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RFC 5320
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Phantom Works
Intended status: Informational June 7, 2008
Expires: December 9, 2008
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-seal-18.txt
Status of this Memo
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This Internet-Draft will expire on December 9, 2008.
Abstract
For the purpose of this document, subnetworks are defined as virtual
topologies that span connected network regions bounded by
encapsulated border nodes. These virtual topologies may span
multiple IP- and/or sub-IP layer forwarding hops, and can introduce
failure modes due to packet duplication and/or links with diverse
Maximum Transmission Units (MTUs). This document specifies a
Subnetwork Encapsulation and Adaptation Layer (SEAL) that
accommodates such virtual topologies over diverse underlying link
technologies.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 5
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 6
4. SEAL Protocol Specification - Tunnel Mode . . . . . . . . . . 7
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 7
4.2. ITE Specification . . . . . . . . . . . . . . . . . . . . 9
4.2.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 9
4.2.2. Accounting for Headers . . . . . . . . . . . . . . . . 10
4.2.3. Segmentation and Encapsulation . . . . . . . . . . . . 11
4.2.4. Sending Probes . . . . . . . . . . . . . . . . . . . . 13
4.2.5. Packet Identification . . . . . . . . . . . . . . . . 14
4.2.6. Sending SEAL Protocol Packets . . . . . . . . . . . . 14
4.2.7. Processing Raw ICMPv4 Messages . . . . . . . . . . . . 14
4.2.8. Processing SEAL-Encapsulated ICMPv4 Messages . . . . . 14
4.3. ETE Specification . . . . . . . . . . . . . . . . . . . . 16
4.3.1. Reassembly Buffer Requirements . . . . . . . . . . . . 16
4.3.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 16
4.3.3. Generating SEAL-Encapsulated ICMPv4 Fragmentation
Needed Messages . . . . . . . . . . . . . . . . . . . 16
4.3.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 18
4.3.5. Decapsulation and Generating Other ICMPv4 Errors . . . 18
5. SEAL Protocol Specification - Transport Mode . . . . . . . . . 19
6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 19
7. End System Requirements . . . . . . . . . . . . . . . . . . . 19
8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Security Considerations . . . . . . . . . . . . . . . . . . . 20
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 20
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 21
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
13.1. Normative References . . . . . . . . . . . . . . . . . . . 21
13.2. Informative References . . . . . . . . . . . . . . . . . . 22
Appendix A. Historic Evolution of PMTUD . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 26
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1. Introduction
As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (frequently
tunnels of one form or another) over an actual network that suppors
the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual
topologies have elements which appear as one hop in the virtual
topology, but are actually multiple IP or sub-IP layer hops. These
multiple hops often have quite diverse properties which are often not
even visible to the end-points of the virtual hop. This introduces
many failure modes that are not dealt with well in current
approaches.
The use of IP encapsulation has long been considered as the means for
creating such virtual topologies. However, the insertion of an outer
IP header reduces the effective path MTU as-seen by the IP layer.
When IPv4 is used, this reduced MTU can be accommodated through the
use of IPv4 fragmentation, but unmitigated in-the-network
fragmentation has been deemed "harmful" through operational
experience and studies conducted over the course of many years
[FRAG][FOLK][RFC4963]. Additionally, classical path MTU discovery
[RFC1191] has known operational issues that are exacerbated by in-
the-network tunnels [RFC2923][RFC4459]. In the following
subsections, we present further details on the motivation and
approach for addressing these issues.
1.1. Motivation
Before discussing the approach, it is necessary to first understand
the problems. In both the Internet and private-use networks today,
IPv4 is ubiquitously deployed as the Layer 3 protocol. The two
primary functions of IPv4 are to provide for 1) addressing, and 2) a
fragmentation and reassembly capability used to accommodate links
with diverse MTUs. While it is well known that the addressing
properties of IPv4 are limited (hence the larger address space
provided by IPv6), there is a lesser-known but growing consensus that
other limitations may be unable to sustain continued growth.
First, the IPv4 header Identification field is only 16 bits in
length, meaning that at most 2^16 packets pertaining to the same
(source, destination, protocol, Identification)-tuple may be active
in the Internet at a given time. Due to the escalating deployment of
high-speed links (e.g., 1Gbps Ethernet), however, this number may
soon become too small by several orders of magnitude. Furthermore,
there are many well-known limitations pertaining to IPv4
fragmentation and reassembly - even to the point that it has been
deemed "harmful" in both classic and modern-day studies (cited
above). In particular, IPv4 fragmentation raises issues ranging from
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minor annoyances (e.g., slow-path processing in routers) to the
potential for major integrity issues (e.g., mis-association of the
fragments of multiple IP packets during reassembly).
As a result of these perceived limitations, a fragmentation-avoiding
technique for discovering the MTU of the forward path from a source
to a destination node was devised through the deliberations of the
Path MTU Discovery Working Group (PMTUDWG) during the late 1980's
through early 1990's (see: Appendix A). In this method, the source
node provides explicit instructions to routers in the path to discard
the packet and return an ICMP error message if an MTU restriction is
encountered. However, this approach has several serious shortcomings
that lead to an overall "brittleness".
In particular, site border routers in the Internet are more and more
being configured to discard ICMP error messages coming from the
outside world. This is due in large part to the fact that malicious
spoofing of error messages in the Internet is made simple since there
is no way to authenticate the source of the messages. Furthermore,
when a source node that requires ICMP error message feedback when a
packets is dropped due to an MTU restriction does not receive the
messages, a path MTU-related black hole occurs. This means that the
source will continue to send packets that are too large and never
receive an indication from the network that they are being discarded.
