Network Working Group F. Templin, Ed.
Internet-Draft Boeing Phantom Works
Intended status: Informational April 21, 2008
Expires: October 23, 2008
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-seal-10.txt
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Abstract
Subnetworks are connected network regions bounded by border routers
that forward unicast and multicast packets over a virtual topology
manifested by tunneling. This virtual topology resembles a "virtual
ethernet", but may span multiple IP- and/or sub-IP layer forwarding
hops that can introduce packet duplication and/or traverse 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
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. SEAL Protocol Specification . . . . . . . . . . . . . . . . . 5
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 5
4.2. ITE Specification . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 7
4.2.2. SEAL Maximum Segment Size (S-MSS) Maintenance . . . . 8
4.2.3. Inner Packet Fragmentation . . . . . . . . . . . . . . 8
4.2.4. SEAL Segmentation and Encapsulation . . . . . . . . . 8
4.2.5. Sending SEAL packets . . . . . . . . . . . . . . . . . 10
4.2.6. Sending S-MSS Probes . . . . . . . . . . . . . . . . . 11
4.2.7. Processing Fragmentation Reports (FRAGREPs) . . . . . 11
4.2.8. Processing ICMP PTBs . . . . . . . . . . . . . . . . . 12
4.3. ETE Specification . . . . . . . . . . . . . . . . . . . . 12
4.3.1. Reassembly Buffer Requirements . . . . . . . . . . . . 12
4.3.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 12
4.3.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 13
4.3.4. Generating Fragmentation Reports (FRAGREPs) . . . . . 13
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 14
6. End System Requirements . . . . . . . . . . . . . . . . . . . 15
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . . 16
Appendix A. Historic Evolution of PMTUD (written 10/30/2002) . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
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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 IP network. 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 an
alternative 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 shown to be 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].
For the purpose of this document, subnetworks are defined as virtual
topologies that span connected network regions bounded by border
routers. Examples include the global Internet interdomain routing
core, Mobile Ad hoc Networks (MANETs) and enterprise networks. These
subnetworks are mainfested by tunnels that may span many underlying
networks and traditional IP subnets, e.g., in the internal
organization of an enterprise network. Subnetwork border routers
support the Internet protocols [RFC0791][RFC2460] and 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 proposes a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for the operation of IP over subnetworks that connect
the Ingress- and Egress Tunnel Endpoints (ITEs/ETEs) of border
routers. 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 without invoking in-the-network IP
fragmentation. The SEAL protocol is specified in the following
sections.
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2. Terminology and Requirements
The term "subnetwork" in this document refers to a virtual topology
that is configured over a connected network region bounded by border
routers and that that appears as a fully-connected shared link, i.e.,
a "Virtual Ethernet (VET)" [I-D.templin-autoconf-dhcp].
The terms "inner" and "outer" respectively refer to the innermost IP
{layer, protocol, header, packet, etc.} *before* any encapsulation,
and the outermost IP {layer, protocol, header, packet etc.} *after*
any encapsulation. Between these inner and outer layers, there may
also be "mid-layer" encapsulations.
The notation IPvX/*/IPvY refers to an inner IPvX packet encapsulated
in any '*' mid-layer headers (including the SEAL header) followed by
an outer IPvY header. The notation "IP" means either IP protocol
version (IPv4 or IPv6).
The following abbreviations correspond to terms used within this
document and elsewhere in common Internetworking nomenclature:
Subnetwork - a connected network region bounded by border routers
SEAL - Subnetwork Encapsulation and Adaptation Layer
VET - Virtual EThernet
MANET - Mobile Ad-hoc Network
ITE - Ingress Tunnel Endpoint
ETE - Egress Tunnel Endpoint
ENCAPS - the size of the outer encapsulating SEAL/*/IPv4 headers
MTU - Maximum Transmission Unit
S-MSS - the per-ETE SEAL Maximum Segment Size
PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "fragmentation
needed" message
DF - the IPv4 header Don't Fragment flag
FRAGREP - a Fragmentation Report message
SEAL-ID - a 32-bit Identification value; randomly initialized and
monotonically incremented for each SEAL-encapsulated packet
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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 use case of subnetwork abstraction
for MANETs, however the domain of applicability also extends to
subnetwork abstractions of enterprise networks, the 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 mid-layer 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 mid-layer for encapsulating inner IP packets within outer UDP/
IPv4 header (e.g., as IP/SEAL/UDP/IPv4) such as for the Teredo domain
of applicability [RFC4380]. For further study, SEAL may also be
useful for "transport-mode" applications, e.g., when the inner layer
includes ordinary protocol data rather than an encapsulated IP
packet.
