Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track February 22, 2010
Expires: August 26, 2010
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-11.txt
Abstract
For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected IP network routing
region and bounded by encapsulating 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.
Status of this Memo
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This Internet-Draft will expire on August 26, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9
4. SEAL with Segmentation and Reassembly (SEAL-SR) Protocol
Specification . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 10
4.2. SEAL-SR Header Format (Mode 1) . . . . . . . . . . . . . . 13
4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
4.3.2. Tunnel Interface Soft State . . . . . . . . . . . . . 15
4.3.3. Inner Fragmentation . . . . . . . . . . . . . . . . . 16
4.3.4. Mid-Layer Encapsulation . . . . . . . . . . . . . . . 16
4.3.5. SEAL Segmentation . . . . . . . . . . . . . . . . . . 17
4.3.6. Outer Encapsulation . . . . . . . . . . . . . . . . . 17
4.3.7. Probing Strategy . . . . . . . . . . . . . . . . . . . 18
4.3.8. Packet Identification . . . . . . . . . . . . . . . . 18
4.3.9. Sending SEAL Protocol Packets . . . . . . . . . . . . 18
4.3.10. Processing Raw ICMP Messages . . . . . . . . . . . . . 19
4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 19
4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 19
4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 20
4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 20
4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 21
4.5. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 21
4.5.1. Generating SCMP Messages . . . . . . . . . . . . . . . 22
4.5.2. Processing SCMP Messages . . . . . . . . . . . . . . . 24
4.6. TE Window Synchronization and Maintenance . . . . . . . . 25
5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol
Specification . . . . . . . . . . . . . . . . . . . . . . . . 27
5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 28
5.2. SEAL-FS Header Format (Version 0) . . . . . . . . . . . . 28
5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 28
5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 28
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5.3.2. Tunnel Interface Soft State . . . . . . . . . . . . . 29
5.3.3. Inner Fragmentation . . . . . . . . . . . . . . . . . 29
5.3.4. Mid-Layer Encapsulation . . . . . . . . . . . . . . . 29
5.3.5. SEAL Segmentation . . . . . . . . . . . . . . . . . . 30
5.3.6. Outer Encapsulation . . . . . . . . . . . . . . . . . 30
5.3.7. Probing Strategy . . . . . . . . . . . . . . . . . . . 30
5.3.8. Packet Identification . . . . . . . . . . . . . . . . 30
5.3.9. Sending SEAL Protocol Packets . . . . . . . . . . . . 30
5.3.10. Processing Raw ICMP Messages . . . . . . . . . . . . . 30
5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 30
5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 30
5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 30
5.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 30
5.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 31
5.5. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 31
5.6. TE Window Synchronization and Maintenance . . . . . . . . 31
6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 31
7. End System Requirements . . . . . . . . . . . . . . . . . . . 31
8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
10. Security Considerations . . . . . . . . . . . . . . . . . . . 32
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 32
12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 33
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . . 34
14.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 38
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 39
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 39
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (including
tunnels of one form or another) over an actual network that supports
the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual
topologies have elements that 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 that are often not
even visible to the endpoints of the virtual hop. This introduces
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 visible to the inner network
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 found 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]. The following subsections
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 IPv4 address
space is rapidly becoming depleted, there is a lesser-known but
growing consensus that other IPv4 protocol limitations have already
or may soon become problematic.
First, the IPv4 header Identification field is only 16 bits in
length, meaning that at most 2^16 unique packets with the same
(source, destination, protocol)-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 for high data rate packet
sources such as tunnel endpoints [RFC4963]. 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 minor annoyances (e.g.,
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in-the-network router fragmentation) to the potential for major
integrity issues (e.g., mis-association of the fragments of multiple
IP packets during reassembly [RFC4963]).
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 D). 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" [RFC2923].
In particular, site border routers in the Internet are being
configured more and more 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
[I-D.ietf-tcpm-icmp-attacks]. Furthermore, when a source node that
requires ICMP error message feedback when a packet 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. This behavior has been
confirmed through documented studies showing clear evidence of path
MTU discovery failures in the Internet today [TBIT][WAND].
The issues with both IPv4 fragmentation and this "classical" method
of path MTU discovery are exacerbated further when IP tunneling is
used [RFC4459]. For example, ingress tunnel endpoints (ITEs) may be
required to forward encapsulated packets into the subnetwork on
behalf of hundreds, thousands, or even more original sources in the
end site. If the ITE allows IPv4 fragmentation on the encapsulated
packets, persistent fragmentation could lead to undetected data
corruption due to Identification field wrapping. If the ITE instead
uses classical IPv4 path MTU discovery, it may be inconvenienced by
excessive ICMP error messages coming from the subnetwork that may be
either suspect or contain insufficient information for translation
into error messages 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 that configures an IPsec
tunnel to transcribe the encrypted packet fragment contained in an
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ICMP error message into a suitable ICMP error message to return to
the original source.
Although recent works have led to the development of a robust end-to-
end MTU determination scheme [RFC4821], this approach requires
tunnels to present a consistent MTU the same as for ordinary links on
the end-to-end path. Moreover, in current practice existing
tunneling protocols mask the MTU issues by selecting a "lowest common
denominator" MTU that may be much smaller than necessary for most
paths and difficult to change at a later date. Due to these many
consideration, a new approach to accommodate tunnels over links with
diverse MTUs is necessary.
1.2. Approach
For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected network routing region
and bounded by encapsulating border nodes. Examples include the
global Internet interdomain routing core, Mobile Ad hoc Networks
(MANETs) and 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 that 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 tunneling network layer protocols (e.g., IP, OSI,
etc.) over IP subnetworks that connect Ingress and Egress Tunnel
Endpoints (ITEs/ETEs) of border nodes. It provides a modular
specification designed to be tailored to specific associated IP
tunneling protocols. A transport-mode of operation is also possible,
and described in Appendix C. SEAL accommodates links with diverse
MTUs, protects against off-path denial-of-service attacks, and
supports efficient duplicate packet detection through the use of a
minimal mid-layer encapsulation.
SEAL specifically treats tunnels that traverse the subnetwork as
unidirectional links that must support network layer services. As
for any link, tunnels that use SEAL must provide suitable networking
services including best-effort datagram delivery, integrity and
consistent handling of packets of various sizes. As for any link
whose media cannot provide suitable services natively, tunnels that
use SEAL employ link-level adaptation functions to meet the
legitimate expectations of the network layer service. As this is
essentially a link level adaptation, SEAL is therefore permitted to
alter packets within the subnetwork as long as it restores them to
their original form when they exit the subnetwork. The mechanisms
described within this document are designed precisely for this
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purpose.
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. As a result, SEAL can naturally tune
its packet sizing parameters to eliminate the in-the-network
fragmentation. This approach is in contrast to existing tunneling
protocol practices which seek to avoid MTU issues by selecting a
"lowest common denominator" MTU that may be overly conservative for
many tunnels and difficult to change even when larger MTUs become
available.
The following sections provide the SEAL normative specifications,
while the appendices present non-normative additional considerations.
2. Terminology and Requirements
The following terms are defined within the scope of this document:
subnetwork
a virtual topology configured over a connected network routing
region and bounded by encapsulating border nodes.
Ingress Tunnel Endpoint
a virtual interface over which an encapsulating border node (host
or router) sends encapsulated packets into the subnetwork.
Egress Tunnel Endpoint
a virtual interface over which an encapsulating border node (host
or router) receives encapsulated packets from the subnetwork.
inner packet
an unencapsulated network layer protocol packet before any mid-
layer or outer encapsulations are added. Note that not only IPv6
and IPv4, but also any other network layer protocol type packet
(e.g., OSI/CLNP [RFC1070]) can appear as an inner packet.