The issues with both IPv4 fragmentation and this "classical" method
of path MTU discovery are exacerbated further when IP-in-IP tunneling
is used. For example, site border routers that are configured as
ingress tunnel endpoints may be required to forward packets into the
subnetwork on behalf of hundreds, thousands, or even more original
sources located within the site. If IPv4 fragmentation were used,
this would quickly wrap the 16-bit Identification field and could
lead to undetected data corruption. If "classical" IPv4
fragmentation were used instead, the site border router may be
bombarded by ICMP error messages coming from the subnetwork which may
be either untrustworthy or insufficiently provisioned to allow
translation into error message to be returned to the original
sources.
The situation is exacerbated further still by IPsec tunnels, since
only the first IPv4 fragment of a fragmented packet contains the
transport protocol selectors (e.g., the source and destination ports)
required for identifying the correct security association rendering
fragmentation useless under certain circumstances. Even worse, there
may be no way for a site border router the configures an IPsec tunnel
to transcribe the encrypted packet fragment contained in an ICMP
error message into a suitable ICMP error message to return to the
original source. Due to these many limitations, a new approach to
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accommodate links with diverse MTUs is necessary.
1.2. Approach
For the purpose of this document, subnetworks are defined as virtual
topologies that span connected network regions bounded by
encapsulating border nodes. Examples include the global Internet
interdomain routing core, Mobile Ad hoc Networks (MANETs) and some
enterprise networks. Subnetwork border nodes forward unicast and
multicast IP packets over the virtual topology across multiple IP-
and/or sub-IP layer forwarding hops which may introduce packet
duplication and/or traverse links with diverse Maximum Transmission
Units (MTUs)
This document introduces a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for tunnel-mode operation of IP over subnetworks that
connect the Ingress- and Egress Tunnel Endpoints (ITEs/ETEs) of
border nodes. Operation in transport mode is also supported when
subnetwork border node upper-layer protocols negotiate the use of
SEAL during connection establishment. SEAL accommodates links with
diverse MTUs and supports efficient duplicate packet detection by
introducing a minimal mid-layer encapsulation.
The SEAL encapsulation introduces an extended Identification field
for packet identification and a mid-layer segmentation and reassembly
capability that allows simplified cutting and pasting of packets.
Moreover, SEAL senses in-the-network IPv4 fragmentation as a "noise"
indication that packet sizing parameters are "out of tune" with
respect to the network path. Instead of experiencing this
fragmentation as a disasterous event, however, SEAL naturally tunes
its packet sizing parameters to eliminate the in-the-network
fragmentation and thereby squelch the noise. The SEAL encapsulation
layer and protocol is specified in the following sections.
2. Terminology and Requirements
The terms "inner", "mid-layer" and "outer" respectively refer to the
innermost IP {layer, protocol, header, packet, etc.} before any
encapsulation, the mid-layer IP {protocol, header, packet, etc.)
after any mid-layer '*' encapsulation and the outermost IP {layer,
protocol, header, packet etc.} after SEAL/*/IPv4 encapsulation.
The term "IP" used throughout the document refers to either Internet
Protocol version (IPv4 or IPv6). Additionally, the notation IPvX/*/
SEAL/*/IPvY refers to an inner IPvX packet encapsulated in any mid-
layer '*' encapsulations followed by the SEAL header followed by any
outer '*' encapsulations followed by an outer IPvY header, where the
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notation "IPvX" means either IP protocol version (IPv4 or IPv6).
The following abbreviations correspond to terms used within this
document and elsewhere in common Internetworking nomenclature:
ITE - Ingress Tunnel Endpoint
ETE - Egress Tunnel Endpoint
PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "Fragmentation
Needed" message
DF - the IPv4 header "Don't Fragment" flag
MHLEN - the length of any mid-layer '*' headers and trailers
OHLEN - the length of the outer encapsulating SEAL/*/IPv4 headers
S_MRU- the per-ETE SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size
SEAL_ID - a 32-bit Identification value; randomly initialized and
monotonically incremented for each SEAL protocol packet
SEAL_PROTO - an IPv4 protocol number used for SEAL
SEAL_PORT - a TCP/UDP service port number used for SEAL
SEAL_OPTION - a TCP option number used for (transport-mode) SEAL
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
3. Applicability Statement
SEAL was motivated by the specific case of subnetwork abstraction for
Mobile Ad-hoc Networks (MANETs), however the domain of applicability
also extends to subnetwork abstractions of enterprise networks, the
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interdomain routing core, etc. The domain of application therefore
also includes the map-and-encaps architecture proposals in the IRTF
Routing Research Group (RRG) (see: http://www3.tools.ietf.org/group/
irtf/trac/wiki/RoutingResearchGroup).
SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation
(e.g., as IPv6/SEAL/IPv4), and appears as a subnetwork encapsulation
as seen by the inner IP layer. SEAL can also be used as a sublayer
for encapsulating inner IP packets within outer UDP/IPv4 header
(e.g., as IPv6/SEAL/UDP/IPv4) such as for the Teredo domain of
applicability [RFC4380]. When it appears immediately after the outer
IPv4 header, the SEAL header is processed exactly as for IPv6
extension headers.
SEAL can also be used in "transport-mode", e.g., when the inner layer
includes upper layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL for
the carriage of protocol data encapsulated as TCP/SEAL/IPv4. In this
sense, the "subnetwork" becomes the entire end-to-end path between
the TCP peers and may potentially span the entire Internet.
The current document version is specific to the use of IPv4 as the
outer encapsulation layer, however the same principles apply when
IPv6 is used as the outer layer.
