable, packet too
big, and payload type unknown. It is critical that the validator these packets
make it receives back up to the ULPs so that they can take appropriate action.

When the ULP packets are sent unmodified, 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. But when the shim on the transmitting side
replaces the ULIDs in the
I2 message IP address fields with some other locators,
then an ICMP error coming back will have a "packet in error" which is indeed one
not a packet that 1) it computed, 2) it computed for the
particular context, and 3) that it isn't a replayed I2 message.

One way for ULP sent. Thus the responder implementation will have to do this is
apply the reverse mapping to maintain a single secret
(S) and a running counter for the Responder Nonce. For each I1
message, "packet in error" before passing the responder can then increase
ICMP error up to the counter, use ULP.

This mapping is different than when receiving ULP packets from the counter
value as
peer, because in that case the responder nonce, and use packets contain CT(local). But the following information as
input
ICMP errors have a "packet in error" with CT(peer) since they were
intended to be received by the one-way function:
o The the secret S
o That Responder Nonce
o The Initiator Context Tag from the I1 message
o The ULIDs from peer. In any case, since the I1 message
o The locators from <Source
Locator, Destination Locator, CT(peer)> has to be unique when
received by the I1 message (strictly only needed if they are
different from peer, the ULIDs)

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 ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Validator: Variable length content whose interpretation is local host should also only be able to find
one context that matches this tuple.

If the responder.

5.14.2 Locator List Option Format ULP packet had been encapsulated in a shim6 payload message,
then this extension header must be removed. The Locator List Option is used result needs to carry all the locators of the
sender. Note be
that the order of ULP receives an ICMP error where the locators is important, since contained "packet in
error" looks as if the
Locator Preferences and shim did not exist.

10. Teardown of the Explorer message refers Host Pair Context

Each host can unilaterally decide when to tear down a host-pair
context. It is RECOMMENDED that hosts not tear down the locators
by using the index in the list.

Note context when
they know that we carry all there is some upper layer protocol that might use the locators in
context. For example, an implementation might know this option even though some
of them can is there is
an open socket which is connected to the ULID(peer). However, there
might be created automatically from cases when 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 |
+-+-+-+-+-+-+-+-+ |
~ ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Locators 1 through N ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Fields:
Locator List Generation: 32-bit unsigned integer. Indicates a
generation number knowledge is not readily available to the
shim layer, for instance for UDP applications which not not connect
their sockets, or any application which retains some higher level
state across (TCP) connections and UDP packets.

Thus it is increased RECOMMENDED that implementations minimize premature
teardown by one for each
new locator list. This observing the amount of traffic that is used sent and received
using the context, and only after it appears quiescent, tear down the
state.

TBD: The Interim meeting discussed whether it was feasible to ensure relax
this so that one can end up with an asymmetric distribution of the
index in the Locator Preferences
context state and Explorer results
refer 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 this (in order to save memory)
but the right version of not-so-busy client would retain the locator list.
Num Locators: 8-bit unsigned integer. context state. The number of locators that
are included
context recover mechanism presented in Section 7.3 would then be
recreate the option. We call this number "N"
below.
Verification Method: N octets. The i'th octet specifies state should the
verification method 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.

11. Updating the i'th locator.
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 Pairs

TBD

The validation issues for the locators carried in the Locator Preferences option can have some flags Update
message are specified in Section 4.4.

12. Various Probe Mechanisms

TBD

13. Rehoming to indicate
whether or not a locator Different Locator Pair

TBD

14. Sending ULP Payloads

When there is known to work. In addition, no context state for the sender
can include 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 notion of preferences. It deferred context
establishment, then the host might make sense need to define
"preferences" as a combination do some accounting (count
the number of priority packets sent and weight the same way
that DNS SRV records has such information. The priority would
provide received) even before there is a way host-
pair context.

If there is a host-pair context for the ULID pair, then the sender
needs to rank verify whether context uses the ULIDs as locators, that is,
whether Lp(peer) == ULID(peer) and within a given priority, Lp(local) == ULID(local).

If this is the
weight would provide a way case, then packets will be sent unmodified by the
shim. If it is not the case, then the logic in Section 14.1 will
need to do be used.

There will also be some load sharing. See [8] for how
SRV defines the interaction of priority and weight. maintenance activity relating to
(un)reachability detection, whether packets are sent with the
original locators or not. The minimum notion details of preferences we need this is to out of scope for
this document and will be able covered is follow-ons to indicate
that [7].

14.1 Sending ULP Payload after a locator Switch

When sending packets, if there is "dead". We can handle this using a single octet
flag for each locator.

We can extend that by carrying a larger "element" host-pair context for each locator.
This document presently also defines 2-octet and 3-octet elements, the ULID
pair, and we can add more information by having even larger elements if
need be.

The locators are not included in the preference list. Instead, ULID pair is no longer used as the
first element refers to locator that was in pair, then
the first element in sender needs to transform the
Locator List option. The generation number carried in this option
and packet. Apart from replacing the Locator List option
IPv6 source and destination fields with a locator pair, an 8-octet
header is used to verify added so that they refer to the
same version of receiver can find 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] ... | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
~ ... ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Locator List Generation: 32-bit unsigned integer. Indicates a
generation number for context and inverse
the locator list to which transformation.

First, the
elements should apply.
Element Len: 8-bit unsigned integer. IP address fields are replaced. The length in octets 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 each
element. This draft defines the cases when ULP
checksums, since the length ULP checksums are carried end-to-end and the ULP
pseudo-header contains the ULIDs which are preserved end-to-end.

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 1, 2, 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 3.
Element[i]: A field with a number of octets defined by ULP
header.

The inserted shim6 Payload extension header includes the Element
Len field. Provides preferences for peer's
context tag.

15. Receiving Packets

As in normal IPv6 receive side packet processing the i'th locator receiver parses
the (extension) headers in order. Should it find a shim6 extension
header it will look at the Locator List option that is type field in use.

When that header. If the Element length equals one, type is
Payload message, then the element consists of only
a flags field. The set of flags is TBD: Assume there will packet must be two
initially: BROKEN and TEMPORARY. The intent passed to the shim6 payload
handling for rewriting. (Otherwise, the shim6 control messages are
handled as specified in other parts of this document.)

The receiver extracts the latter is to
allow context tag from the distinction between more stable addresses payload message
header, and less stable
addresses when shim6 is combined uses this together with IP mobility, when we might have
more stable home locators, the IPv6 source and less stable care-of-locators.

When destination
address fields to find a host-pair context. If no context is found,
the Element length equals two, receiver SHOULD generate a No Such Context error message (see
Section 8).

With the context in hand, the element consists of a 1
octet flags field followed by a 1 octet priority field. The priority
has receiver can now replace the same semantics as IP address
fields with the priority ULIDs kept in DNS SRV records.

When the Element length equals three, context. Finally, the Payload
extension header is removed from the packet (so that the element consists of a 1
octet flags field followed ULP doesn't
get confused by a 1 octet priority field, it), and a 1 octet
weight field. The weight has the same semantics as the weight next header value in DNS
SRV records.