Internet protocol numbers that identify inner packets are found in
the IANA Internet Protocol registry [RFC3232].
mid-layer packet
a packet resulting from adding mid-layer encapsulating headers to
an inner packet.
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outer IP packet
a packet resulting from adding an outer IP header to a mid-layer
packet.
packet-in-error
the leading portion of an invoking data packet encapsulated in the
body of an error control message (e.g., an ICMPv4 [RFC0792] error
message, an ICMPv6 [RFC4443] error message, etc.).
IP, IPvX, IPvY
used to generically refer to either IP protocol version, i.e.,
IPv4 or IPv6.
The following abbreviations correspond to terms used within this
document and elsewhere in common Internetworking nomenclature:
PTB - a Packet Too Big message recognized by the inner network
layer, e.g., an ICMPv6 "Packet Too Big" message [RFC4443], an
ICMPv4 "Fragmentation Needed" message [RFC0792], etc.
DF - the IPv4 header "Don't Fragment" flag [RFC0791]
MHLEN - the length of any mid-layer headers and trailers
OHLEN - the length of the outer encapsulating headers and
trailers, including the outer IP header, the SEAL header and any
other outer headers and trailers.
HLEN - the sum of MHLEN and OHLEN
S_MRU - the SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size
SCMP - the SEAL Control Message Protocol
SEAL_ID - a 48-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-FS - SEAL with Fragmentation Sensing
SEAL-SR - SEAL with Segmentation and Reassembly
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ITE - Ingress Tunnel Endpoint
ETE - Egress Tunnel Endpoint
TE - Tunnel Endpoint (i.e., either ingress or egress)
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]. When used
in lower case (e.g., must, must not, etc.), these words MUST NOT be
interpreted as described in [RFC2119], but are rather interpreted as
they would be in common English.
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, ISP
networks, SOHO networks, the interdomain routing core, and many
others. In particular, SEAL is a natural complement to the
enterprise network abstraction manifested through the VET mechanism
[I-D.templin-intarea-vet] and the RANGER architecture
[I-D.templin-ranger][I-D.russert-rangers].
SEAL can be used as a network sublayer for encapsulation of an inner
packet within outer encapsulating headers. For example, for IPvX in
IPvY encapsulation (e.g., as IPv4/SEAL/IPv6), the SEAL header appears
as a subnetwork encapsulation as seen by the inner IP layer. SEAL
can also be used as a sublayer within a UDP data payload (e.g., as
IPv4/UDP/SEAL/IPv6 similar to Teredo [RFC4380]), where UDP
encapsulation is typically used for operation over subnetworks that
give preferential treatment to the "core" Internet protocols (i.e.,
TCP and UDP). The SEAL header is processed the same as for IPv6
extension headers, i.e., it is not part of the outer IP header but
rather allows for the creation of an arbitrarily extensible chain of
headers in the same way that IPv6 does.
This document specifies two modes of operation for the SEAL protocol
known as "SEAL with Fragmentation Sensing (SEAL-FS)" and "SEAL with
Segmentation and Reassembly (SEAL-SR)". SEAL-FS provides a minimal
mechanism through which the egress tunnel endpoint (ETE) acts as a
passive observer that simply informs the ingress tunnel endpoint
(ITE) of any MTU limitations. SEAL-FS therefore determines the
tunnel MTU based on the MTU of the smallest link in the path. It is
useful for determining an appropriate MTU for tunnels between
performance-critical routers connected to high data rate subnetworks
such as the Internet DFZ, as well as for other uses in which packet
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segmentation and reassembly would present too great of a burden for
the routers or end systems.
SEAL-SR is a functional superset of SEAL-FS, and requires that the
tunnel endpoints support segmentation and reassembly of packets that
are too large to traverse the tunnel without fragmentation. SEAL-SR
determines the tunnel MTU based on the largest packet the ETE is
capable of receiving rather than on the MTU of the smallest link in
the path. Therefore, SEAL-SR can transport packets that are much
larger than the underlying links themselves can carry in a single
piece, i.e., even if IPv6 jumbograms are used [RFC2675].
SEAL-SR tunnels may be configured over paths that include not only
ordinary physical links, but also virtual links that may include
SEAL-FS tunnels or even other SEAL-SR tunnels. An example
application would be linking two geographically remote supercomputer
centers with large MTU links by configuring a SEAL_TE tunnel across
the Internet. A second example would be support for sub-IP
segmentation over low-end links, i.e., especially over wireless
transmission media such as IEEE 802.15.4, broadcast radio links in
Mobile Ad-hoc Networks (MANETs), Very High Frequency (VHF) civil
aviation data links, etc.
Many other use case examples for both SEAL-FS and SEAL-SR are
anticipated, and will be identified as further experience is gained.
4. SEAL with Segmentation and Reassembly (SEAL-SR) Protocol
Specification
This section specifies the fully-functioned mode of SEAL known as
"SEAL with Segmentation and Reassembly (SEAL-SR)"; a minimal mode
known as "SEAL with Fragmentation Sensing (SEAL-FS)" is specified in
Section 5. SEAL-SR is a superset of SEAL-FS, and differs only in its
segmentation and reassembly requirements. SEAL-SR and SEAL-FS are
distinguished simply by a mode value in the SEAL header. The
following sections therefore specify SEAL-SR, but use the simple term
"SEAL" since the same formats and mechanisms apply also to SEAL-FS.
4.1. Model of Operation
SEAL is an encapsulation sublayer that supports a multi-level
segmentation and reassembly capability for the transmission of
unicast and multicast packets across an underlying IP subnetwork with
heterogeneous links. First, the ITE can use IPv4 fragmentation to
fragment inner IPv4 packets before SEAL encapsulation if necessary.
Secondly, the SEAL layer itself provides a simple cutting-and-pasting
capability for mid-layer packets to avoid IP fragmentation on the
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outer packet. Finally, ordinary IP fragmentation is permitted on the
outer packet after SEAL encapsulation and is used to detect and tune
out any in-the-network fragmentation.
SEAL-enabled ITEs encapsulate each inner packet in mid-layer headers
and trailers, segment the resulting mid-layer packet into multiple
segments if necessary, then append a SEAL header and (if necessary) a
UDP header to each segment. The ITE then adds the outer
encapsulation headers to each segment. For example, a single-segment
inner IPv6 packet encapsulated in any mid-layer headers and trailers,
the SEAL header, any outer headers and trailers and an outer IPv4
header would appear as shown in Figure 1:
+--------------------+
~ outer IPv4 header ~
+--------------------+
I ~ other outer hdrs ~
n +--------------------+
n ~ SEAL Header ~
e +--------------------+ +--------------------+
r ~ mid-layer headers ~ ~ mid-layer headers ~
+--------------------+ +--------------------+
I --> | | --> | |
P --> ~ inner IPv6 ~ --> ~ inner IPv6 ~
v --> ~ Packet ~ --> ~ Packet ~
6 --> | | --> | |
+--------------------+ +--------------------+
P ~ mid-layer trailers ~ ~ mid-layer trailers ~
a +--------------------+ +--------------------+
c ~ outer trailers ~
k Mid-layer packet +--------------------+
e after mid-layer encaps.
t Outer IPv4 packet
after SEAL and outer encaps.