4. SEAL Protocol Specification - Tunnel Mode
4.1. Model of Operation
SEAL supports the encapsulation of inner IP packets in mid-layer and
outer encapsulating headers/trailers. For example, an inner IPv6
packet would appear as IPv6/*/SEAL/*/IPv4 after mid-layer and outer
encapsulations, where '*' denotes zero or more additional
encapsulation sublayers. Ingres Tunnel Endpoints (ITEs) add mid-
layer '*' and outer SEAL/*/IPv4 encapsulations to the inner packets
they inject into a subnetwork, where the outermost IPv4 header
contains the source and destination addresses of the subnetwork
entry/exit points (i.e., the ITE/ETE), respectively. SEAL uses a new
Internet Protocol type and a new encapsulation sublayer for both
unicast and multicast. The ITE encapsulates an inner IP packet in
mid-layer and outer encapsulations as shown in Figure 1:
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+-------------------------+
| |
~ Outer */IPv4 headers ~
| |
I +-------------------------+
n | SEAL Header |
n +-------------------------+ +-------------------------+
e ~ Any mid-layer * headers ~ ~ Any mid-layer * headers ~
r +-------------------------+ +-------------------------+
| | | |
I --> ~ Inner IP ~ --> ~ Inner IP ~
P --> ~ Packet ~ --> ~ Packet ~
| | | |
P +-------------------------+ +-------------------------+
a ~ Any mid-layer trailers ~ ~ Any mid-layer trailers ~
c +-------------------------+ +-------------------------+
k ~ Any outer trailers ~
e +-------------------------+
t
(After mid-layer encaps.) (After SEAL/*/IPv4 encaps.)
Figure 1: SEAL Encapsulation
where the SEAL header is inserted as follows:
o For simple IPvX/IPv4 encapsulations (e.g.,
[RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
the inner IP and outer IPv4 headers as: IPvX/SEAL/IPv4.
o For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
SEAL header is inserted between the {AH,ESP} header and outer IPv4
headers as: IPvX/*/{AH,ESP}/SEAL/IPv4.
o For IP encapsulations over transports such as UDP, the SEAL header
is inserted immediately after the outer transport layer header,
e.g., as IPvX/*/SEAL/UDP/IPv4.
SEAL-encapsulated packets include a 32-bit SEAL_ID formed from the
concatenation of the 16-bit ID Extension field in the SEAL header as
the most-significant bits, and with the 16-bit ID value in the outer
IPv4 header as the least-significant bits. (For tunnels that
traverse IPv4 Network Address Translators, the SEAL_ID is instead
maintained only within the 16-bit ID Extension field in the SEAL
header.) Routers within the subnetwork use the SEAL_ID for duplicate
packet detection, and ITEs/ETEs use the SEAL_ID for SEAL segmentation
and reassembly.
SEAL enables a multi-level segmentation and reassembly capability.
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First, the ITE can use IPv4 fragmentation to fragment inner IPv4
packets with DF=0 before SEAL encapsulation to avoid lower-level
segmentation and reassembly. Secondly, the SEAL layer itself
provides a simple mid-layer cutting-and-pasting of mid-layer packets
to avoid IPv4 fragmentation on the outer packet. Finally, ordinary
IPv4 fragmentation is permitted on the outer packet after SEAL
encapsulation and used to detect and dampen any in-the-network
fragmentation as quickly as possible.
The following sections specifiy the SEAL-related operations of the
ITE and ETE, respectively:
4.2. ITE Specification
4.2.1. Tunnel Interface MTU
The ITE configures a tunnel virtual interface over one or more
underlying links that connect the border node to the subnetwork. The
tunnel interface must present a fixed MTU to the inner IP layer
(i.e., Layer 3) as the size for admission of inner IP packets into
the tunnel. Since the tunnel interface may support a potentially
large set of ETEs, however, care must be taken in setting a greatest-
common-denominator MTU for all ETEs while still upholding end system
expectations.
Due to the ubiquitous deployment of standard Ethernet and similar
networking gear, the nominal Internet cell size has become 1500
bytes; this is the de facto size that end systems have come to expect
will either be delivered by the network without loss due to an MTU
restriction on the path or a suitable PTB message returned. However,
the network may not always deliver the necessary PTBs, leading to
MTU-related black holes [RFC2923]. The ITE therefore requires a
means for conveying 1500 byte (or smaller) packets to the ETE without
loss due to MTU restrictions and without dependence on PTB messages
from within the subnetwork.
In common deployments, there may be many forwarding hops between the
original source and the ITE. Within those hops, there may be
additional encapsulations (IPSec, L2TP, etc.) such that a 1500 byte
packet sent by the original source might grow to a larger size by the
time it reaches the ITE for encapsulation as an inner IP packet.
Similarly, additional encapsulations on the path from the ITE to the
ETE could cause the encapsulated packet to become larger still and
trigger in-the-network fragmentation. In order to preserve the end
system expectations, the ITE therefore requires a means for conveying
these larger packets to the ETE even though there may be links within
the subnetwork that configure a smaller MTU.
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The ITE should therefore set a tunnel virtual interface MTU of 1500
bytes plus extra room to accommodate any additional encapsulations
that may occur on the path from the original source (i.e., even if
the underlying links do not support an MTU of this size). The ITE
can set larger MTU values still, but should select a value that is
not so large as to cause excessive PTBs coming from within the tunnel
interface (see: Sections 4.2.2 and 4.2.6). The ITE can also set
smaller MTU values, however care must be taken not to set so small a
value that original sources would experience an MTU underflow. In
particular, IPv6 sources must see a minimum path MTU of 1280 bytes,
and IPv4 sources should see a minimum path MTU of 576 bytes.
The inner IP layer consults the tunnel interface MTU when admitting a
packet into the interface. For inner IPv4 packets larger than the
tunnel interface MTU and with the IPv4 Don't Fragment (DF) bit set to
0, the inner IPv4 layer uses IPv4 fragmentation to break the packet
into fragments no larger than the tunnel interface MTU then admits
each fragment into the tunnel as an independent packet. For all
other inner packets (IPv4 or IPv6), the ITE admits the packet if it
is no larger than the tunnel interface MTU; otherwise, it drops the
packet and sends an ICMP PTB message with an MTU value of the tunnel
interface MTU to the source.