5.14.4 CGA Parameter Data Structure Option Format

This option contains the CGA parameter data structure (hereafter
called the PDS). When HBA preceding
header is used set to validate be the locators, actual protocol number for the PDS
contains payload. Then
the HBA multiprefix extension. When CGA 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 used using some heuristic for determining when to validate perform a
deferred context establishment, then the locators, in addition host might need to do some
accounting (count the CGA PDS, number of packets sent and received) for
packets that does not have a shim6 extension header. But the signature will need to
be included as a CGA Signature option.

Fields:
CGA Parameter Data Structure: Variable length content. Content
defined in [5].

5.14.5 CGA Signature Option Format

When CGA is used
for validation this depends on what heuristics the implementation has chosen.

16. Initial Contact

TBD Describe what inital contact is (basically some non-shim
communication starts between two ULIDs), and what the implications
are of one or more failures. Basic option is to rely on the application retrying
and RFC 3484bis ordering of source and destination ULIDs.

17. Open Issues

The following open issues are known:
o Forking the locators in context state. On the
PDS, then mailing list we've discussed
the message in question will need to contain this option.

Fields:
CGA Signature: Variable length content. Content defined in [5].

5.14.6 ULID Pair Option Format

It isn't clear whether we need this option. It depends whether fork the context state, so that different ULP streams
can be sent using different locator pairs. No protocol extensions
are needed if any forking is done independently by each endpoint.
But if we want A to be able to setup a context for a ULID pair when tell B that ULID
pair can't certain traffic (a
5-tuple?) should be used forked, then we need a way to communicate. Thus the IPv6 addresses convey this in
the
context establishment shim6 protocol. The hard part would not be the ULIDs.

Fields:
Reserved: 48-bit field. Reserved for future use. Zero on
transmit. MUST defining what
selectors can be ignored on receipt.
Sender ULID: A 128-bit IPv6 address.
Receiver ULID: A 128-bit IPv6 address.

5.14.7 Packet In Error Option Format

Fields:
Packet: A variable length field which contains specified for the packet in
error starting with filter which determines which
traffic uses which of the IPv6 header.

5.14.8 Explorer Results Option Format

TBD: This needs forks. So the question is whether we
really need signaling for forking, or whether it is sufficient to indicate which explorer messages (sequence
numbers, source and destination locators?) that have been recently
received, in order
allow each endpoint to detect do its own selection of which locator pairs work when there pair
it is
no using for which traffic.
o If we allow forking, it seems like the mechanism for reachability
detection, whether it is CUD or FBD, must be applied separately
for each locator pair which works that is in both directions. use. Without forking a single
locator pair will be in use for each host-pair context, hence
things would be simpler.
o What happens when a host runs out of N bit context tags? When indicating
locators is
it makes sense safe for a host to use the offset in reuse a context tag? With the Locator List (that
was carries in unilateral
teardown one end might discard the Locator List option), since this takes less space
than including context state long before the locators themselves.

TBD: add that data and
other shim control messages are included in
the learned results.

p
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 = 8 |0| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Locator List Generation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Locator List Generation |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Explorer Results ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Sender Locator List Generation: The generation number for the
sender's locator list to which end.
o Should a host explicitly fail communication when a ULID becomes
invalid (based on RFC 2462 lifetimes or DHCPv6), or should we let
the indices below
refer.
Receiver Locator List Generation: The generation number for communication continue using the
receiver's locator list invalidated ULID (it can
certainly work since other locators will be used).
o Should we rename "host-pair context" to which the indices below
refer.
Explorer Results: This field contains a list of elements, where each
element indicates one locator be "ULID-pair context"?
If we've decided this is per ULID pair that might make sense.

o We need to pick some initial retransmit timers for which the
sender of I1 and I2. Is
4 seconds ok?
o Should we require that the option has recently received a message.
Each result occupies 32 bits. The list should R1 verifier be
ordered usable for some minimum
time so that the most recently heard locator pairs
are first. SHOULD NOT include locator pairs that were
last received more than some number of seconds ago.
p
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender Index | Receiver Index| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Fields:
Sender Index: 8-bit unsigned Integer. The Index is relative initiator knows for how long time it can safely
retransmit I2 before it needs to go back to sending I1 again?
o Should we expand the
sender's locator list.
Receiver Index: 8-bit unsigned Integer. The Index is relative context tag from 32 to 47 bits?
o Should we make the receiver not use the source locator list.
Sequence Number: 16-bit unsigned Integer. The Sequence number of to find the
explorer message in
context, but instead only use the context tag? (and optionally,
the destination locator). This would provide some flexibility for
the future. The potential downside, which we would need to
understand, is packet injection. *If* there is ingress filtering,
then we get some extra checking by including the source locator pair < sender
index, receiver index> was last heard. If this
locator pair was last heard in a message other than
the lookup. But an
Explore message, then this number on-path attacker can inject packets at will,
whether the source locator is zero.

6. Conceptual Model part of the lookup or not. An off-
path attacker would have a Host

This section describes hard time to guess a conceptual model of one possible data
structure organization that hosts will maintain for the purposes of
shim6. The described organization is provided 47-bit number.
o Include locator list in R1 message to facilitate the
explanation of how the shim6 protocol should behave. This document
does not mandate that implementations adhere deal with R2 being dropped?
o Should we allow a host to this model as long as
their external behavior is consistent intentionally discard the context state,
with the assumption that described in this
document.

6.1 Conceptual Data Structures

The key conceptual data structure for the shim6 protocol peer is the host
pair context. responsible to maintain it,
and detect failures? This is might be useful in asymetric case, e.g.
a data structures server which contains the following
information:
o The peer ULID; ULID(peer)
o The local ULID; ULID(local)
o The list serves lots of peer locators, with their preferences; Ls(peer)
o clients, but it can't recover from
all failures. For each peer locator, instance, if the client doesn't send anything
for a bit whether it has been validated using
HBA, while, and a bit whether when the server starts to send the locator has been probed pair
doesn't work any more. In this case the server can do nothing
since it doesn't have a context with alternate locators, and the
client can't possibly know that the server might be having
problems reaching it.
o When does a host need 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 list? Immediately
i.e. before accepting packets from those locators and the preferences; Ls(local)
o The preferred local locator - used as source; Lp(local)
o The context tag used to transmit control messages and ULP packets
- allocated by the peer; CT(peer)
o The context source
address? Or before sending packets to expect in received control those locators? There are
some issues if it isn't verified immediately since it allows an
on-path attacker to send bogus update messages and extension
headers - allocated by which can not be
verified; that would potentially make the local host; CT(local)
o Reachability state for host no longer accept
packets from the actual locator pairs.
o During pair exploration, information about the explore messages that have been sent and received.

The receiver finds the context by looking it up using <Source
Locator, Destination Locator, CT(local)>, where the context tag peer is in
the shim header. The sender needs to be able using, and when
it tries to find verify the context
state when a ULP packet locators it would find that they are "bad"
and has no alternate peer locator it can use. This is passed down from the ULP. In that case
even if the lookup key is peer has sent a locator list as long as the pair attacker
has sent a more recent one.

18. Implications Elsewhere

The general shim6 approach, as well as the specifics of ULIDs.