Figure 1: SEAL Encapsulation - Single Segment
In a second example, an inner IPv4 packet requiring three SEAL
segments would appear as three separate outer IPv4 packets, where the
mid-layer headers are carried only in segment 0 and the mid-layer
trailers are carried in segment 2 as shown in Figure 2:
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+------------------+ +------------------+
~ outer IPv4 hdr ~ ~ outer IPv4 hdr ~
+------------------+ +------------------+ +------------------+
~ other outer hdrs ~ ~ outer IPv4 hdr ~ ~ other outer hdrs ~
+------------------+ +------------------+ +------------------+
~ SEAL hdr (SEG=0) ~ ~ other outer hdrs ~ ~ SEAL hdr (SEG=2) ~
+------------------+ +------------------+ +------------------+
~ mid-layer hdrs ~ ~ SEAL hdr (SEG=1) ~ | inner IPv4 |
+------------------+ +------------------+ ~ Packet ~
| inner IPv4 | | inner IPv4 | | (Segment 2) |
~ Packet ~ ~ Packet ~ +------------------+
| (Segment 0) | | (Segment 1) | ~ mid-layer trails ~
+------------------+ +------------------+ +------------------+
~ outer trailers ~ ~ outer trailers ~ ~ outer trailers ~
+------------------+ +------------------+ +------------------+
Segment 0 (includes Segment 1 (no mid- Segment 2 (includes
mid-layer hdrs) layer encaps) mid-layer trails)
Figure 2: SEAL Encapsulation - Multiple Segments
The SEAL header itself is inserted according to the specific
tunneling protocol. Examples include the following:
o For simple encapsulation of an inner network layer packet within
an outer IPvX header (e.g., [RFC1070][RFC2003][RFC2473][RFC4213],
etc.), the SEAL header is inserted between the inner packet and
outer IPvX headers as: IPvX/SEAL/{inner packet}.
o For IPsec encapsulations [RFC4301], the SEAL header is inserted
between the {AH,ESP} headers and outer IP headers as: IPvX/SEAL/
{AH,ESP}/{inner packet}. Here, the {AH, ESP} headers and trailers
are seen as mid-layer encapsulations.
o For encapsulations over transports such as UDP (e.g., [RFC4380]),
the SEAL header is inserted between the outer transport layer
header and the mid-layer packet, e.g., as IPvX/UDP/SEAL/{mid-layer
packet}. Here, the UDP header is seen as an "other outer header".
SEAL-encapsulated packets include a SEAL_ID to uniquely identify each
packet. Routers within the subnetwork use the SEAL_ID for duplicate
packet detection, and TEs use the SEAL_ID for SEAL segmentation/
reassembly and protection against off-path attacks. The following
sections specify the SEAL header format and SEAL-related operations
of the ITE and ETE, respectively.
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4.2. SEAL-SR Header Format (Mode 1)
The SEAL mode 1 header (i.e., the SEAL-SR header) is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MODE|A|F|M|RSV| NEXTHDR/SEG | SEAL_ID (bits 48 - 32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL_ID (bits 31 - 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SEAL Mode 1 Header Format
where the header fields are defined as:
MODE (3)
a 3-bit value that encodes the SEAL protocol mode. This section
describes Mode 1 of the SEAL protocol, i.e., the MODE field
encodes the value 1.
A (1)
the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes
to receive an explicit acknowledgement from the ETE.
F (1)
the "First Segment" bit. Set to 1 if this SEAL protocol packet
contains the first segment (i.e., Segment #0) of a mid-layer
packet.
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.
RSV (2)
a 2-bit Reserved field. Set to 0 for the purpose of this
specification.
NEXTHDR/SEG (8) an 8-bit field. When 'F'=1, encodes the next header
Internet Protocol number the same as for the IPv4 protocol and
IPv6 next header fields. When 'F'=0, encodes a segment number of
a multi-segment mid-layer packet. (The segment number 0 is
reserved.)
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SEAL_ID (48)
a 48-bit Identification field.
4.3. ITE Specification
4.3.1. 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 Layer 3 as the size for
admission of inner packets into the tunnel. Since the tunnel
interface may support a large set of ETEs that accept widely varying
maximum packet sizes, however, a number of factors should be taken
into consideration when selecting a tunnel interface MTU.
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 ICMP Packet Too Big (PTB)
message returned. When the 1500 byte packets sent by end systems
incur additional encapsulation at an ITE, however, they may be
dropped silently since the network may not always deliver the
necessary PTBs [RFC2923].
The ITE should therefore set a tunnel virtual interface MTU of at
least 1500 bytes plus extra room to accommodate any additional
encapsulations that may occur on the path from the original source.
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. 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 ITE can alternatively set an indefinite MTU on the tunnel virtual
interface such that all inner packets are admitted into the interface
without regard to size. For ITEs that host applications, this option
must be carefully coordinated with protocol stack upper layers, since
some upper layer protocols (e.g., TCP) derive their packet sizing
parameters from the MTU of the outgoing interface and as such may
select too large an initial size. This is not a problem for upper
layers that use conservative initial maximum segment size estimates
and/or when the tunnel interface can reduce the upper layer's maximum
segment size (e.g., the size advertised in the TCP MSS option) based
on the per-neighbor MTU.
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The inner network layer protocol consults the tunnel interface MTU
when admitting a packet into the interface. For inner IPv4 packets
with the IPv4 Don't Fragment (DF) bit set to 0, if the packet is
larger than the tunnel interface MTU the inner IPv4 layer uses IPv4
fragmentation to break the packet into fragments no larger than the
tunnel interface MTU. The ITE then admits each fragment into the
tunnel as an independent packet.
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 a PTB error message to the source with the MTU value set to
the tunnel interface MTU. The message must contain as much of the
invoking packet as possible without the entire message exceeding the
network layer minimum MTU (e.g., 576 bytes for IPv4, 1280 bytes for
IPv6, etc.).
Note that when the tunnel interface sets an indefinite MTU the ITE
unconditionally admits all packets into the interface without
fragmentation. In light of the above considerations, it is
RECOMMENDED that the ITE configure an indefinite MTU on the tunnel
virtual interface and handle any per-neighbor MTU mismatches within
the tunnel virtual interface (e.g., by reducing the size advertised
in the TCP MSS option).
4.3.2. Tunnel Interface Soft State
For each ETE, the ITE maintains soft state within the tunnel
interface (e.g., in a neighbor cache) used to support inner
fragmentation and SEAL segmentation for packets admitted into the
tunnel interface. The soft state includes the following:
o a Mid-layer Header Length (MHLEN); set to the length of any mid-
layer encapsulation headers and trailers (e.g., AH, ESP, NULL,
etc.) that must be added before SEAL segmentation.
o an Outer Header Length (OHLEN); set to the length of the outer IP,
SEAL and other outer encapsulation headers and trailers.
o a total Header Length (HLEN); set to MHLEN plus OHLEN.
o a SEAL Maximum Segment Size (S_MSS); initialized to a value that
is no larger than the largest MTU of all underlying IP interfaces.
The ITE decreases or increases S_MSS based on any SCMP Segment
Report messages received (see Section 4.5).
o a SEAL Maximum Reassembly Unit (S_MRU). The ITE initializes S_MRU
to "infinity" and decreases or increases S_MRU based on any SCMP
Segment Report messages received (see Section 4.5). When
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(S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256) as the effective
S_MRU value. The ITE maintains S_MRU as a 32-bit value such that
inner packets larger than 64KB (e.g., IPv6 jumbograms [RFC2675])
can be accommodated when appropriate for a given subnetwork.
Note that S_MSS and S_MRU include the length of the outer and mid-
layer encapsulating headers and trailers (i.e., HLEN), since the ETE
must retain the headers and trailers during reassembly.