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
4.1. Model of Operation
Ingres Tunnel Endpoints (ITEs) insert a SEAL header in the IP/*/
IPv4-encapsulated 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 defines a new IP protocol type and a new mid-
layer encapsulation for both unicast and multicast inner IP packets.
The ITE inserts a SEAL header during encapsulation as shown in
Figure 1:
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+-------------------------+
| |
~ Outer */IPv4 headers ~
| |
+-------------------------+
| SEAL Header |
+-------------------------+ +-------------------------+
~ Any mid-layer * headers ~ ~ Any mid-layer * headers ~
+-------------------------+ +-------------------------+
| | | |
~ Inner IP ~ ---> ~ Inner IP ~
~ Packet ~ ---> ~ Packet ~
| | | |
+-------------------------+ +-------------------------+
~ Any mid-layer trailers ~ ~ Any mid-layer trailers ~
+-------------------------+ +-------------------------+
~ Any outer trailers ~
+-------------------------+
Figure 1: SEAL Encapsulation
where the SEAL header is inserted as follows:
o For simple IP/IPv4 encapsulations (e.g.,
[RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
the inner IP and outer IPv4 headers as: IP/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: IP/*/{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 IP/*/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. 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.
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 inner IP packets
to avoid IPv4 fragmentation on the outer packet. Finally, ordinary
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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 router 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 provides a virtual point-to-
multipoint abstraction between the ITE and a potentially large set of
ETEs, however, care must be taken in setting the MTU 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 be delivered by the network without loss due to an MTU
restriction on the path, or a suitable ICMP 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, with
(2KB-ENCAPS) serving as the nominal worst-case upper bound.
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 expectation of delivery for 1500 byte and smaller original
packets, the ITE therefore requires a means for conveying them to the
ETE even though there may be links within the subnetwork that
configure a smaller MTU.
The ITE upholds the 1500-byte-and-smaller packet delivery expectation
by setting a tunnel virtual interface MTU of 1500 bytes plus extra
room to accommodate any additional encapsulations that may occur on
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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 (e.g., up to the maximum MTU size of the underlying
links), but should select a value that is not so large as to cause
excessive internally-generated ICMP PTBs coming from within the
tunnel interface (see: Section 4.2.4).
4.2.2. SEAL Maximum Segment Size (S-MSS) Maintenance
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 path
MTU discovery cache). The ITE initializes S-MSS to the MTU of the
underlying link minus ENCAPS, and decreases or increases S-MSS based
on any Fragmentation Report (FRAGREP) messages received (see: Section
4.2.7).
4.2.3. Inner Packet Fragmentation
The ITE performs inner packet fragmentation *before* it admits an
inner packet into the tunnel interface.
For inner IPv4 packets larger than 1500 bytes and with the IPv4 Don't
Fragment (DF) bit set to 0, the ITE uses IPv4 fragmentation to break
the packet into 1500 byte IPv4 fragments, with the final fragment
possibly smaller than the first fragment. The IPv4 layer then admits
each fragment into the tunnel as an independent inner IPv4 packet.
These IPv4 fragments will ultimately be reassembled by the final
destination. (Note that inner fragmentation may not be available for
certain ITE types, e.g., for tunnel-mode IPsec.)
For all other inner packets, 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 to the source.
4.2.4. SEAL Segmentation and Encapsulation
The ITE performs SEAL segmentation and encapsulation *after* it
admits an inner packet into the tunnel interface.
For inner IP packets larger than (2KB-ENCAPS) and also larger than
S-MSS, the ITE drops the packet and sends an ICMP PTB message back to
the source. Otherwise, the ITE encapsulates the packet in any mid-
layer '*' headers (for '*' other than the SEAL header). Next, if the
inner IP packet plus '*' headers 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
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byte of the previous segment, i.e., the segments MUST NOT overlap.
Note that this SEAL segmentation and encapsulation ignores the DF bit
in the inner IPv4 header or (in the case of IPv6) ignores the fact
that the network is not permitted to perform IPv6 fragmentation.
This segmentation process is a mid-layer (not an IP layer) operation
employed by the ITE to adapt the inner IP packet to the subnetwork
path characteristics, and the ETE will restore the inner 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 encapsulates each segment in a SEAL header formatted 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID Extension |R|M|CTL|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 16-bit ID field in the outer IPv4
header; encodes the most-significant 16 bits of a 32 bit SEAL-ID
value.
R (1)
Reserved.
M (1)
the "More Segments" bit. Set to 1 if this SEAL-encapsulated
packet contains a non-final segment of a multi-segment inner IP
packet.
CTL (2)
a 2-bit "Control" field that identifies the type of SEAL-
encapsulated packet as follows:
'00' - a Fragmentation Report (FRAGREP).
'01' - a non-probe.
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'10' - an implicit probe.
'11' - an explicit probe.