7. Establishing Host Pair Contexts

Host pair contexts are established this proposed
solution, has implications elsewhere. The key implications are:
o Applications that perform referrals, or callbacks using a 4-way exchange, which
allows IP
addresses as the responder 'identifiers' can still function in limited ways,
as described in [18]. But in order for such applications 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 be
able to take advantage of the multiple locators with for redundancy,
the peer.

In some cases applications need to be modified to either use fully qualified
domain names as the 4-way exchange is not necessary, for instance when
both ends try 'identifiers', or they need to setup pass all the context at
locators as the same time, or when
recovering 'identifiers' i.e., the 'identifier' from the
applications perspective becomes a context that has been garbage collected or lost at
one set of the hosts.

7.1 Normal context establishment

The normal context establishment consists IP addresses instead of
a 4 message exchange in single IP address.
o Firewalls that today pass limited traffic, e.g., outbound TCP
connections, would presumably block the order of I1, R1, I2, R2.

Initiator Responder

------------- I1 -------------->

<------------ R1 ---------------

------------- I2 -------------->

<------------ R2 ---------------

Figure 26

7.2 Concurrent context establishment

When both ends try to initiate a context for shim6 protocol. This
means that even when shim6 capable hosts are communicating, the same ULID pair, then
we might end up with crossing I1 messages, or since
messages would be dropped, hence the no state hosts would not discover that
their peer is
created when receiving shim6 capable. This is in fact a feature, since if
the hosts managed to establish a host-pair context, then the I1,
firewall would probably drop the "different" packets that are sent
after a host might send failure (those using the shim6 payload message with a I1 TCP
packet inside it). Thus stateful firewalls that are modified to
allow shim6 messages through should also be modified to allow the
payload messages through after having
sent a R1 message.

Since a host remembers failure. This presumably implies
that it has sent an I1, it can respond the firewall needs to an
I1 from track the peer (for set of locators in use by
looking at the same ULID), with a R2.

Initiator Responder

-\
---\
---\ /---
--- I1 ---\ /---
---\
/--- I1 ---/ ---\
/--- -->
<---

-\
---\
---\ /---
--- R2 ---\ /---
---\
/--- R2 ---/ ---\
/--- -->
<---
Figure 27

If a host has received an I1 and sent an R1, then a ULP can trigger
it shim6 exchanges. Such firewalls might even want to send an I1 message itself, since it doesn't retain any state
when receiving
verify the I1 message. Thus while one end is sending an I1 locators using the HBA/CGA verification themselves.
o Signaling protocols for QoS or other is sending an I2.

Initiator Responder

-\
---\
---\
--- I1 ---\
---\
---\
-->

/---
/---
---
/--- R1--/
/---
<---

-\
---\
---\ /---
--- I2---\ /---
---\
/--- I1 ---/ ---\
/--- -->
<---

-\
---\
---\ /---
--- R2 ---\ /---
---\
/--- R2 ---/ ---\
/--- -->
<---

Figure 28

7.3 Context recovery

Due to garbage collection, we can end up with one end things that involve having and
using
devices in the context state, network path look at IP addresses and the other end not having any state. We port numbers,
or IP addresses and Flow Labels, need to be able invoked on the hosts
when the locator pair changes due to recover this state at a failure. At that point in
time those protocols need to inform the end devices that has lost it,
before a new pair of
IP addresses will be used for the flow. Note that this is the
case even though we can use it.

This no longer overload the flow label as a context
tag; the in-path devices need can arise in two cases: to know about the use of the new
locators even though the flow label stays the same.
o MTU implications. The communication is working using path MTU mechanisms we use are robust
against different packets taking different paths through the ULID pair as
Internet, by computing a minimum over the recently observed path
MTUs. When shim6 fails over from using one locator pair to
another pair, but this means that packets might travel over a problem arises, and
different path through the end 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 has retained the
context state decides shim, at least for uncommon payload types, will
add an 8 octet extension header (the payload message) 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 explore alternate locator pairs.
The communication is working using a locator pair that the ULPs is not an implementation matter.

19. Security Considerations

This document satisfies the
ULID pair, hence concerns specified in [17] as follows:
o TBD: Using HBA or CGA for ...

Some of the ULP packets sent from a peer that has
retained residual threats in this proposal are:

o An attacker which arrives late on the path (after the context state has
been established) can use the shim payload header.
In both cases No Such Context error to cause one
peer to recreate the result is context, and at that point in time the peer without state receives a
shim message for which it has
attacker can observe all of the exchange. But this doesn't seem
to context open any new doors for the <source locator,
destination locator, context tag>.

In both of those case we attacker since such an attacker can recover the context by having
observe the node
which doesn't have a context state, send back an R1bis [TBD] message,
and have this complete a recover with a I2 Context tags that are being used, and R2 message.

If one end has garbage collected or lost the context state, once known it might
try
can use those to create the context state (for send bogus messages.
o An attacker which is present on the same ULID pair), by sending
an I1 message. The peer path so that it can simply reply with an R2 message in this
case.

7.4 Context confusion

Since each end might garbage collect find out
the context state we tags, can have
the case when one end generate a No Such Context error after it
has retained moved off the context state and tries path. For this packet to be effective it needs
to have a source locator which belongs to use
it, while the other end has lost context, thus there
can not be "too much" ingress filtering between the state. We discussed this in attackers new
location and the
previous section on recovery. communicating peers. But for this doesn't seem to be
that severe, because once the error causes the same reasons, when one
host retains context tag X for ULID to be torn
down and re-established, a new pair <A1, B1>, the other end
might end up allocating that of context tag for another ULID pair, e.g.,
<A3, B1> between tags will be used,
which will not be known to the same hosts. In attacker. If this case is still a
concern, we can not use could require a 2-way handshake "did you really loose
the
recovery mechanisms since there needs state?" in response to the error message.
o It might be separate possible for an attacker to try random 32-bit context
tags and see if they can cause disruption for
the communication
between two ULID pairs.

This type of "confusion" hosts. We can be observed in two cases (assuming it make this harder by using a larger
context tag; 47 bits is
A the largest that has retained fit in the state 8-octet
payload header. If this isn't sufficient, one could use an even
larger tag in the shim6 control messages, and B has dropped it):
B decides use the low-order 47
bits in the payload header.

20. IANA Considerations

IANA needs to create allocate a context for ULID pair <A3, B1&gt, and
allocates X as its context tag new IP Next Header value for this, and sends an I1 to A.
A decides this protocol.

IANA also needs to create record a context CGA message type for ULID pair <A3, B1&gt, and starts
the exchange by sending I1 to B. When B receives this protocol in the I2 message,
it allocates X as
[CGA] namespace, 0x4A30 5662 4858 574B 3655 416F 506A 6D48.

TBD: the context tag IANA rules for the shim6 message types and option types.

21. Possible Protocol Extensions

During the development of this context.
In both cases, A can detect that B has allocated X for ULID pair <A3,
B1&gt even though that A still X protocol, several issues have been
brought up as CT(peer) for ULID pair <A1,
B1&gt. Thus A can detect important one to address, but are ones that B must have lost the context for <A1,
B1&gt.