4.3.3. Inner Fragmentation
After an inner packet/fragment has been admitted into the tunnel
interface the ITE uses the following algorithm to determine whether
the packet can be accommodated and (if so) whether (further) inner IP
fragmentation is needed:
o if the inner packet is unfragmentable (e.g., an IPv6 packet, an
IPv4 packet with DF=1, etc.), and the packet is larger than (S_MRU
- HLEN), the ITE drops the packet and sends a PTB message to the
original source with an MTU value of (S_MRU - HLEN); else,
o if the inner packet is fragmentable (e.g., an IPv4 packet with
DF=0), and the packet is larger than (MIN(S_MRU, S_MSS) - HLEN),
the ITE uses inner fragmentation to break the packet into
fragments no larger than (MIN(S_MRU, S_MSS) - HLEN); else,
o if the inner packet is a non-IP protocol packet, packet sizing
considerations specific to the inner protocol are observed; else,
o the ITE processes the packet without inner fragmentation.
(Note that in the above the ITE must also track whether the tunnel
interface is using header compression. If so, the ITE must include
the length of the uncompressed headers and trailers when calculating
the total length of the packet.)
4.3.4. Mid-Layer Encapsulation
After inner IP fragmentation (if necessary), the ITE next
encapsulates each inner packet/fragment in the MHLEN bytes of mid-
layer headers and trailers. (For example, when IPsec ESP is used
[RFC4301], the ITE performs the necessary security transformations on
the inner packet/fragment then adds an ESP header and trailer.) The
ITE then presents the mid-layer packet for SEAL segmentation and
outer encapsulation.
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4.3.5. SEAL Segmentation
After mid-layer encapsulation, if the length of the resulting mid-
layer packet plus OHLEN is greater than S_MSS, the ITE must
additionally perform SEAL segmentation. To do so, it breaks the mid-
layer packet into N segments (N <= 256) that are no larger than
(S_MSS - OHLEN) bytes each. Each segment, except the final one, MUST
be of equal length. The first byte of each segment MUST begin
immediately after the final byte of the previous segment, i.e., the
segments MUST NOT overlap. The ITE SHOULD generate the smallest
number of segments possible, e.g., it SHOULD NOT generate 6 smaller
segments when the packet could be accommodated with 4 larger
segments.
Note that this SEAL segmentation ignores the fact that the mid-layer
packet may be unfragmentable outside of the subnetwork. 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 reassembly. Therefore, the fact that the packet
may have been segmented within the subnetwork is not observable
outside of the subnetwork.
4.3.6. Outer Encapsulation
Following SEAL segmentation, the ITE next encapsulates each segment
in a SEAL header formatted as specified in Section 4.2 with MODE=1,
RSV=0. For the first segment, the ITE sets F=1, then sets NEXTHDR to
the Internet Protocol number of the encapsulated inner packet, and
finally sets M=1 if there are more segments or sets M=0 otherwise.
For each non-initial segment of an N-segment mid-layer packet (N <=
256), the ITE sets (F=0; M=1; SEG=1) in the SEAL header of the first
non-initial segment, sets (F=0; M=1; SEG=2) in the next non-initial
segment, etc., and sets (F=0; M=0; SEG=N-1) in the final segment.
(Note that the value SEG=0 is not used, since the initial segment
encodes a NEXTHDR value and not a SEG value.)
The ITE next encapsulates each segment in the requisite outer headers
and trailers according to the specific encapsulation format (e.g.,
[RFC2003], [RFC2473], [RFC4213], [RFC4380], etc.), except that it
writes 'SEAL_PROTO' in the protocol field of the outer IP header
(when simple IP encapsulation is used) or writes 'SEAL_PORT' in the
outer destination service port field (e.g., when IP/UDP encapsulation
is used). The ITE finally sets the A bit if necessary as specified
in Section 4.3.7, sets the packet identification values as specified
in Section 4.3.8 and sends the packets as specified in Section 4.3.9.
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4.3.7. Probing Strategy
All SEAL encapsulated packets sent by the ITE are considered implicit
probes, and will elicit SCMP Segment Report messages from the ETE
(see: Section 4.5) with a new value for S_MSS if any IP fragmentation
occurs in the path. Thereafter, the ITE can periodically reset S_MSS
to a larger value (e.g., the underlying IP interface MTU minus OHLEN
bytes) to detect path MTU increases.
The ITE also sends explicit probes, periodically, to verify that the
ETE is still reachable and to manage a window of SEAL_IDs (see:
Section 4.6). The ITE sets A=1 in the SEAL header of a segment 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 NEXTHDR field to
a value of "No Next Header" (see Section 4.7 of [RFC2460]). The
probe will elicit an SCMP Segment Report message from the ETE as an
acknowledgement. Finally, the ITE MAY send "expendable" outer IP
probe packets (see Section 4.3.9) as explicit probes in order to
generate PTB messages from routers on the path to the ETE.
In all cases, the ITE MUST be conservative in its use of the A bit in
order to limit the resultant control message overhead.
4.3.8. Packet Identification
The ITE maintains a randomly-initialized 48-bit SEAL_ID value as per-
ETE soft state (e.g., in the neighbor cache) and monotonically
increments it for each successive SEAL protocol packet it sends to
the ETE. For each successive SEAL protocol packet, the ITE writes
the current SEAL_ID value into the header field of the same name in
the SEAL header.
4.3.9. Sending SEAL Protocol Packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
the header of each outer IPv4 packet to ensure that they will be
delivered to the ETE even if they are fragmented within the
subnetwork. (The ITE can instead set DF=1 for "expendable" outer
IPv4 packets (e.g., for NULL packets used as probes -- see Section
4.3.7), but these may be lost due to an MTU restriction). For outer
IPv6 packets, the "DF" bit is always implicitly set to 1; hence, they
will not be fragmented within the subnetwork.
The ITE 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.
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4.3.10. Processing Raw ICMP Messages
The ITE may receive "raw" ICMP error messages [RFC0792][RFC4443] from
either the ETE or routers within the subnetwork that comprise an
outer IP header, followed by an ICMP header, followed by a portion of
the SEAL packet that generated the error (also known as the "packet-
in-error"). The ITE can use the SEAL_ID encoded in the packet-in-
error as a nonce to confirm that the ICMP message came from either
the ETE or an on-path router, and can use any additional information
to determine whether to accept or discard the message.
The ITE should specifically process raw ICMPv4 Protocol Unreachable
messages and ICMPv6 Parameter Problem messages with Code
"Unrecognized Next Header type encountered" as a hint that the ETE
does not implement the SEAL protocol; specific actions that the ITE
may take in this case are out of scope.
4.4. ETE Specification
4.4.1. Reassembly Buffer Requirements
ETEs MUST be capable of performing IP-layer and SEAL-layer reassembly
for SEAL protocol IP packets of at least 2KB in length. Hence, ETEs:
o MUST configure an outer IP reassembly buffer size of at least 2KB.
o MUST configure a SEAL layer reassembly buffer size (i.e., S_MRU)
of at least 2KB.
o MUST be capable of discarding outer packets that (after
reassembly) are larger than S_MRU.
Note that the ETE must retain the outer IP, SEAL and other outer
headers and trailers during both IP-layer and SEAL-layer reassembly
for the purpose of associating the fragments/segments of the same
packet. Therefore, S_MRU includes the length of the outer and mid-
layer encapsulating headers and trailers (i.e., HLEN).
Note also that the ETE can maintain S_MRU as a per-ITE value as
opposed to a single value to be applied for all ITEs. In that case,
the ETE can manage each per-ITE S_MRU value separately (e.g., to
reduce congestion caused by excessive segmentation from specific
ITEs) but should seek to maintain as stable a value as possible for
each ITE.