4.2.2. Accounting for Headers
As for any transport layer protocol, ITEs use the MTU of the
underlying IPv4 interface, the length of any mid-layer '*' headers
and trailers, and the length of the outer SEAL/*/IPv4 headers to
determine the maximum-sized upper layer payload. For example, when
the underlying IPv4 interface advertises an MTU of 1500 bytes and the
ITE inserts a minimum-length (i.e., 20 byte) IPv4 header, the ITE
sees a maximum payload size of 1480 bytes. When the ITE inserts IPv4
header options, the size is further reduced by as many as 40
additional bytes (the maximum length for IPv4 options) such that as
few as 1440 bytes may be available for the upper layer payload. When
the ITE inserts additional '*' encapsulations, the available MTU for
the upper layer payload is reduced further still.
The ITE must additionally account for the length of the SEAL header
itself as an extra encapsulation that further reduces the size
available for the upper layer payload. The length of the SEAL header
is not incorporated in the IPv4 header length, therefore the network
does not observe the SEAL header as an IPv4 option. In this way, the
SEAL header is inserted after the IPv4 options but before the upper
layer payload in exactly the same manner as for IPv6 extension
headers.
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4.2.3. Segmentation and Encapsulation
For each ETE, the ITE maintains the length of any mid-layer '*'
encapsulation headers and trailers (e.g., for '*' = AH, ESP, NULL,
etc.) in a variable 'MHLEN' and maintains the length of the outer
SEAL/*/IPv4 encapsulation headers in a variable 'OHLEN'. The ITE
maintains a SEAL Maximum Segment Size (S_MSS) value for each ETE as
soft state within the tunnel interface (e.g., in the IPv4 destination
cache). The ITE initializes S_MSS to the minimum of (the underlying
IPv4 interface MTU minus OHLEN) and 128 bytes, and decreases or
increases S_MSS based on any ICMPv4 Fragmentation Needed messages
received (see: Section 4.2.6). The ITE additionally maintains a SEAL
Maximum Reassembly Unit (S_MRU) value for each ETE, initialized to a
value no larger than 2KB.
The ITE performs segmentation and encapsulation on inner packets that
have been admitted into the tunnel interface. For inner IPv4 packets
with the DF bit set to 0, if the length of the inner packet is larger
than (S_MRU - MHLEN) the ITE uses IPv4 fragmentation to break the
packet into IPv4 fragments no larger than (S_MRU - MHLEN). For
unfragmentable inner packets (e.g., IPv6 packets, IPv4 packets with
DF=1, etc.), if the length of the inner packet is larger than
(MAX(S_MRU, S_MSS) - MHLEN), the ITE drops the packet and sends an
ICMP PTB message with an MTU value of (MAX(S_MRU, S_MSS) - MHLEN)
back to the original source.
The ITE then encapsulates each inner packet/fragment in the MHLEN
bytes of mid-layer '*' headers and trailers. For each such resulting
mid-layer packet, if the length of the mid-layer packet is no larger
than S_MRU but is larger than S_MSS, the ITE breaks it into N
segments (N <= 16) that are no larger than S_MSS bytes each. Each
segment except the final one MUST be of equal length, while the final
segment MUST be no larger than the initial segment. The first byte
of each segment MUST begin immediately after the final byte of the
previous segment, i.e., the segments MUST NOT overlap.
Note that this SEAL segmentation is used only for mid-layer packets
that are no larger than S_MRU; mid-layer packets that are larger than
S_MRU are instead encapsulated as a single segment. Note also that
this SEAL segmentation ignores the fact that the mid-layer packet may
be unfragmentable. This segmentation process is a mid-layer (not an
IP layer) operation employed by the ITE to adapt the mid-layer packet
to the subnetwork path characteristics, and the ETE will restore the
packet to its original form during decapsulation. Therefore, the
fact that the packet may have been segmented within the subnetwork is
not observable after decapsulation.
The ITE next encapsulates each segment in a SEAL header formatted as
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID Extension |P|R|D|M|Segment| Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SEAL Header Format
where the header fields are defined as follows:
ID Extension (16)
a 16-bit extension of the ID field in the outer IPv4 header;
encodes the most-significant 16 bits of a 32 bit SEAL_ID value.
P (1)
the "Probe" bit. Set to 1 if the ITE wishes to receive an
explicit acknowledgement from the ETE.
R (1)
the "Report Fragmentation" bit. Set to 1 if the ITE wishes to
receive a report from the ETE if any IPv4 fragmentation occurs.
D (1)
the "Dont Reassemble" bit. Set to 1 if the reassembled SEAL
protocol packet is to be discarded by the ETE if any IPv4
reassemly is required.
M (1)
the "More Segments" bit. Set to 1 if this SEAL protocol packet
contains a non-final segment of a multi-segment mid-layer packet.
Segment (4)
a 4-bit Segment number. Encodes a segment number between 0 - 15.
Next Header (8) an 8-bit field that encodes an Internet Protocol
number the same as for the IPv4 protocol and IPv6 next header
fields.
For single-segment mid-layer packets, the ITE encapsulates the
segment in a SEAL header with (M=0; Segment=0). For N-segment mid-
layer packets (N <= 16), the ITE encapsulates each segment in a SEAL
header with (M=1; Segment=0) for the first segment, (M=1; Segment=1)
for the second segment, etc., with the final segment setting (M=0;
Segment=N-1). For each encapsulated segment, the ITE sets D=0 in the
SEAL header if the ETE is to accept the packet even if it arrives as
multiple IPv4 fragments; for example, the ITE may set D=0 in the SEAL
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header of each segment for all mid-layer packets no larger than
S_MRU. The ITE instead sets D=1 in the SEAL header if the ETE is to
discard the packet if it arrives as multiple IPv4 fragments; in
particular, the ITE should set D=1 in the SEAL header of each segment
for all mid-layer packets larger than S_MRU.