The solution do not need
to this issue is TBD. The know possibilities are:
Have A forcibly destroy be in the context for <A1, B1&gt, so that it base protocol itself but can
accept instead be done as
extensions to the new context protocol. The key ones are:
o Is there need for <A3, B1&gt.
Have A accept keeping the context for <A3, B1&gt, forget about list of locators private between the old
context,
two communicating endpoints? We can potentially accomplish that
when using CGA but initiate a new (replacement) context for <A1, B1&gt
by sending an I1 message. That I1 through R2 exchange will make B
allocate a new context tag for <A1, B1&gt.
Avoid the problem by changing not with HBA, but it comes at the context tag allocation so that A cost of doing
some public key encryption and B allocates half of the bits (16 each) decryption operations as part of
the context tags, so
that even if one end looses state, establishment. The suggestion is to leave this for a
future extension to the peer can make sure protocol.

o Defining some form of end-to-end "compression" mechanism that
removes the
context tags need for each context are unique.

7.5 Sending I1 messages

When including the Shim6 Payload extension header
when the locator pair is not the shim layer decides to setup a context for a ULID pair, it
starts by allocating pair.

22. Change Log

The following changes have been made since draft-ietf-shim6-proto-00:
o Removed the use of the flow label and initializing the context state for its end.
As part overloading of this it assigns its context tag to the context. Then it
can send an I1 message.

If IP
protocol numbers. Instead, when the host does locator pair is not receive the ULID
pair, the ULP payloads will be carried with an I2 or R2 message in response 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
I1 message, then it needs hosts, without having to retransmit overload the I1 message. The
retransmissions should use a retransmission timer flow
label or the IP protocol numbers.
o Grew the context tag from 20 bits to 32 bits, with binary
exponential backoff the possibility
to grow it to 47 bits. This implies changes to avoid creating congestion issues for the
network when lots of hosts perform this.

If, after several retransmissions, there message
formats.
o Almost by accident, the new shim6 message format is no response, then most
likely very close to
the peer does not implement HIP message format.
o Adopted the shim6 protocol, or there could
be a firewall that blocks HIP format for the protocol. In options, since this case it makes sense
for the host to remember it easier
to not try again describe variable length options. The original, ND-style,
option format requires internal padding in the options to establish a host pair
context with make
them 8 octet length in total, while the HIP format handles that ULID. However, any such negative caching should
retained for a limit time;
using the option length field.
o Removed some of the control messages, and renamed the other ones.
o Added a few minutes would be appropriate, to
allow things "generation" number to recover should the host not be reachable at all when Locator List option, so that
the shim tries peers can ensure that the preferences refer to establish the context.

If right
"version" of the host receives an ICMP error with "payload type unknown" Locator List.
o In order for FBD and exploration to work when there the included packet 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 I1 packet it just sent, then this forking
of the context is now causing a
more reliable indication that new context to be established for
the peer ULID does not implement shim6.

7.6 Receiving I1 messages

If same ULID; the new context having a new context tag. The
original context is referred to as the host looks up a "default" context for the
ULID pair pair.
o Added more background material and textual descriptions.

23. Acknowledgements

Over the peer's (not
its) context tag. If it finds such a context, the it needs to verify
that the locators years many people active in the message multi6 and shim6 WGs have
contributed ideas a suggestions that are reflected in fact part this draft.

Thanks to Marcelo Bagnulo for providing comments on earlier versions
of this draft.

Appendix A. Design Alternatives

This document has picked a certain set of design choices in order to
try to work out a bunch of the locator sets
that details, and stimulate discussion.
But as has been discussed on the mailing list, there are recorded other
choices that make sense. This appendix tries to enumerate some
alternatives.

Appendix A.1 Context granularity

TBD

Appendix A.2 Demultiplexing of data packets in the existing shim6 communications

Once a Host-pair context state. If this is not the
case, then established between two hosts, packets
may carry locators that differ from the I1 message MUST be silently ignored. (This can only
happen when there is an ULID pair option in ULIDs presented to the I1 message.) If ULPs
using the
locators are ok, then established context. One of main functions of the host can respond with an R2 message as if
it had received an I2 message and not an I1 message.

If there SHIM6
layer is no existing context state, then the host forms a verifier
and sends this back to perform the peer in an I2 message. No state is
created on the host in this case.

7.7 Receiving R1 messages

When the host receives an R1 message, it verifies that mapping between the nonce
matches what it sent in locators used to forward
packets through the I1 message, network and the ULIDs presented to the ULP. In
order to perform that it has context state translation for incoming packets, the ULID pair. It then sends an I2 message, SHIM6
layer needs to first identify which includes of the
verifier option that was in incoming packets need to
be translated and then perform the R1 message. The I2 message also
includes A's locator list mapping between locators and ULIDs
using the CGA parameter set. If CGA (and not
HBA) associated context. Such operation is called
demultiplexing. It should be noted that because any address can be
used to verify the both as a locator list, then A also signs things and
includes as a CGA signature option.

The host may receive an R1[bis] TBD message that was not sent ULID, additional information other
than the addresses carried in
response packets, need to an I1 message but instead sent as be taken into account
for this operation.

For example, if a result of context
recovery. The difference between an R1bis host has address A1 and an R1 message is that
the former use the context tag of the responder??? TBD how there are
handled A2 and whether they are identical to an R1.

7.8 Retransmitting I2 messages

If the initiator does not receive an R2 message after sending an I2
message it MAY retransmit the I2 message. But since the verifier
option might have starts communicating
with a limited lifetime, that is, the peer with addresses B1 and B2, then some communication
(connections) might reject
verifier options that use the pair <A1, B1> as ULID and others might
use e.g., <A2, B2>. Initially there are too old to avoid replay attacks, no failures so these address
pairs are used as locators i.e. in the
initiator SHOULD fall back to retransmitting IP address fields in the I1 message
packets on the wire. But when there is no response to one or a few I2 messages.

7.9 Receiving I2 messages

The responder checks that the nonce and failure the verifier option is
consistent with what it shim6 layer on
A might have sent in a recent R1 message (by
verifying decide to send packets that used <A1, B1> as ULIDs using <A2,
B2> as the hash it computed.) If locators. In this is ok, then case B needs to be able to rewrite the host checks
if it already has context state
IP address field for the ULID pair some packets and not others, but the CT(peer).
If it has such state, packets all
have the I2 message was probably a retransmission. same locator pair.

In this case the host sends an R2 message.

If there is no context state, order to accomplish the responder allocates demultiplexing operation successfully,
data packets carry a context tag
(CT(local)) and creates the context state for the context. It
records the peer's locator set as well as its own locator set in that allows the
context. It MAY verify receiver of the peers locator set at this point in time,
but
packet to determine the requirement is that a locator MUST shim context to be verified before the
host starts sending packets used to that locator, thus perform the host MAY defer
operation.

Two mechanisms for carrying the verification until later.

The host forms an R2 message with its locators and its context tag,
and includes tag information have been
considered in depth during the necessary options so that shim protocol design. Those carrying
the context tag in the peer can verify flow label field of the
locators.

R2 messages are never retransmitted. If IPv6 header and the R2 message is lost, then
usage of a new extension header to carry the initiator context tag. In this
appendix we will retransmit either describe the I2 or I1 message. Either
retransmission will cause pros and cons of each approach and
justify the responder selected option.