Segment (4)
a 4-bit Segment number. Encodes a segment number between 0 - 15.
Next Header (8) an 8-bit field that encodes an IP protocol number
the same as for the IPv4 protocol and IPv6 next header fields.
For single-segment inner IP packets, the ITE encapsulates the segment
in a SEAL header with (M=0; Segment=0). For N-segment inner 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).
The ITE next sets CTL in the SEAL header of each segment as specified
in Section 4.2.6, then writes the IP protocol number corresponding to
the inner packet in the SEAL 'Next Header' field. Finally, the ITE
encapsulates the segment in the requisite */IPv4 outer headers
according to the specific encapsulation format (e.g., [RFC2003],
[RFC4213], etc.) then sets packet identification values as described
below.
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-encapsulated 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 tunnels that may traverse an IPv4 Network Address Translator
(NAT), 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-encapsulated
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.
4.2.5. Sending SEAL packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
the outer IPv4 header of every outer packet it sends.
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The ITE then sends each outer packet that encapsulates a segment of
the same inner packet into the tunnel in canonical order, i.e.,
Segment 0 first, then Segment 1, etc. and finally Segment N-1.
4.2.6. Sending S-MSS Probes
When S-MSS is larger than 128, the ITE sends each data packet as an
implicit probe to detect any in-the-network IPv4 fragmentation. The
ITE sets CTL='10' in the SEAL header and DF=0 in the outer IPv4
header of each SEAL-encapsulated packet, and will receive FRAGREP
messages from the ETE if fragmentation occurs. When S-MSS=128, the
ITE instead sets CTL='01' in the SEAL header to avoid generating
FRAGREPs for unavoidable in-the-network fragmentation.
The ITE additionally sends explicit probes periodically to manage a
window of SEAL-IDs of outstanding probes that allows the ITE to
validate any FRAGREPs it receives. The ITE sends explicit probes by
setting CTL='11' in the SEAL header and DF=0 in the IPv4 header,
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".
The ITE should also send explicit probes that are larger than S-MSS
periodically to detect increases in the path MTU to the ETE; the ITE
can send a large probe using either a NULL packet or an ordinary data
packet that is padded at the end by setting the outer IPv4 length
field to a larger value than the packet's true length. When the ETE
receives an explicit probe, it will return a FRAGREP message whether
or not any in-the-network fragmentation occured.
4.2.7. Processing Fragmentation Reports (FRAGREPs)
When the ITE receives a potential FRAGREP message, it first verifies
that the message was formatted correctly (see: Section 4.3.4) and
that the SEAL-ID embedded in the encapsulated IPv4 first-fragment is
within the current window of outstanding probes. If the FRAGREP is
valid, the ITE advances the probe window and sets a variable 'LEN' to
the value in the first-fragment's IPv4 length field. If (LEN-ENCAPS)
is smaller than S-MSS and the first-fragment was also the final
fragment, the ITE discards the FRAGREP. Otherwise, it re-calculates
S-MSS as follows:
if (LEN-ENCAPS) is greater than S-MSS or LEN is at least 576
set S-MSS to (LEN-ENCAPS)
else
set S-MSS to the maximum of S-MSS/2 and 128
endif
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Finally, if the length field of the inner IP header encapsulated
within the first-fragment contains a value larger than (2KB-ENCAPS),
and the length field of the first-fragment header contains a still
larger value, the ITE discards the FRAGREP. Otherwise, it
encapsulated the inner IP packet portion embedded within the first-
fragment in an ICMP PTB to send back to the original source, with the
MTU field set to the maximum of 2KB-ENCAPS and (length of the first-
fragment minus ENCAPS).
(NB: The "576" in the S-MSS calculation above is the nominal minimum
MTU for typical IPv4 links and accounts for normal-case IPv4 first
fragments, while the "else" clause includes a "limited halving"
factor that accounts for unusual cases in which the ETE receives a
small IPv4 first-fragment [RFC1812]. This limited halving may
require multiple iterations of sending probes and receiving FRAGREPs,
but will rapidly converge to a stable value for S-MSS.)
4.2.8. Processing ICMP PTBs
SInce the ITE sends all SEAL-encapsulated packets with DF=0, it
unconditionally ignores any ICMP PTBs pertaining to SEAL-encapsulated
packets that it receives from within the tunnel.
4.3. ETE Specification
4.3.1. Reassembly Buffer Requirements
ETEs MUST be capable of using IPv4-layer reassembly to reassemble
SEAL-encapsulated outer packets of at least 2KB bytes, and MUST also
be capable of using SEAL-layer reassembly to reassemble inner IP
packets of (2KB-ENCAPS).
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.
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
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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.).