The ITE next sets the P and R bits in the SEAL header of each segment
according to whether the packet is to be used as an explicit/implicit
probe as specified in Section 4.2.4, then writes the Internet
Protocol number corresponding to the mid-layer packet in the SEAL
'Next Header' field. Next, the ITE encapsulates the segment in the
requisite */IPv4 outer headers according to the specific
encapsulation format (e.g., [RFC2003], [RFC4213], [RFC4380], etc.),
except that it writes 'SEAL_PROTO' in the protocol field of the outer
IPv4 header (when simple IPv4 encapsualtion is used) or writes
'SEAL_PORT' in the outer destination service port field (e.g., when
UDP/IPv4 encapsulation is used). The ITE finally sets packet
identification values as specified in Section 4.2.5 and sends the
packets as specified in Section 4.2.6.
4.2.4. Sending Probes
When S_MSS is larger than 128, the ITE sends ordinary encapsulated
data packets as implicit probes to detect in-the-network IPv4
fragmentation and to determine new values for S_MSS. The ITE sets
R=1 in the SEAL header and DF=0 in the outer IPv4 header of each
segment of a SEAL-segmented packet to be used as an implicit probe,
and will receive ICMPv4 Fragmentation Needed messages from the ETE if
any IPv4 fragmentation occurs. When S_MSS is no larger than 128, the
ITE instead sets R=0 in the SEAL header to avoid generating
fragmentation reports for unavoidable in-the-network fragmentation.
The ITE should send explicit probes periodically to manage a window
of SEAL_IDs of outstanding probes as a means to validate any ICMPv4
messages it receives. The ITE sets P=1 in the SEAL header of each
segment of a SEAL-segmented packet to be used as an explicit probe,
where the probe can be either an ordinary data packet or a NULL
packet created by setting the 'Next Header' field in the SEAL header
to a value of "No Next Header" (see: [RFC2460], Section 4.7.
The ITE should further send explicit probes periodically to detect
increases in S_MSS by resetting S_MSS to the minimum of (the
underlying IPv4 interface MTU minus OHLEN) and 128 bytes, and/or
sending explicit probes that are larger than the current S_MSS.
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4.2.5. Packet Identification
For the purpose of packet identification, the ITE maintains a 32-bit
SEAL_ID value as per-ETE soft state, e.g. in the IPv4 destination
cache. The ITE randomly-initializes SEAL_ID when the soft state is
created and monotonically increments it (modulo 2^32) for each
successive SEAL protocol packet it sends to the ETE. For each
packet, the ITE writes the least-significant 16 bits of the SEAL_ID
value in the ID field in the outer IPv4 header, and writes the most-
significant 16 bits in the ID Extension field in the SEAL header.
For packets that may traverse IPv4 Network Address Translators
(NATs), the ITE instead maintains SEAL_ID as a 16-bit value that it
randomly-initializes when the soft state is created and monotonically
increments (modulo 2^16) for each successive SEAL protocol packet.
For each packet, the ITE writes SEAL_ID in the ID extension field of
the SEAL header and writes a random 16-bit value in the ID field in
the outer IPv4 header. This requires that both the ITE and ETE
participate in this alternate scheme.
4.2.6. Sending SEAL Protocol Packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
the outer IPv4 header of every outer packet it sends. For
"expendable" packets (e.g., for NULL packets used as probes - see:
Section 4.2.6), the ITE may optionally set DF=1.
The ITE then sends each outer packet that encapsulates a segment of
the same mid-layer packet into the tunnel in canonical order, i.e.,
Segment 0 first, followed by Segment 1, etc. and finally Segment N-1.
4.2.7. Processing Raw ICMPv4 Messages
The ITE may receive "raw" ICMPv4 error messages from routers within
the subnetwork that comprise an outer IPv4 header followed by an
ICMPv4 header followed by a portion of the SEAL packet that generated
the error (also known as the "packet-in-error"). For such messages,
the ITE can use the 32-bit SEAL ID encoded in the packet-in-error as
a nonce to confirm that the ICMP message came from an on-path router
within the subnetwork. The ITE MAY process raw ICMPv4 messages as
soft errors indicating that the path to the ETE may be failing, but
it discards any raw ICMPv4 Fragmentation Needed messages for which
the IPv4 header of the packet-in-error has DF=0.
4.2.8. Processing SEAL-Encapsulated ICMPv4 Messages
In addition to any raw ICMPv4 messages, the ITE may receive SEAL-
encapsulated ICMPv4 messages from subnetwork border nodes that
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comprise outer ICMPv4/*/SEAL/*/IPv4 headers followed by a portion of
the SEAL-encapsulated packet-in-error. The ITE can use the 32-bit
SEAL ID encoded in the packet-in-error as well as information in the
outer IPv4 and SEAL headers as nonces to confirm that the ICMP
message came from a legitimate ETE. The ITE then verifies that the
SEAL_ID encoded in the packet-in-error is within the current window
of transmitted SEAL_IDs for this ETE. If the SEAL_ID is outside of
the window, the ITE discards the message; otherwise, it advances the
window and processes the message.
The ITE processes SEAL-encapsulated ICMPv4 messages other than ICMPv4
Fragmentation Needed exactly as specified in [RFC0792]. For SEAL-
encapsulated ICMPv4 Fragmentation Needed messages, if the IPv4 length
of the packet-in-error minus OHLEN is larger than S_MSS the ITE sets
S_MSS to this new value. For SEAL-encapsulated ICMPv4 Fragmentation
Needed messages with MF=1 in the IPv4 header of the packet-in-error,
the ITE instead sets S_MSS to this new value if the value is no
smaller than (576 - OHLEN) and sets S_MSS to MAX(S_MSS/2, 128) if the
value is smaller than (576 - OHLEN).