Appendix A.2.1 Flow-label

A possible approach is to find carry the context state and
respond with an R2 message.

7.10 Receiving R2 messages

The initiator can receive an R2 message tag in response to either an I1
or an I2 message, but the handling Flow Label
field of the R2 IPv6 header. This means that when a shim6 context is the same in both
cases.
established, a Flow Label value is associated with this context (and
perhaps a separate flow label for each direction).

The host first verifies simplest approach that the nonce does this is to have the same as the one
it sent (in triple <Flow
Label, Source Locator, Destination Locator> identify the I1 or I2 message). If it doesn't match, context at
the R2
message receiver.

The problem with this approach is silently dropped.

Then that because the host records locator sets are
dynamic, it is not possible at any given moment to be sure that two
contexts for which the information from same context tag is allocated will have
disjoint locator sets during the R2 message in lifetime of the
context state. It records contexts.

Suppose that Node A has addresses IPA1, IPA2, IPA3 and IPA4 and that
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 peer's locator set in the context. It
MAY verify 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 peers locator set at this point in time, but the
requirement associated to IPB2 is that a
just IPB2.

Because the locator MUST be verified before sets of the host starts
sending packets to Context #1 and Context # 2 are
disjoint, hosts could think that locator, thus the host MAY defer the
verification until later.

8. No Such Content Errors

TBD

The Interim Meeting discussed ways to recover the same context state at
one end 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 other end sees triple <Flow
Label, Source Locator, Destination Locator> would not identify a failure (and starts sending Explore
messages). The discussed
unique context anymore and correct demultiplexing is no longer
possible.

A possible approach to overcome this limitation is simply not to use a R1 (or R1bis) message
repeat the Flow Label values for any communication established in response to a message with an unknown context, which would cause
the context
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 recreated.

9. Handling ICMP Error Messages differentiated because it has a different Flow
Label value.

The routers in the path as well as the destination might generate
various ICMP error messages, problem with such as host unreachable, packet too
big, and payload type unknown. It approach is critical that these packets
make it back up 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 ULPs so that 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 take appropriate action.

When be lifted
by marking the ULP packets are sent unmodified, that is, while belong to shim sessions from those that
do not. These marking would require at least a bit in the initial
locators=ULIDs are working, this introduces no new concerns; an
implementation's existing mechanism IPv6
header that is not currently available, so more creative options
would be required, for delivering these errors instance using new Next Header values to
indicate that the ULP will work. But when the shim on the transmitting side
replaces packet belongs to a shim6 enabled communication and
that the ULIDs Flow Label carries context information as proposed in the IP address fields
now expire NOID draft. . However, even if this is done, this
approach is incompatible with some other locators,
then an ICMP error coming back will have a "packet in error" the deferred establishment capability
of the shim protocol, which is
not a packet that the ULP sent. Thus preferred function, since it
suppresses the implementation will have delay due to
apply the reverse mapping shim context establishment prior to
initiation of the "packet in error" before passing the
ICMP error up communication and it also allows nodes to define at
which stage of the ULP.

This mapping is different than when receiving ULP packets from the
peer, because in communication they decide, based on their own
policies, that case the packets contain CT(local). But the
ICMP errors have a "packet in error" with CT(peer) since they were
intended given communication requires to be received protected by the peer.
shim.

In any case, since the <Source
Locator, Destination Locator, CT(peer)> has order to be unique when
received by cope with the peer, identified limitations, an alternative
approach that does not constraints the local host should also only be able to find
one context flow label values used by
communications that matches this tuple.

If are using ULIDs equal to the ULP packet had been encapsulated 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 [15]). The packets sent after a shim6 payload message,
then failure when a
different locator pair is used would use a completely different flow
label, and this extension header must be removed. The result needs to flow label could be
that the ULP receives an ICMP error where allocated by the contained "packet in
error" looks receiver as if part
of the shim did not exist.

10. Teardown context establishment. Since it is allocated during the
context establishment, the receiver of the Host Pair Context

Each host "failed over" packets can unilaterally decide when
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 tear down a host-pair
context. It is RECOMMENDED that hosts not tear down the shim context when
they know that there is some upper layer protocol has a
certain locator sets associated, the same value cannot be used for
other communications that might use the
context. For example, an implementation might know this is there is an open socket which address pair that is connected to contained in
the ULID(peer). However, there
might be cases when locator sets of the knowledge context. This is not readily available to a constraint in the
shim layer, for instance for UDP applications which not not connect
their sockets, or any application which retains some higher level
state across (TCP) connections and UDP packets.

Thus it
potential Flow Label allocation strategies.

A possible workaround to this constraint is RECOMMENDED to mark shim packets that implementations minimize premature
teardown by observing
require translation, in order to differentiate them from regular IPv6
packets, using the amount artificial Next Header values described above. In
this case, the Flow Label values constrained are only those of traffic the
packets that are being translated by the shim. This last approach
would be the preferred approach if the context tag is sent and received
using to be carried
in the context, and Flow Label field. This is not only after because it appears quiescent, tear down imposes the
state.

11. Updating
minimum constraints to the Locator Pairs

TBD

12. Various Probe Mechanisms

TBD

13. Rehoming Flow Label allocation strategies, limiting
the restrictions only to a Different Locator Pair

TBD

14. Payload Packets before a Switch

When there is no context state for those packets that need to be translated by
the ULID pair on shim, but also because Context Loss detection mechanisms greatly
benefit from the sender, there
is no effect on how ULP fact that shim data packets are sent. If identified as such,
allowing the host is using some
heuristic for determining when receiving end to perform identify if a deferred shim context
establishment, then the host might need associated
to do some accounting (count
the number of packets sent and received) even before there is a host-
pair context. This need received packet is suppose to count packets might also appear on the
receive side, depending on what heuristics exist, as it will be discussed in
the implementation has
chosen.

If there Context Loss detection appendix below.

Appendix A.2.2 Extension Header

Another approach is to carry the context tag in a host-pair new Extension
Header. These context for the ULID pair, then tags are allocated by the sender
needs to verify whether context uses receiving end during
the ULIDs as locators, shim6 protocol initial negotiation, implying that is,
whether Lp(peer) == ULID(peer) and Lp(local) == ULID(local).

If this is the case, then each context
will have two context tags, one for each direction. Data packets
will be sent unmodified by demultiplexed using the
shim. If context tag carried in the Extension
Header. This seems a clean approach since it is does not the case, then the logic overload
existing fields. However, it introduces additional overhead in Section 15 will need
to be used.

There will also be some maintenance activity relating the
packet due to
(un)reachability detection, whether packets are sent with the
original locators or not. additional header. The details of this additional overhead
introduced is out of scope for
this document and will 8 octets. However, it should be covered is follow-ons to [6].

15. Payload Packets after a Switch

When sending packets, if there noted that the context
tag is only required when a host-pair context for the ULID
pair, and locator other than the ULID pair is no longer one used as the locator pair, then
the sender needs to transform ULID
is contained in the packet. Apart from replacing Packets where both the
IPv6 source and
destination address fields with contain the ULIDs do not require a locator pair, an 8-octet context
tag, since no rewriting is necessary at the receiver. This approach
would reduce the overhead, because the additional header is added so that only
required after a failure. On the receiver can find 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.