For SEAL-encapsulated packets that are larger than 2KB and that
arrive as multiple IPv4 fragments, the ETE uses the IPv4 first
fragment to generate a FRAGREP as specified in Section 4.3.4. The
ETE then discards all non-initial IPv4 fragemnts and decapsulates the
inner packet from the first fragment only. If the entire inner
packet is a single-segment SEAL packet that was fully-contained
within the IPv4 first fragment (i.e., all non-initial IPv4 fragments
contained only padding bytes), the ETE forwards the inner packet as-
normal; otherwise it drops the packet. This ensures that tunnel is
consistent in its handling of large inner packets.
4.3.3. SEAL-Layer Reassembly
After IPv4-layer reassembly, the ETE performs SEAL-layer reassembly
through simple in-order concatenation of the encapsulated segments
from N consecutive SEAL-encapsulated packets from the same inner
packet. These packets contain Segment numbers 0 through N-1 with
M=0/1 in final and non-final segments, respectively, and with
consecutive SEAL-ID values encoded in the 32-bit concatenation of the
ID Extension field in the SEAL header and the ID field in the IPv4
header. That is, for an N-segment inner 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 an IPv4 Network Address Translator (NAT), the ETE
instead uses only the 16-bit value in the ID extension field in the
SEAL header as a 16-bit SEAL-ID value, and uses mod 2^16 arithmetic
to associate the segments of the same packet.)
SEAL-layer reassembly requires the ETE to maintain a cache of
recently received SEAL packets 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.
4.3.4. Generating Fragmentation Reports (FRAGREPs)
When the ETE receives the IPv4 first-fragment of a SEAL packet that
was delivered as multiple IPv4 fragments and with CTL='10' in the
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SEAL header, it sends a FRAGREP message back to the ITE. The ETE
also sends a FRAGREP for any SEAL packet with CTL='11', i.e., even if
the packet was not fragmented and while treating the unfragmented
packet the same as a first-fragment.
The ETE prepares the FRAGREP message by encapsulating the leading 256
bytes (or up to the end) of the first-fragment in outer SEAL/*/IPv4
headers as shown in Figure 3:
+-------------------------+ -
| | \
~ Outer */IPv4 headers ~ |
~ of FRAGREP ~ > FRAGREP headers
| | |
+-------------------------+ |
| SEAL Header of FRAGREP | /
+-------------------------+ -
| | \
~ IP/*/SEAL/*/IPv4 ~ |
~ hdrs of first-fragment ~ |
| | > First 256 bytes (or up to
+-------------------------+ | the end) of first-fragment
| | |
~ Data of first-fragment ~ |
| | /
+-------------------------+ -
Figure 3: Fragmentation Report (FRAGREP) Message
The ETE next sets CTL='00', Segment=0 and M=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
according to the specific encapsulation type. The ETE then sets the
FRAGREP's destination address to the source address of the first-
fragment and sets the FRAGREP's 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 FRAGREP's
source address to an address assigned to the underlying IPv4
interface. Finally, the ETE sends the FRAGREP to the ITE.
5. 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
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segmentation and reassembly such that an MTU of at least 576 can
still be presented to the IP layer.
6. End System Requirements
SEAL provides robust mechanisms for returning ICMP 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].
7. Router Requirements
IPv4 routers within the subnetwork observe the requirements in
[RFC1812], and are strongly encouraged to implement IPv4
fragmentation such that the first fragment is the largest and
approximately the size of the underlying link MTU.
8. IANA Considerations
SEAL will use the IANA-assigned value of 253 as an IP protocol value
for experimentation purposes [RFC3692]; therefore, this document has
no actions for IANA.
9. 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 FRAGREP 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 FRAGREPs.
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.
10. Acknowledgments
Path MTU determination through the report of fragmentation
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experienced by the final destination was first proposed by Charles
Lynn of BBN on the TCP-IP mailing list in May 1987. An historical
analysis of the evolution of path MTU discovery appears in
http://www.tools.ietf.org/html/draft-templin-v6v4-ndisc-01 and is
reproduced in Appendix A of this document.
The following individuals are acknowledged for helpful comments and
suggestions: Jari Arkko, Fred Baker, Teco Boot, Iljitsch van Beijnum,
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.
11. References
11.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[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.
11.2. 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.
[I-D.ietf-manet-smf]
Macker, J. and S. Team, "Simplified Multicast Forwarding
for MANET", draft-ietf-manet-smf-07 (work in progress),
February 2008.
[I-D.templin-autoconf-dhcp]
Templin, F., Russert, S., and S. Yi, "The MANET Virtual
Ethernet (VET) Abstraction",
draft-templin-autoconf-dhcp-14 (work in progress),
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April 2008.
[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.
[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.
[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.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
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[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 (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)
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].
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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
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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