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4.4.2. IP-Layer Reassembly
ETEs perform standard IP-layer reassembly for SEAL protocol IP
fragments, and should maintain a conservative reassembly cache high-
and low-water mark. When the size of the reassembly cache exceeds
this high-water mark, the ETE should actively discard incomplete
reassemblies (e.g., using an Active Queue Management (AQM) strategy)
until the size falls below the low-water mark. The ETE should also
actively discard any pending reassemblies that clearly have no
opportunity for completion, e.g., when a considerable number of new
fragments have been received before a fragment that completes a
pending reassembly has arrived.
When the ETE processes the IP first fragment (i.e., one with MF=1 and
Offset=0 in the IP header) of a fragmented SEAL packet, it sends an
SCMP Segment Report message back to the ITE (see Section 4.5).
4.4.3. SEAL-Layer Reassembly
Following IP reassembly of a SEAL segment, the ETE adds the segment
to a SEAL-Layer pending-reassembly queue according to the (Source,
Destination, SEAL_ID)-tuple found in the outer IP and SEAL headers.
The ETE performs SEAL-layer reassembly through simple in-order
concatenation of the encapsulated segments of the same mid-layer
packet from N consecutive SEAL packets. SEAL-layer reassembly
requires the ETE to maintain a cache of recently received segments
for a hold time that would allow for nominal inter-segment delays.
When a SEAL reassembly times out, the ETE discards the incomplete
reassembly and returns an SCMP "Time Exceeded" message to the ITE
(see Section 4.5). As for IP-layer reassembly, the ETE should also
maintain a conservative reassembly cache high- and low-water mark and
should 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.
When the ETE receives a SEAL packet with an incorrect value in the
SEAL header, it discards the packet and returns an SCMP "Parameter
Problem" message (see Section 4.5). If the ETE receives a SEAL
packet for which a segment with the same (Source, Destination,
SEAL_ID)-tuple is already in the queue, it must determine whether to
accept the new segment and release the old, or drop the new segment.
If accepting the new segment would cause an inconsistency with other
segments already in the queue (e.g., differing segment lengths), the
ETE drops the segment that is least likely to complete the
reassembly. If the ETE accepts a new SEAL segment that would cause
the reassembled outer packet to be larger than S_MRU following
reassembly, it discontinues the reassembly and sends an SCMP Segment
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Report message with the MTU field set to S_MRU (see Section 4.5).
After all segments are gathered, the ETE reassembles the packet by
concatenating the segments encapsulated in the N consecutive SEAL
packets beginning with the initial segment (i.e., SEG=0) and followed
by any non-initial segments 1 through N-1. That is, for an N-segment
mid-layer packet, reassembly entails the concatenation of the SEAL-
encapsulated packet segments with (F=1, M=1, SEAL_ID=j) in the first
SEAL header, followed by (F=0, M=1, SEG=1, SEAL_ID=(j+1)) in the next
SEAL header, followed by (F=0, M=1, SEG=2, SEAL_ID=(j+2)), etc., up
to (F=0, M=0, SEG=(N-1), SEAL_ID=(j + N-1)) in the final SEAL header.
(Note that modulo arithmetic based on the length of the SEAL_ID field
is used).
4.4.4. Decapsulation and Delivery to Upper Layers
Following IP- and SEAL-layer reassembly, the ETE discards the outer
headers and trailers and performs any mid-layer transformations
(e.g., IPsec ESP) on the mid-layer packet. The ETE next discards the
mid-layer headers and trailers, and delivers the inner packet to the
upper-layer protocol indicated either in the SEAL NEXTHDR field or
the next header field of the mid-layer packet (i.e., if the packet
included mid-layer encapsulations). The ETE instead silently
discards the inner packet if it was a NULL packet (see Section
4.3.9).
4.5. The SEAL Control Message Protocol (SCMP)
SEAL uses a companion SEAL Control Message Protocol (SCMP) that
implements the same message format as the Internet Control Message
Protocol for IPv6 (ICMPv6) [RFC4443]. SCMP messages are further
identified by the NEXTHDR value '58' (i.e., the official IANA
protocol number for ICMPv6), however the SCMP message is *not*
immediately preceded by an inner IPv6 header as is the case for
ICMPv6 messages. Instead, SCMP messages appear immediately following
either the SEAL header or mid-layer header (i.e., if the packet
included mid-layer encapsulations). Therefore, this differing header
arrangement is the sole means by which TEs differentiate SCMP
messages from ordinary ICMPv6 messages. Unlike ICMPv6 messages, SCMP
messages are used only for the purpose of conveying information
between TEs, i.e., they are used only for information sharing within
the tunnel and not beyond the tunnel.
SCMP messages use the same message types specified for ordinary
ICMPv6 messages in [RFC4443][RFC4861]. SCMP can also be used to
carry other ICMPv6 message types (e.g., [RFC4191], etc.) in manners
that are outside the scope of this document. SCMP messages are
formatted as shown in Figure 4:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Message Body ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking SEAL data |
~ packet as possible without the SCMP ~
| packet exceeding 576 bytes (*) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(*) the invoking SEAL packet segment (i.e., the "packet-in-error")
is only included for SCMP messages sent in response to data packets
Figure 4: SCMP Message Format
As for ICMPv4 and ICMPv6 messages, the SCMP message begins with a 4
byte header that includes 8-bit Type and Code fields followed by a
16-bit Checksum field. The SCMP message header is followed by the
message body which is followed by the leading portion of the invoking
packet-in-error (when present) beginning with the packet's outer IP
header. The Checksum is calculated the same as specified for ICMPv4
messages in [RFC0792], i.e., the checksum does not include a pseudo-
header of the outer IP header since the SEAL_ID gives sufficient
assurance against mis-delivery.
4.5.1. Generating SCMP Messages
The TE prepares the SCMP message exactly as specified for the
corresponding ICMPv6 message, then encapsulates it in any mid-layer
headers and trailers. For example, if IPsec ESP is used, the TE
encapsulates the SCMP message directly within the mid-layer ESP
headers and trailers, i.e., it does not encapsulate the SCMP message
within an inner header. The TE next encapsulates the mid-layer
packet in the SEAL header, any other outer headers and finally in the
outer IP header. The SCMP message format is shown in Figure 5.
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+--------------------+
~ outer IPv4 header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
S +--------------------+ +--------------------+
C ~ mid-layer headers ~ ~ mid-layer headers ~
M +--------------------+ +--------------------+
P --> ~ SCMP message header~ --> ~ SCMP message header~
--> +--------------------+ --> +--------------------+
M --> ~ SCMP message body ~ --> ~ SCMP message body ~
e --> +--------------------+ --> +--------------------+
s --> ~ packet-in-error ~ --> ~ packet-in-error ~
s +--------------------+ +--------------------+
a ~ mid-layer trailers ~ ~ mid-layer trailers ~
g +--------------------+ +--------------------+
e ~ outer trailers ~
SCMP Message +--------------------+
after mid-layer encaps.
SCMP Message after
SEAL and outer encaps.
Figure 5: SCMP Message Encapsulation
During outer encapsulation, the TE sets the outer IP destination and
source addresses of the SCMP packet to the source and destination
addresses (respectively) of the invoking SEAL data packet (i.e., the
packet-in-error). If the destination address in the packet-in-error
was multicast, the TE instead sets the outer IP source address of the
SCMP packet to an address assigned to the underlying IP interface.
The TE finally sets the NEXTHDR field in either the SEAL header or
the mid-layer header (if present) to the value '58', i.e., the
official IANA protocol number for the ICMPv6 protocol.