Note that in the above, 576 accounts for the nominal minimum MTU for
common IPv4 links. When an ETE returns a packet-in-error with MF=1
and with length smaller than 576, the ITE performs a "limited
halving" of S_MSS to account for IPv4 links with unusually small MTUs
or cases in which the ETE otherwise receives an undersized IPv4
first-fragment. This limited halving may require multiple iterations
of sending probes and receiving ICMPv4 Fragmentation Needed messages,
but will soon converge to a stable S_MSS value. When performing this
limited having, it is important that the ITE adjust its S_MSS size
based on the first ICMPv4 Fragmentation Needed message and refrain
from reducing S_MSS until ICMPv4 Fragmentation Needed messages
pertaining to packets sent under the new S_MSS are received. For
example, the ITE should not halve the S_MSS repeatedly based on a
flurry of ICMPv4 Fragmentation Needed messages all pertaining to
packets sent under the same S_MSS.
After deterimining a new value for S_MSS, if the IPv4 header of the
packet-in-error has MF=1 and its SEAL header has D=1 the ITE MAY
transcribe the message into an ICMP PTB message to send back to the
original source. To do so, the ITE discards the SEAL/*/IPv4 headers
plus any mid-layer '*' headers/trailers of the packet-in-error then
encapsulates the remaining inner IP packet portion in a PTB message
with the MTU field set to MAX((S_MRUS, S_MSS) - MHLEN). Note that
this may not be possible when the inner IP packet portion was
encrypted (e.g. via IPsec/ESP), and is otherwise not entirely
necessary since the ITE will discard subsequent large packets and
send back an ICMP PTB *before* encapsulating them and sending to the
ETE. Transcribing ICMPv4 Fragmentation Needed messages into ICMP
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PTBs is therefore offered only as an optional optimization.
4.3. ETE Specification
4.3.1. Reassembly Buffer Requirements
ETEs MUST be capable of using IPv4-layer reassembly to reassemble
SEAL protocol outer IPv4 packets of at least (2KB + OHELN) and MUST
also be capable of using SEAL-layer reassembly to reassemble mid-
layer packets of at least (2KB + OHLEN). The term OHLEN is included
to account for the length of the SEAL/*/IPv4 header, which must be
retained for the purpose of associating the fragments/segments of the
same packet. Note that the term S_MRU used in section 4.2 omits
OHLEN for the purpose of specification clarity.
4.3.2. IPv4-Layer Reassembly
The ETE performs IPv4 reassembly as-normal, and should maintain a
conservative high- and low-water mark for the number of outstanding
reassemblies pending for each ITE. When the size of the reassembly
buffer exceeds this high-water mark, the ETE actively discards
incomplete reassemblies (e.g., using an Active Queue Management (AQM)
strategy) until the size falls below the low-water mark. The ETE
should also use a reduced IPv4 maximum segment lifetime value (e.g.,
15 seconds), i.e., the time after which it will discard an incomplete
IPv4 reassembly for a SEAL protocol packet.
After reassembly, the ETE either accepts or discards the reassembled
packet based on the current status of the IPv4 reassembly cache
(congested vs uncongested). The SEAL_ID included in the IPv4 first-
fragment provides an additional level of reassembly assurance, since
it can record a distinct arrival timestamp useful for associating the
first-fragment with its corresponding non-initial fragments. The
choice of accepting/discarding a reassembly may also depend on the
strength of the upper-layer integrity check if known (e.g., IPSec/ESP
provides a strong upper-layer integrity check) and/or the corruption
tolerance of the data (e.g., multicast streaming audio/video may be
more corruption-tolerant than file transfer, etc.). In the limiting
case, the ETE may choose to discard all IPv4 reassemblies and process
only the IPv4 first-fragment for SEAL-encapsulated error generation
purposes (see the following sections).
4.3.3. Generating SEAL-Encapsulated ICMPv4 Fragmentation Needed
Messages
During IPv4-layer reassembly, the ETE determines whether the packet
belongs to the SEAL protocol by checking for SEAL_PROTO in the outer
IPv4 header (i.e., for simple IPv4 encapsulation) or for SEAL_PORT in
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the outer */IPv4 header (e.g., for '*'=UDP). When the ETE processes
the IPv4 first-fragment (i.e, one with DF=1 and Offset =0 in the IPv4
header) of a SEAL protocol IPv4 packet with (R=1; Segment=0) in the
SEAL header, it sends a SEAL-encapsulated ICMPv4 Fragmentation Needed
message back to the ITE with the MTU value set to 0.
When the ETE processes a SEAL protocol IPv4 packet with (P=1;
Segment=0) for which no IPv4 reassembly was required, it sends a
SEAL-encapsulated ICMPv4 Fragmentation Needed message back to the ITE
with the MTU value set to 0. Note therefore that when the P bit is
set the R bit is "don't-care" and that the ETE only sends a single
IPv4 Fragmentation Needed message, i.e., it does not send two
separate messages (one for the first fragment and a second for the
reassembled whole IPv6 packet).