Appendix A.3 Context Loss Detection

In this appendix we will present different approaches considered to
detect context loss and inverse
the transformation.

First, potential context recovery strategies. The
scenario being considered is the IP address fields following: Node A and Node B are replaced. The IPv6 source address
field
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 Lp(local) 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 destination address field is set context needs to
Lp(peer). NOTE be recovered/
reestablished.

Mechanisms for detecting context. loss

These mechanisms basically consist in that this MUST NOT cause any recalculation each end of the ULP
checksums, since context
periodically sends a packet containing context-specific information
to the ULP checksums are carried end-to-end and other end. Upon reception of such packets, the ULP
pseudo-header contains receiver
verifies that the ULIDs which are preserved end-to-end.

The sender skips any "routing sub-layer extension headers" required context exists. In case that the
ULP might have included, thus context
does not exist, it skips any hop-by-hop extension
header, any routing header, and any destination options header that
is followed by sends a routing header. After any such headers packet notifying the shim6
extension header will be added. This might problem to the
sender.

An obvious alternative for this would be before a Fragment
header, to create a Destination Options header, specific context
keepalive exchange, which consists in periodically sending packets
with this purpose. This option was considered and discarded because
it seemed an ESP or AH header, or overkill to define a ULP
header.

The inserted shim6 Payload extension header includes new packet exchange to deal with
this issue.

An alternative is to piggyback the peer's context tag.

The receiver parses the (extension) headers loss detection function in order. Should it find
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 shim6 extension header node receives one
of such packets, it will look at the type field in that
header. If the type is Payload message, then verify if the context exists. However, shim
control frequency may not be adequate for context loss detection
since control packet must exchanges can be
passed to the shim6 payload handling very limited for rewriting. (Otherwise, 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
shim6 control messages are handled as specified ULIDs in other parts of
this document.)

The receiver extracts the address fields) do not need
context tag information since they do not need any shim processing.
Packets that carry locators that differ from the payload message
header, and uses this together with the IPv6 source and destination
address fields ULIDs carry context
information.

However, we need to find make a host-pair context. If no context is found, distinction here between the receiver SHOULD generate a No Such Context error message (see
Section 8).

With different
approaches considered to carry the context tag, in hand, the receiver can now replace the IP address
fields with the ULIDs kept 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 context. Finally, case where the Payload
extension header context tag is removed from the packet (so that carried in the ULP doesn't
get confused by it), 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 next header value in the preceding
header
associated context is set not able to be the actual protocol number for detect that the payload. Then packet is
associated with a missing context. The result is that the packet can
will be passed unchanged to the protocol identified by the next
header value (which might be some function associated with the IP
endpoint sublayer, or upper layer protocol, which in turn
will probably silently discard it due to a ULP).

16. Open Issues checksum error. The following open issues are known:
o Is there need for keeping the list of locators private between
resulting behavior is that the
two communicating endpoints? We can potentially accomplish context loss is undetected. This is
one additional reason to discard an approach that
when using CGA but not with HBA, but it comes at carries the cost of doing
some public key encryption context
tag in the Flow Label field and decryption operations does not explicitly mark the shim
packets as part of such. On the context establishment.
o Forking other hand, approaches that mark shim data
packets (like the context state. On Extension Header or the mailing list we've discussed Flow Label with new Next
Header values approaches) allow the need receiver to fork detect if the context state, so that different ULP streams
can be sent using different locator pairs. No protocol extensions
are needed if any forking is done independently by each endpoint.
But if we want A
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 able to tell B noted that certain traffic (a
5-tuple?) should be forked, then we need only those packets that
carry a way to convey this in locator that differs form the shim6 protocol. The hard part would be defining what
selectors can ULID are marked. This
basically means that context loss will be specified for detected after an outage
has occurred i.e. alternative locators are being used.

Summarizing, the filter which determines which
traffic proposed context loss detection mechanisms uses which shim
control packets and payload packets to detect context loss. Shim
control packets detect context loss during the whole lifetime of the forks. So
context, but the question expected frequency in some cases is whether we
really need signaling for forking, or whether very low. On
the other hand, payload packets 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 sufficient to
allow each endpoint to do its own selection of which locator pair detected
after a failure i.e. once that it is using for which traffic.
o If we allow forking, it seems like the actually needed. Because of
that, a mechanism for reachability
detection, whether it recovering from context loss is CUD or FBD, must be applied separately
for each locator pair that required if
this approach is in use. Without forking a single
locator pair will be in use used.

Overall, the mechanism for each host-pair context, hence
things detecting lost context would be simpler.
o The specified mechanism (of relying on No Such Context errors)
doesn't always detect work as
follows: the loss of end that still has the context on available sends a message
referring to the peer when context. Upon the
original ULID=locators are used. See Section 17 for other
options.

o In reception of such message, the Locator List option, do we need to indicate which locators
need to be validated using HBA vs. CGA? Or it could tell which
locators are in
end that has lost the HBA extension in context identifies the PDS, situation and assume any
others need CGA validation.
o What happens when a host runs out of N bit notifies
the context tags? When is
it safe for a host loss event to reuse a context tag? With the unilateral
teardown one other end might discard by sending a packet
containing the lost context state long before information extracted from the
other end.

17. Design Alternatives

This document has picked a certain set of design choices in order received
packet.

One option is to
try simply send an error message containing the received
packets (or at least as much of the received packet that the MTU
allows to work out a bunch fit in). One of the details, and stimulate discussion.
But as has been discussed on goals of this notification is to allow
the mailing list, there are other
choices end that make sense. This section tries to enumerate some
alternatives.

17.1 State Cleanup

This document uses a timer based cleanup mechanism, as specified in
Section 10.

An alternative would be still retains context state, to use an explicit CLOSE mechanism, akin reestablish the
lost context. The mechanism to reestablish the one specified loss context consists
in HIP [22]. If an explicit CLOSE handshake and
associated timer performing the 4-way initial handshake. This is used, then there would no longer a time consuming
exchange and at this point time may be critical since we are
reestablishing a need for context that is currently needed (because context
loss detection may occur after a failure). So, another option is to
replace the No Context Error error message due to by a peer having garbage collected
its end 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 context. However, there is end
that still potentially a need
to have has the context state can finish the context establishment
exchange and restore the lost context.

Appendix A.4 Securing locator sets

The adoption of a No Context Error message in protocol like SHIM that allows the case binding of a complete state
loss
given ULID with a set of locators opens the peer (also known doors for different types
of redirection attacks as a crash followed by a reboot). Only
if we assume that described in [17]. The goal in terms of
security for the reboot takes at least design of the CLOSE timer, or that
it shim protocol is ok to not provide complete service until CLOSE timer minutes
after the crash, can we completely do away with to introduce any
new vulnerability in the No Context Error
message.

17.2 Detecting Context Loss

This document specifies that context loss Internet architecture. It is detected by receiving a
No Such Context error message from non-goal to
provide additional protection than the peer. Such messages are
generated currently available in response the
single-homed IPv6 Internet.