4.5.1.1. Generating SCMP Segment Report Messages
An ETE generates an SCMP Segment Report message in the following
cases:
o Case 1: the ETE receives a SEAL data packet that would cause the
reassembled outer packet to be larger than S_MRU following SEAL
reassembly.
o Case 2: the ETE receives the IP first fragment (i.e., one with
MF=1 and Offset=0 in the IP header) of a fragmented SEAL data
packet.
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o Case 3: the ETE receives a SEAL data packet with A=1 in the SEAL
header.
When the ETE receives a SEAL data packet that matches one or more of
the above cases, it prepares an SCMP Segment Report message using the
same format specified for an ICMPv6 Packet Too Big message (see:
[RFC4443], Section 3.2), and writes the S_MRU value for this ITE in
the MTU field of the message. The ETE then includes as much of the
invoking SEAL data packet as possible as the packet-in-error without
the resulting SCMP packet exceeding 576 bytes. The ETE then writes
the value '0' in the Code field of the SCMP Segment Report message
header for Case 1, or writes the value '1' in the Code field for
Cases 2 and 3. The ETE then encapsulates the SCMP Segment Report
message in any mid-layer and outer headers and trailers as shown in
Figure 5 then sends the resulting SCMP packet back to the ITE.
After it sends the SCMP Segment Report message, the ETE next accepts
or discards the SEAL data packet according to the specific case. For
Case 1, the ETE discards the SEAL data packet and schedules any
reassembly resources for deletion. For Cases 2 and 3, the ETE
accepts the SEAL data packet even though it also returned an SCMP
Segment Report message to the ITE.
4.5.2. Processing SCMP Messages
Each TE processes any SCMP messages it receives from the tunnel far
end. In order to detect spoofed messages from off-path attackers,
each ITE maintains a window of outstanding SEAL_IDs of packets that
it has recently sent to each ETE, and each ETE maintains a window of
SEAL_IDs of packets that it has recently received from each ITE. The
windows are managed as described in Section 4.6.
For each SCMP message, the TE first verifies that the SEAL_ID is
within the current window of packets that it has received from the
tunnel far end (but see Section 4.6). When the SCMP message contains
an invoking SEAL data packet as a packet-in-error, the TE also
verifies that the SEAL_ID in the packet-in-error is within the is
within the current window of packets that it has sent to the tunnel
far end. The TE then verifies that the checksum in the SCMP message
header is correct. If the SEAL_ID is outside of the window and/or
the checksum is incorrect, the ITE discards the message; otherwise,
it processes the message the same as for ordinary ICMPv6 messages.
4.5.2.1. Processing SCMP Segment Report Messages
An ITE may receive an SCMP Segmentation Report message after it sends
a SEAL data packet that matches one or more of the cases in Section
4.5.1.1. When the ITE receives a Segment Report message, it records
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the value in the MTU field of the message as the new S_MRU value in
its soft state for this ETE. The ITE then processes the message
further according to the value in the Code field.
For SCMP Segmentation Report messages with Code=0, the ITE examines
the packet-in-error to determine whether it can be translated into a
PTB message to send back to the original source. If so, the ITE can
optionally send a translated PTB message to the original source
For SCMP Segmentation Report messages with Code=1, the ITE examines
the IP header of the packet-in-error. If the packet-in-error is not
an IP first fragment, and if the packet-in-error length is greater
than the current S_MSS value, the ITE records the length as the new
S_MSS value in its soft state for this ETE. If the packet in-error
is a first fragment, however, the ITE determines a new S_MSS value
according to the packet-in-error length as follows:
o If the length is no less than 576 (i.e., the nominal minimum MTU
for IPv4 links), the ITE records the length as the new S_MSS
value.
o If the length is less than the current S_MSS value and also less
than 576, the ITE can discern that IP fragmentation is occurring
but it cannot determine the true MTU of the restricting link due
to a router on the path generating runt first fragments.
In this latter case, the ITE must search for a reduced S_MSS value
through an iterative searching strategy that parallels (Section 5 of
[RFC1191]). This searching strategy may require multiple iterations
in which the ITE sends SEAL data packets using a reduced S_MSS and
receives additional SCMP Segment Report messages. During this
process, it is essential that the ITE reduce S_MSS based on the first
SCMP Segment Report message received under the current S_MSS size,
and refrain from further reducing S_MSS until SCMP Segment Report
messages pertaining to packets sent under the new S_MSS are received.
Finally, the ITE examines the SEAL header of the packet-in-error to
determine whether the message constitutes a reply to an explicit
probe (see: Section 4.3.7). The ITE then discards the SCMP message.
4.6. TE Window Synchronization and Maintenance
Each ITE uses the SEAL_ID in the packets it sends not only to support
the segmentation and reassembly procedures, but also as a sequence
number of packets that it has recently sent to the ETE. Similarly,
each ETE uses the SEAL_ID in the packets it receives as a sequence
number of packet that it has recently received from the ITE. This
arrangement requires a synchronization of sequence numbers between
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tunnel endpoints as follows.
When an ITE needs to synchronize with a new ETE (i.e., one for which
it has no neighbor cache entry), it first creates a neighbor cache
entry for the ETE and chooses a random 48-bit value to use as its
initial sequence number, i.e., an initial "SEAL_ID(ITE)" which it
records in the neighbor cache entry. The ITE then creates an SCMP
"Router Solicitation (RS)" or "Neighbor Solicitation (NS)" message
that includes a Nonce option (see: [RFC3971], Section 5.3) that
encodes SEAL_ID(ITE). The ITE then sets SEAL_ID in the SCMP message
header to 0 as an indication that this RS/NS message is being used
for the purpose of SEAL_ID synchronization. The ITE finally sends
the RS/NS message to the ETE.
When the ETE receives the RS/NS message, it notices that the SEAL_ID
in the message is 0 and considers the message as a potential window
synchronization request. The ETE then chooses a random 48-bit value
to use as its initial sequence number, i.e., an initial
"SEAL_ID(ETE)" which it stores in a minimal temporary fast path data
structure that caches only the IP source address of the SCMP message,
SEAL_ID(ITE) and SEAL_ID(ETE). (For efficiency purposes, the data
structure should be indexed, e.g., by a secret hash of the IP source
address). The ETE then creates an SCMP "Router Advertisement (RA)"
or "Neighbor Advertisement (NA)" message that includes the Nonce
option taken from the corresponding RS/NS. The ETE writes the value
SEAL_ID(ETE) into the SEAL_ID field of the SCMP message and sends the
message back to the ITE.
When the ITE receives the RA/NA, it first verifies that the value
encoded in the Nonce option matches SEAL_ID(ITE). If so, the ITE
records the value SEAL_ID(ETE) in the neighbor cache entry. (If
instead the ITE does not receive a timely RA/NA response, it
retransmits the initial RS/NS message for a total of 3 tries before
giving up.) After the ITE receives a valid RA/NA message, it then
uses SEAL_ID(ITE) as the SEAL _ID of subsequent SEAL packets that it
sends to this ETE and uses SEAL_ID(ETE) as the SEAL_ID to match
against subsequent SEAL packets that it receives from this ETE.
After the ITE receives the RA/NA message, it sends either an
unsolicited RA/NA or an ordinary data packet back to the ETE. When
the ETE receives the data packet, it first checks its neighbor cache
to see if there is a matching neighbor cache entry. If there is a
neighbor cache entry, and the SEAL_ID of the packet is within the
current window, the ETE accepts the packet. If there is no neighbor
cache entry, the ETE instead checks the fast path cache to see if the
packet is a match for an in-progress window synchronization event.