The ETE prepares the ICMPv4 Fragmentation Needed message by
encapsulating as much of the first fragment (or the whole IPv4
packet) as possible in outer */SEAL/*/IPv4 headers without the length
of the message exceeding 576 bytes as shown in Figure 3:
+-------------------------+ -
| | \
~ Outer */SEAL/*/IPv4 hdrs~ |
| | |
+-------------------------+ |
| ICMPv4 Header | |
|(Dest Unreach; Frag Need)| |
+-------------------------+ |
| | > Up to 576 bytes
~ IP/*/SEAL/*/IPv4 ~ |
~ hdrs of packet/fragment ~ |
| | |
+-------------------------+ |
| | |
~ Data of packet/fragment ~ |
| | /
+-------------------------+ -
Figure 3: SEAL-encapsulated ICMPv4 Fragmentation Needed Message
The ETE next sets D=0, P=0, R=0, M=0 and Segment=0 in the outer SEAL
header, sets the SEAL_ID the same as for any SEAL packet, then sets
the SEAL Next Header field and the fields of the outer */IPv4 headers
the same as for ordinay SEAL encapsulation. The ETE then sets outer
IPv4 destination address to the source address of the first-fragment
and sets the outer IPv4 source address to the destination address of
the first-fragment. If the destination address in the first-fragment
was multicast, the ETE instead sets the outer IPv4 source address to
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an address assigned to the underlying IPv4 interface. The ETE
finally sends the SEAL-encapsulated ICMPv4 message to the ITE the
same as specified in Section 4.2.5, except that the ETE may send the
messages subject to rate limiting since it is not entirely critical
that all fragmentation be reported to the ITE.
4.3.4. SEAL-Layer Reassembly
Following IPv4 reassembly of a SEAL protocol packet, the ETE adds the
SEAL packet to a SEAL-Layer pending-reassembly queue (if necessary).
If the packet arrived as multiple IPv4 fragments and with D=1 in the
SEAL header, the ETE marks the packet and/or pending reassembly queue
as "discard following reassembly". The ETE also marks the packet as
"discard following reassembly" if the (Next Header, P, R, D) fields
of the packet's SEAL header differ from their respective values in
other SEAL segments already in the queue, i.e., the (Next Header, P,
R, D)-tuple serves as a reassembly nonce.
The ETE performs SEAL-layer reassembly for multi-segment mid-layer
packets through simple in-order concatenation of the encapsulated
segments from N consecutive SEAL protocol packets from the same mid-
layer packet. SEAL-layer reassembly requires the ETE to maintain a
cache of recently received SEAL packet segments for a hold time that
would allow for reasonable inter-segment delays. The ETE uses a SEAL
maximum segment lifetime of 15 seconds for this purpose, i.e., the
time after which it will discard an incomplete reassembly. However,
the ETE should also actively discard any pending reassemblies that
clearly have no opportunity for completion, e.g., when a considerable
number of new SEAL packets have been received before a packet that
completes a pending reassembly has arrived.
The ETE reassembles the mid-layer packet segments in SEAL protocol
packets that contain Segment numbers 0 through N-1, with M=1/0 in
non-final/final segments, respectively, and with consecutive SEAL_ID
values. That is, for an N-segment mid-layer packet, reassembly
entails the concatenation of the SEAL-encapsulated segments with
(Segment 0, SEAL_ID i), followed by (Segment 1, SEAL_ID ((i + 1) mod
2^32)), etc. up to (Segment N-1, SEAL_ID ((i + N-1) mod 2^32)). (For
tunnels that may traverse IPv4 NATs, the ETE instead uses only a 16-
bit SEAL_ID value, and uses mod 2^16 arithmetic to associate the
segments of the same packet.)
4.3.5. Decapsulation and Generating Other ICMPv4 Errors
Following SEAL-layer reassembly, if the packet had the value "No Next
Header" in the SEAL header's Next Header field, or if the packet was
marked "discard following reassembly" and IPv4 fragmentation was
experienced, the ETE silently discards the reassembled mid-layer
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packet. Otherwise, the ETE decapsulates the inner packet and
processes it as normal. If the ETE determines that the decapsulated
inner packet cannot be processed further it drops the packet,
prepares an appropriate SEAL-encapsulated ICMPv4 error message and
sends the error message back to the ITE exactly as for ICMPv4
Fragmentation Needed messages (See: Section 4.3.3).
5. SEAL Protocol Specification - Transport Mode
Section 4 specifies the operation of SEAL in "tunnel mode", i.e.,
when there is both an inner and outer IP layer and with a SEAL
encapsulation layer between. However, the SEAL protocol can also be
used in a "transport mode" of operation in which the inner layer
corresponds to a transport layer protocol (e.g., UDP, TCP, etc.)
instead of an inner IP layer.
For example, two TCP endpoints connected to the same subnetwork
region can negotiate the use of transport-mode SEAL for a connection
by inserting a 'SEAL_OPTION' TCP option during the connection
establishment phase. If both TCPs agree on the use of SEAL, their
protocol messages will be carriaged as TCP/SEAL/IPv4 and the
connection will be serviced by the SEAL protocol using TCP (nstead of
an encapsulating tunnel endpoint) as the transport layer protocol.
The SEAL protocol for transport mode otherwise observes the same
specifications as for Section 4.
6. Link Requirements
Subnetwork designers are strongly encouraged to follow the
recommendations in [RFC3819] when configuring link MTUs, where all
IPv4 links SHOULD configure a minimum MTU of 576 bytes. Links that
cannot configure an MTU of at least 576 bytes (e.g., due to
performance characteristics) SHOULD implement transparent link-layer
segmentation and reassembly such that an MTU of at least 576 can
still be presented to the IPv4 layer.
7. End System Requirements
SEAL provides robust mechanisms for returning PTB messages to the
original source, however end systems that send unfragmentable IP
packets larger than 1500 bytes are strongly encouraged to use
Packetization Layer Path MTU Discovery per [RFC4821].
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8. Router Requirements
IPv4 routers within the subnetwork are strongly encouraged to
implement IPv4 fragmentation such that the first fragment is the
largest and approximately the size of the underlying link MTU.
9. IANA Considerations
SEAL_PROTO, SEAL_PORT and SEAL_OPTION are taken from their respective
range of experimental values documented in [RFC3692][RFC4727]. These
values are for experimentation purposes only, and not to be used for
any kind of deployments (i.e., they are not to be shipped in any
products). This document therefore has no actions for IANA.