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 shim6 message randomly generated bit string that contain a is negotiated during the peer's
context tag, including the shim6 Payload messages, when the receiver
doesn't have matching context. They are also generated establishment phase and then it is included in response following
signaling messages. By this mean, it would be possible to data packets after a locator switch (because such payload packets
are identified as such by using the payload message header).

This approach has verify
that the disadvantage party that it doesn't detect was involved in the loss of
context state when initial handshake is the original ULIDs are used as locators, because
there might be no shim6 messages exchanged if same
party that is introducing new locators. Moreover, before using a new
locator, an exchange is performed through the reachability
detection manages to suppress any extra messages.

The Interim Meeting discussed ways to recover new locator, verifying
that the context state party located at
one end when the other end sees a failure (and starts sending Explore
messages). The discussed approach new locator knows the cookie i.e. that
it is to use a R1 (or R1bis) message
in response to a message with an unknown context, which would cause the context to be recreated.

18. Implications Elsewhere

The general shim6 approach, as well as same party that performed the specifics of initial handshake.

While this proposed
solution, has implications elsewhere. The key implications are:
o Applications 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 perform referrals, or callbacks using IP
addresses as once was on the 'identifiers'
path, it discovers the cookie by sniffing any signaling message.
After that, the attacker can leave the path and still function in limited ways, perform a
redirection attack, since as described in [17]. But he is in order for such applications to be
able to take advantage possession of the multiple locators for redundancy, cookie, he
can introduce a new locator in the applications need to be modified to either use fully qualified
domain names as locator set and he can also
successfully perform the 'identifiers', or they need reachability exchange if he is able to pass all
receive packets at the
locators as new locator. The difference with the 'identifiers' i.e., current
single-homed IPv6 situation is that in the 'identifier' from current situation the
applications perspective becomes a set of IP addresses instead
attacker needs to be on-path during the whole lifetime 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 attack,
while in this new situation where only cookie protection if provided,
an attacker that even when shim6 capable hosts are communicating, once was on the path can perform attacks after he
has left the on-path location.

Moreover, because the I1
messages would be dropped, hence cookie is included in signaling messages, the hosts would not
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
their peer is shim6 capable. This negotiated during the initial handshake but
that is in fact used as a feature, since if
the hosts managed key to establish a host-pair context, then protect following messages. With this
technique, the
firewall would probably drop attacker must be present on the "different" 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 are sent
after a failure (those using
the shim6 payload message with attacker must be on path at a TCP
packet inside it). Thus stateful firewalls that are modified very specific moment (the
establishment phase) to
allow shim6 messages through should also actually be modified able to allow launch the
payload messages through after a failure. This presumably implies
that attack. While
this seems to substantially improve the firewall needs situation, it should be noted
that, depending on protocol details, an attacker may be able to track force
the set recreation of locators in use the initial handshake (for instance by
looking at blocking
messages and making the shim6 exchanges. Such firewalls might even want to
verify parties think that the locators using context has been
lost), so the HBA/CGA verification themselves.
o Signaling protocols for QoS or other things resulting situation may not differ that involve having
devices in much from the network path look at IP addresses and port numbers,
or IP addresses and Flow Labels, need to be invoked on
cookie based approach.

Another option that was discussed during the hosts
when design of the locator pair changes due protocol
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 failure. At locator
set that point in
time those protocols need can be used to inform exchange packets. The mechanism provided by
IPSec to securely bind the devices address used with the cryptographic keys
is the usage of digital certificates. This implies that a new pair an IPSec
based solution would require that the generation of
IP addresses will be used digital
certificates that bind the key and the ULID by a common third trusted
party for both parties involved in the flow. Note communication. Considering
that this is the
case even though we no longer overload scope of application of the flow label as shim protocol is global, this
would imply a context
tag; global public key infrastructure. The major issues
with this approach are the in-path devices need deployment difficulties associated with a
global PKI.

Finally two different technologies were selected to know about protect the use shim
protocol: HBA [6] and CGA [5]. 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 new
locators even though capability of generating all the flow label stays
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 same.
o MTU implications. prefix-set
available into the interface identifier part. The path MTU mechanisms we use are robust
against different packets taking different paths through result is that the
Internet, by computing a minimum over
binding between all the recently observed path
MTUs. When shim6 fails over from using one locator pair to
another pair, this means available addresses is encoded within the
addresses themselves, providing hijacking protection. Any peer using
the shim protocol node can efficiently verify that packets might travel over a
different path through the Internet, hence alternative
addresses proposed for continuing the path MTU might be
quite different. Perhaps such a path change would be a good hint communication are bound to the path MTU mechanism to try
initial address through a larger MTU?

The fact simple hash calculation. A limitation of
the HBA technique is that once generated the shim, at least for uncommon payload types, will
add an 8 octet extension header (the payload message) after a
locator switch, can address set is fixed and
cannot be changed without also affect changing all the usable path MTU for addresses of the ULPs. HBA
set. In this case other words, the MTU change HBA technique does not support dynamic
addition of address to a previously generated HBA set. An advantage
of this approach is local that it requires only hash operations to the sending host, thus
conveying the change verify a
locator set, imposing very low computational cost to the ULPs is an implementation matter.

19. Security Considerations

This document satisfies protocol.

In a CGA based approach the concerns specified in [16] address used as follows:
o TBD: Using HBA or ULID is a CGA for ...

Some that
contains a hash of a public key in its interface identifier. The
result is a secure binding between the residual threats 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 proposal are:
o An attacker which arrives late on case is the following: the ULID used for the
communication is securely bound to the key pair because it contains
the path (after hash of the context has
been established) can use public key, and the No Such Context error to cause one
peer locator set is bound to recreate the context, and at that point in time
public key through the
attacker can observe all signature. The CGA approach then supports
dynamic addition of new locators in the exchange. But this doesn't seem locator set, since in order
to open any do that, the node only needs to sign the new doors for locator with the attacker since such an attacker can
observe
private key associated with the Context tags CGA used as ULID. A limitation of
this approach is that are being used, and once known it
can use those to send bogus messages.
o An attacker which 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 present on securely bound to a locator set that
can only be defined by the path owner of the ULID.

So, the design decision adopted was that both mechanisms HBA and CGA
are supported, so that it can find out when only stable address sets are required,
the context tags, nodes can generate a No Such Context error after it
has moved off benefit from the path. For low computational cost offered by HBA
while when dynamic locator sets are required, this packet to can be effective it needs
to have achieved
through CGAs with an additional cost. Moreover, because HBAs are
defined as a source locator which belongs to CGA extension, the context, thus there addresses available in a node can not
simultaneously be "too much" ingress filtering between the attackers new
location CGAs and HBAs, allowing the communicating peers. But this doesn't seem to be
that severe, because once usage of the error causes HBA and
CGA functionality when needed without requiring a change in the
addresses used.

Appendix A.5 Host-pair context establishment exchange

Two options were considered for the host-pair context to be torn
down establishment
exchange: a 2-way handshake and re-established, a new pair of context tags will be used,
which will not be known to 4-way handshake.