If the packet matches (i.e., if there is a fast path cache entry with
a matching SEAL_ID (ITE), the ETE accepts the packet and also creates
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a new neighbor cache entry or updates the existing neighbor cache
entry. If there is no matching fast path cache entry, the ETE
instead discards the packet.
By maintaining the fast path cache, the ETE is able to mitigate
buffer exhaustion attacks that may be launched by off-path attackers
[RFC4987]. The ETE will receive positive confirmation that the
synchronization request came from an on-path ITE after it receives
the "third leg" of this three-way handshake as described above. The
ITE and ETE should maintain neighbor cache entries as long as traffic
is flowing through the tunnel, but should delete the neighbor cache
entries after a nominal idle time (e.g., 120 seconds). The ETE
should also purge fast-path cache entries for which no window
synchronization messages are received within a nominal idle time
(e.g., 10 seconds).
When a TE receives a SEAL packet with non-zero SEAL_ID, it checks in
its neighbor cache to determine whether the SEAL_ID is within the
current window, and discards any packets that are outside the window.
Since packets may be lost and since SEAL presents only a best effort
(i.e., and not reliable) link model, however, the TE should accept
any packet with a SEAL_ID that is within 32768 (plus or minus) of the
most recently acknowledged SEAL_ID. For this reason, the ITE must
set the A bit in the SEAL headers of packet periodically (e.g., once
for every 4096 packets) so that the ETE's last acknowledged SEAL_ID
tracks with the flow of packets.
5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol Specification
This section specifies a minimal mode of SEAL known as "SEAL with
Fragmentation Sensing (SEAL-FS)". SEAL-FS observes the same protocol
specifications as for "SEAL with Segmentation and Reassembly
(SEAL-SR)" (see Section 4) except that the ETE unilaterally drops any
SEAL-FS packets that arrive as multiple IP fragments and/or multiple
SEAL segments.
SEAL-FS can be considered for use by associated tunneling protocol
specifications when there is operational assurance that "marginal"
links are rare, e.g., when it is known that the vast majority of
links configure MTUs that are appreciably larger than a constant
value 'M' (e.g., 1500 bytes). SEAL-FS can also be used in instances
when it is acceptable for the ITE to return PTB messages for packet
sizes smaller than 'M', however SEAL-SR should be used instead if
excessive PTB messages would result.
With respect to Section 4, the SEAL-FS protocol corresponds to
SEAL-SR as follows:
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5.1. Model of Operation
SEAL-FS follows the same model of operation as for SEAL-SR as
described in Section 4.1 except as noted in the following sections.
5.2. SEAL-FS Header Format (Version 0)
The SEAL-FS header is 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MODE|A| RSV | NEXTHDR | SEAL_ID (bits 47 - 32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL_ID (bits 31 - 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: SEAL Version 1 Header Format
where the header fields are defined as:
MODE (3)
a 3-bit value that encodes the SEAL protocol mode. This section
describes Mode 0 of the SEAL protocol, i.e., the MODE field
encodes the value '0'.
A (1)
the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes
to receive an explicit acknowledgement from the ETE.
RSV (4)
a 4-bit Reserved field. Set to 0 for the purpose of this
specification.
NEXTHDR (8)
an 8-bit field that encodes the next header Internet Protocol
number the same as for the IPv4 protocol and IPv6 next header
fields.
SEAL_ID (48)
a 48-bit Identification field.
5.3. ITE Specification
5.3.1. Tunnel Interface MTU
SEAL-FS observes the SEAL-SR specification found in Section 4.3.1.
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5.3.2. Tunnel Interface Soft State
SEAL-FS observes the SEAL-SR specification found in Section 4.3.2.
5.3.3. Inner Fragmentation
SEAL-FS observes the SEAL-SR specification found in Section 4.3.3,
except that the inner fragmentation algorithm is adjusted to avoid
all outer IP fragmentation and SEAL segmentation within the tunnel.
For this purpose, the SEAL-FS ITE maintains S_MSS as a value that
would be unlikely to incur fragmentation within the tunnel, e.g., 576
bytes for IPv4 and 1280 bytes for IPv6. The ITE may also set S_MSS
to a larger value if there is assurance that the vast majority of
links that may occur within the tunnel configure a larger MTU, and/or
may use explicit probes (e.g., dummy packets with the 'A' bit set in
the SEAL header) to dynamically discover a larger S_MSS value.
The ITE uses S_MRU and S_MSS in the following algorithm to determine
when to discard, fragment or admit the inner packets into the tunnel
without inner fragmentation:
o if the inner packet is unfragmentable (e.g., an IPv6 packet, an
IPv4 packet with DF=1, etc.), and the packet is larger than
(MIN(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends a
PTB message to the original source with an MTU value of
(MIN(S_MRU, S_MSS) - HLEN) the same as described in Section 4.3.2;
else,
o if the inner packet is fragmentable (e.g., an IPv4 packet with
DF=0), and the packet is larger than (MIN(S_MRU, S_MSS) - HLEN),
the ITE uses inner fragmentation to break the packet into
fragments no larger than (MIN(S_MRU, S_MSS) - HLEN); else,
o the ITE admits the packet without inner fragmentation.
If the inner packet is unfragmentable, and the packet will be sent
in-the-clear with no mid-layer encryption, the ITE can instead employ
a stateless strategy by simply encapsulating and sending the packet
without regard to its length. The ITE can then translate any SCMP
Segment Report messages it receives from the ETE into PTB messages to
return to the original source (where the translation is based on the
packet-in-error within the SCMP Segment Report message). In this
method, the ITE need not retain per-ETE S_MRU and S_MSS state.
5.3.4. Mid-Layer Encapsulation
SEAL-FS observes the SEAL-SR soft state specifications found in
Section 4.3.4.
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5.3.5. SEAL Segmentation
SEAL-FS does not observe the SEAL-SR segmentation procedures in
Section 4.3.5, since all SEAL-FS packets are sent within a single
SEAL segment.
5.3.6. Outer Encapsulation
SEAL-FS observes the SEAL-SR specification found in Section 4.3.6,
except that it uses the header format defined in this section and
with the MODE field set to '0'.
5.3.7. Probing Strategy
SEAL-FS observed the SEAL-SR specification found in Section 4.3.7.
5.3.8. Packet Identification
SEAL-FS observes the SEAL-SR specifications found in Section 4.3.8.
5.3.9. Sending SEAL Protocol Packets
SEAL-FS observes the SEAL-SR specification found in Section 4.3.9.
5.3.10. Processing Raw ICMP Messages
SEAL-FS observes the SEAL-SR specification found in Section 4.3.10.
5.4. ETE Specification
5.4.1. Reassembly Buffer Requirements
SEAL-FS does not maintain reassembly buffers, but still maintains a
value for S_MRU as the largest outer packet size the ETE can receive.
5.4.2. IP-Layer Reassembly
SEAL-FS uses SEAL-protocol IP first fragments solely for the purpose
of generating SCMP Segment Report messages, but otherwise discards
all SEAL-protocol packets that arrive as multiple IP fragments.
5.4.3. SEAL-Layer Reassembly
SEAL-FS does not observe the SEAL-SR reassembly procedures in Section
4.4.3, since all SEAL-FS packets are sent within a single SEAL
segment.
As for SEAL-SR, SEAL-FS returns a Parameter Problem for SEAL packets
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with unrecognized values in the SEAL header.
5.4.4. Decapsulation and Delivery to Upper Layers
SEAL-FS observes the SEAL-SR specification found in Section 4.4.4.
5.5. The SEAL Control Message Protocol (SCMP)
SEAL-FS observes the SEAL-SR SCMP protocol specification found in
Section 5, however the ETE unconditionally drops any SEAL protocol
packets that arrive as multiple IP fragments.