10. Security Considerations
Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-
overlapping.
An amplification/reflection attack is possible when an attacker sends
IPv4 first-fragments with spoofed source addresses to an ETE,
resulting in a stream of ICMPv4 Fragmentation Needed messages
returned to a victim ITE. The encapsulated segment of the spoofed
IPv4 first-fragment provides mitigation for the ITE to detect and
discard spurious ICMPv4 Fragmentation Needed messages.
The SEAL header is sent in-the-clear (outside of any IPsec/ESP
encapsulations) the same as for the IPv4 header. As for IPv6
extension headers, the SEAL header is protected only by L2 integrity
checks and is not covered under any L3 integrity checks.
11. Related Work
Section 3.1.7 of [RFC2764] provides a high-level sketch for
supporting large tunnel MTUs via a tunnel-level segmentation and
reassembly capability to avoid IP level fragmentation, which is in
part the same approach used by tunnel-mode SEAL. SEAL could
therefore be considered as a fully-functioned manifestation of the
method postulated by that informational reference, however SEAL also
supports other modes of operation including transport-mode and
duplicate packet detection.
Section 3 of[RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU,
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however the SEAL protocol specifies a mid-layer segmentation and
reassembly capability that is distinct from both inner and outer
fragmentation.
Section 4 of [RFC2460] specifies a method for inserting and
processing extension headers between the base IPv6 header and
transport layer protocol data. The SEAL header is in fact inserted
and processed in exactly the same manner.
The concepts of path MTU determination through the report of
fragmentation and extending the IP Identification field were first
proposed in deliberations of the TCP-IP mailing list and the Path MTU
Discovery Working Group (MTUDWG) during the late 1980's and early
1990's. SEAL supports a report fragmentation capability using bits
in an extension header (the original proposal used a spare bit in the
IP header) and supports ID extension through a 16 bit field in an
extension header (the original proposal used a new IP option). An
historical analysis of the evolution of these concepts as well as the
development of the eventual path MTU discovery mechanism for IP
appears in Appendix A of this document.
12. Acknowledgments
The following individuals are acknowledged for helpful comments and
suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Teco Boot,
Bob Braden, Brian Carpenter, Steve Casner, Ian Chakeres, Remi Denis-
Courmont, Aurnaud Ebalard, Gorry Fairhurst, Joel Halpern, John
Heffner, Bob Hinden, Christian Huitema, Joe Macker, Matt Mathis, Dan
Romascanu, Dave Thaler, Joe Touch, Magnus Westerlund, Robin Whittle,
James Woodyatt and members of the Boeing PhantomWorks DC&NT group.
Path MTU determination through the report of fragmentation was first
proposed by Charles Lynn on the TCP-IP mailing list in 1987.
Extending the IP identification field was first proposed by Steve
Deering on the MTUDWG mailing list in 1989.
13. References
13.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
13.2. Informative References
[FOLK] C, C., D, D., and k. k, "Beyond Folklore: Observations on
Fragmented Traffic", December 2002.
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
October 1987.
[MTUDWG] "IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November
1989 - February 1995.".
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, July 1988.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, February 2000.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
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[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[TCP-IP] "TCP-IP mailing list archives,
http://www-mice.cs.ucl.ac.uk/multimedia/mist/tcpip, May
1987 - May 1990.".
Appendix A. Historic Evolution of PMTUD
(Taken from 'draft-templin-v6v4-ndisc-01.txt'; written 10/30/2002):
The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
and numerous proposals in the late 1980's through early 1990. The
initial problem was posed by Art Berggreen on May 22, 1987 in a
message to the TCP-IP discussion group [TCP-IP]. The discussion that
followed provided significant reference material for [FRAG]. An IETF
Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
with charter to produce an RFC. Several variations on a very few
basic proposals were entertained, including:
1. Routers record the PMTUD estimate in ICMP-like path probe
messages (proposed in [FRAG] and later [RFC1063])
2. The destination reports any fragmentation that occurs for packets
received with the "RF" (Report Fragmentation) bit set (Steve
Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal)
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3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 proposal
(straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)
4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
1990)
5. Fragmentation avoidance by setting "IP_DF" flag on all packets
and retransmitting if ICMPv4 "fragmentation needed" messages
occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191]
by Mogul and Deering).
Option 1) seemed attractive to the group at the time, since it was
believed that routers would migrate more quickly than hosts. Option
2) was a strong contender, but repeated attempts to secure an "RF"
bit in the IPv4 header from the IESG failed and the proponents became
discouraged. 3) was abandoned because it was perceived as too
complicated, and 4) never received any apparent serious
consideration. Proposal 5) was a late entry into the discussion from
Steve Deering on Feb. 24th, 1990. The discussion group soon
thereafter seemingly lost track of all other proposals and adopted
5), which eventually evolved into [RFC1191] and later [RFC1981].
In retrospect, the "RF" bit postulated in 2) is not needed if a
"contract" is first established between the peers, as in proposal 4)
and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
Feb 19. 1990. These proposals saw little discussion or rebuttal, and
were dismissed based on the following the assertions:
o routers upgrade their software faster than hosts
o PCs could not reassemble fragmented packets
o Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets
o Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)
o the 16-bit IP_ID field could wrap around and disrupt reassembly at
high packet arrival rates
The first four assertions, although perhaps valid at the time, have
been overcome by historical events leaving only the final to
consider. But, [FOLK] has shown that IP_ID wraparound simply does
not occur within several orders of magnitude the reassembly timeout
window on high-bandwidth networks.
(Authors 2/11/08 note: this final point was based on a loose
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interpretation of [FOLK], and is more accurately addressed in
[RFC4963].)
Author's Address
Fred L. Templin (editor)
Boeing Phantom Works
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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