A key goal for the attacker. If design of this is still exchange was that protection
against DoS attacks. The attack under consideration was basically a
concern, we could require
situation where an attacker launches a 2-way handshake "did you really loose
the state?" in response great amount of host-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 error message.
o It might be possible for an attacker
receiver does not creates any state associate to try random 32-bit context
tags and see if they can cause disruption for communication
between two hosts. We can make this harder by using a larger given context tag; 47 bits is
until the largest that fit reception of the second packet which contains a prior
contact proof in the 8-octet
payload header. If form of a token. At this isn't sufficient, one could use an even
larger tag in point the shim6 control messages, and use receiver can
verify that at least the low-order 47
bits in address used by the payload header.

20. IANA Considerations

IANA needs initiator is at some
extent valid, since the initiator is able to allocate 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 new IP Next Header value 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 protocol.

TBD:
limitation has a reduced impact in the IANA rules for performance of the shim6 message types and option types.

21. Change Log

The following changes protocol.
(It may however have been made since draft-ietf-shim6-proto-00:
o Removed a greater impact in the use situation of context
recover as discussed earlier, but in this case, it is possible to
perform optimizations to reduce the flow label and the overloading number of packets as described
above)

The other option considered was a 2-way handshake with the IP
protocol numbers. Instead, when
possibility to fall back to a 4-way handshake in case of attack. In
this approach, the locator host pair is establishment exchange normally consists
in a 2-packet exchange and it does not verify that the ULID
pair, the ULP payloads will be carried with an 8 octet extension
header. The belief is initiator has
performed a prior contact before creating context state. In case
that it a DoS attack is possible to remove these extra
bytes by defining future shim6 extensions that exchange more
information between detected, the hosts, without having responder falls back to a 4-way
handshake similar to overload the flow
label or the IP protocol numbers.
o Grew one described previously in order to prevent
the context tag from 20 bits detected attack to 32 bits, proceed. The main difficulty with the possibility this attack
is how to grow detect that a responder is currently under attack. It
should be noted, that because this is 2-way exchange, it is not
possible to 47 bits. This implies changes 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 message
formats.
o Almost by accident, current impact of
DoS attacks and the new shim6 message format is very close to low impact of the HIP message format.
o Adopted 4-way exchange in the HIP format shim
protocol thanks to the deferred context establishment capability, a
4-way exchange would be adopted for the options, since this makes it easier base protocol.

Appendix A.6 Updating locator sets

There are two possible approaches to describe variable length options. the addition and removal of
locators: atomic and differential approaches. The original, ND-style,
option format requires internal padding atomic approach
essentially send the complete locators set each time that a variation
in the options 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 make
them 8 octet length in total, be exchanged each time while the HIP format handles differential approach
requires re-synchronization of both ends through changes i.e. that
using
both ends have the option length field.
o Removed some same idea about what the current locator set is.

Because of the control messages, and renamed difficulties imposed by the other ones.
o Added synchronization
requirement, the atomic approach was selected.

Appendix A.7 State Cleanup

There are essentially two approaches for discarding an existing state
about locators, keys and identifiers of a "generation" number to correspondent node: a
coordinated approach and an unilateral approach.

In the Locator List option, so that unilateral approach, each node discards the peers can ensure that information about
the preferences refer to other node without coordination with the right
"version" 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 Locator List.
o nodes has
discarded the state while the other node still hasn't. In order for FBD and exploration this case,
a No-Context error message may be required to work when there inform about the
situation and possibly a recovery mechanism is also needed.

A coordinated approach would use of an explicit CLOSE mechanism, akin
to the
context is forked, that one specified in HIP [23]. If an explicit CLOSE handshake and
associated timer is different ULP messages are sent over
different locator pairs, things are a lot easier if used, then there is only
one current locator pair used would no longer be a need for each context. Thus
the forking No Context Error message due to a peer having garbage collected
its end of the context context. However, there is now causing still potentially a new context need
to be established for
the same ULID; the new context having have a new context tag. The
original context is referred to as No Context Error message in the "default" context for case of a complete state
loss of the
ULID pair.
o Added more background material and textual descriptions.

22. Acknowledgements

Over peer (also known as a crash followed by a reboot). Only
if we assume that the years many people active in reboot takes at least the multi6 and shim6 WGs have
contributed ideas a suggestions CLOSE timer, or that are reflected in this draft.

Thanks
it is ok to Marcelo Bagnulo for providing comments on earlier versions
of this draft.

23. not provide complete service until CLOSE timer minutes
after the crash, can we completely do away with the No Context Error
message.

24. References

23.1

24.1 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] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, March 2005.

[6] Bagnulo, M., "Hash Based Addresses (HBA)",
draft-ietf-shim6-hba-00 (work in progress), July 2005.

[6]

[7] Beijnum, I., "Shim6 Reachability Detection",
draft-ietf-shim6-reach-detect-00 (work in progress), July 2005.

[7]

[8] Arkko, J., "Failure Detection and Locator Pair Exploration
Design for IPv6 Multihoming",
draft-ietf-shim6-failure-detection-01 (work in progress),
October 2005.

23.2

24.2 Informative References

[8]

[9] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.

[9]

[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.

[10]

[11] Narten, T. and R. Draves, "Privacy Extensions for Stateless
Address Autoconfiguration in IPv6", RFC 3041, January 2001.

[11]

[12] Draves, R., "Default Address Selection for Internet Protocol
version 6 (IPv6)", RFC 3484, February 2003.

[12]

[13] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.

[13]

[14] Abley, J., Black, B., and V. Gill, "Goals for IPv6 Site-
Multihoming Architectures", RFC 3582, August 2003.

[14]

[15] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, "IPv6
Flow Label Specification", RFC 3697, March 2004.

[15]

[16] Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
Addresses", RFC 4193, October 2005.

[16]

[17] Nordmark, E., "Threats relating to IPv6 multihoming solutions",
draft-ietf-multi6-multihoming-threats-03 (work in progress),
January 2005.

[17]

[18] Nordmark, E., "Shim6 Application Referral Issues",
draft-ietf-shim6-app-refer-00 (work in progress), July 2005.

[18]

[19] Abley, J., "Shim6 Applicability Statement",
draft-ietf-shim6-applicability-00 (work in progress),
July 2005.

[19]

[20] Huston, G., "Architectural Commentary on Site Multi-homing
using a Level 3 Shim", draft-ietf-shim6-arch-00 (work in
progress), July 2005.

[20]

[21] Bagnulo, M. and J. Arkko, "Functional decomposition of the
multihoming protocol", draft-ietf-shim6-functional-dec-00 (work
in progress), July 2005.

[21]

[22] Nordmark, E. and M. Bagnulo, "Multihoming L3 Shim Approach",
draft-ietf-shim6-l3shim-00 (work in progress), July 2005.

[22]

[23] Moskowitz, R., "Host Identity Protocol", draft-ietf-hip-base-03
(work in progress), June 2005.

[23]

[24] Lear, E. and R. Droms, "What's In A Name:Thoughts from the
NSRG", draft-irtf-nsrg-report-10 (work in progress),
September 2003.

Author's Address

Erik Nordmark
Sun Microsystems
17 Network Circle
Menlo Park, CA 94025
USA

Phone: +1 650 786 2921
Email: erik.nordmark@sun.com

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