5.6. TE Window Synchronization and Maintenance
SEAL-FS observes the SEAL-SR specification found in Section 4.6.
6. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
7. End System Requirements
SEAL provides robust mechanisms for returning PTB messages; however,
end systems that send unfragmentable IP packets larger than 1500
bytes are strongly encouraged to implement their own end-to-end MTU
assurance, e.g., using Packetization Layer Path MTU Discovery per
[RFC4821].
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, i.e.,
they should avoid generating runt first fragments.
9. IANA Considerations
The IANA is instructed to allocate an IP protocol number for
'SEAL_PROTO' in the 'protocol-numbers' registry.
The IANA is instructed to allocate a Well-Known Port number for
'SEAL_PORT' in the 'port-numbers' registry.
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The IANA is instructed to establish a "SEAL Protocol" registry to
record SEAL Mode values. This registry should be initialized to
include the Mode values defined in Sections 4.2 and 5.2, and the Code
and Type values defined in Section 4.4.5.
10. Security Considerations
Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-
overlapping. This condition is naturally enforced due to the fact
that each consecutive SEAL segment begins at offset 0 with respect to
the previous SEAL segment.
An amplification/reflection attack is possible when an attacker sends
IP first fragments with spoofed source addresses to an ETE, resulting
in a stream of SCMP messages returned to a victim ITE. The SEAL_ID
in the encapsulated segment of the spoofed IP first fragment provides
mitigation for the ITE to detect and discard spurious SCMP messages.
The SEAL header is sent in-the-clear (outside of any IPsec/ESP
encapsulations) the same as for the outer IP and other outer headers.
In this respect, the threat model is no different than for IPv6
extension headers. As for IPv6 extension headers, the SEAL header is
protected only by L2 integrity checks and is not covered under any L3
integrity checks.
SCMP messages carry the SEAL_ID of the packet-in-error. Therefore,
when an ITE receives an SCMP message it can unambiguously associate
it with the SEAL data packet that triggered the error.
Security issues that apply to tunneling in general are discussed in
[I-D.ietf-v6ops-tunnel-security-concerns].
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 SEAL. SEAL could therefore be
considered as a fully functioned manifestation of the method
postulated by that informational reference.
Section 3 of [RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU;
however, the SEAL protocol specifies a mid-layer segmentation and
reassembly capability that is distinct from both inner and outer
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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 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). A
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 D of this document.
12. SEAL Advantages over Classical Methods
The SEAL approach offers a number of distinct advantages over the
classical path MTU discovery methods [RFC1191] [RFC1981]:
1. Classical path MTU discovery always results in packet loss when
an MTU restriction is encountered. Using SEAL, IP fragmentation
provides a short-term interim mechanism for ensuring that packets
are delivered while SEAL adjusts its packet sizing parameters.
2. Classical path MTU may require several iterations of dropping
packets and returning PTB messages until an acceptable path MTU
value is determined. Under normal circumstances, SEAL determines
the correct packet sizing parameters in a single iteration.
3. Using SEAL, ordinary packets serve as implicit probes without
exposing data to unnecessary loss. SEAL also provides an
explicit probing mode not available in the classic methods.
4. Using SEAL, ETEs encapsulate SCMP error messages in outer and
mid-layer headers such that packet-filtering network middleboxes
will not filter them the same as for "raw" ICMP messages that may
be generated by an attacker.
5. The SEAL approach ensures that the tunnel either delivers or
deterministically drops packets according to their size, which is
a required characteristic of any IP link.
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6. Most importantly, all SEAL packets have an Identification field
that is sufficiently long to be used for duplicate packet
detection purposes and to associate ICMP error messages with
actual packets sent without requiring per-packet state; hence,
SEAL avoids certain denial-of-service attack vectors open to the
classical methods.
13. Acknowledgments
The following individuals are acknowledged for helpful comments and
suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver
Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner,
Ian Chakeres, Noel Chiappa, Remi Denis-Courmont, Aurnaud Ebalard,
Gorry Fairhurst, Dino Farinacci, Joel Halpern, Sam Hartman, John
Heffner, Thomas Henderson, Bob Hinden, Christian Huitema, Eliot Lear,
Darrel Lewis, Joe Macker, Matt Mathis, Erik Nordmark, Dan Romascanu,
Dave Thaler, Joe Touch, Margaret Wasserman, Magnus Westerlund, Robin
Whittle, James Woodyatt, and members of the Boeing Research &
Technology NST 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.
14. References
14.1. 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.
[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.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
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Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
14.2. Informative References
[FOLK] Shannon, C., Moore, D., and k. claffy, "Beyond Folklore:
Observations on Fragmented Traffic", December 2002.
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
October 1987.
[I-D.ietf-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-06 (work in progress), January 2010.
[I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-10 (work in progress),
January 2010.
[I-D.ietf-v6ops-tunnel-security-concerns]
Hoagland, J., Krishnan, S., and D. Thaler, "Security
Concerns With IP Tunneling",
draft-ietf-v6ops-tunnel-security-concerns-01 (work in
progress), October 2008.
[I-D.russert-rangers]
Russert, S., Fleischman, E., and F. Templin, "RANGER
Scenarios", draft-russert-rangers-01 (work in progress),
September 2009.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-09 (work in progress),
February 2010.
[I-D.templin-ranger]
Templin, F., "Routing and Addressing in Next-Generation
EnteRprises (RANGER)", draft-templin-ranger-09 (work in
progress), October 2009.
[MTUDWG] "IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November
1989 - February 1995.".
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[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, July 1988.
[RFC1070] Hagens, R., Hall, N., and M. Rose, "Use of the Internet as
a subnetwork for experimentation with the OSI network
layer", RFC 1070, February 1989.
[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.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[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.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[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.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[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
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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.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
[TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
October 2004.
[TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List,
http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May
1987 - May 1990.".
[WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and
Debugging Path MTU Discovery Failures", October 2005.
Appendix A. Reliability
Although a SEAL tunnel may span an arbitrarily-large subnetwork
expanse, the IP layer sees the tunnel as a simple link that supports
the IP service model. Since SEAL-SR supports segmentation at a layer
below IP, SEAL-SR therefore presents a case in which the link unit of
loss (i.e., a SEAL segment) is smaller than the end-to-end
retransmission unit (e.g., a TCP segment).
Links with high bit error rates (BERs) (e.g., IEEE 802.11) use
Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366] to increase
packet delivery ratios, while links with much lower BERs typically
omit such mechanisms. Since SEAL-SR tunnels may traverse
arbitrarily-long paths over links of various types that are already
either performing or omitting ARQ as appropriate, it would therefore
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often be inefficient to also require the tunnel to perform ARQ.
When the SEAL-SR ITE has knowledge that the tunnel will traverse a
subnetwork with non-negligible loss due to, e.g., interference, link
errors, congestion, etc., it can solicit Segment Reports from the ETE
periodically to discover missing segments for retransmission within a
single round-trip time. However, retransmission of missing segments
may require the ITE to maintain considerable state and may also
result in considerable delay variance and packet reordering.
SEAL-SR may also use alternate reliability mechanisms such as Forward
Error Correction (FEC). A simple FEC mechanism may merely entail
gratuitous retransmissions of duplicate data, however more efficient
alternatives are also possible. Basic FEC schemes are discussed in
[RFC5445].
The use of ARQ and FEC mechanisms for improved reliability are for
further study.
Appendix B. Integrity
Each link in the path over which a SEAL tunnel is configured is
responsible for link layer integrity verification for packets that
traverse the link. As such, when a multi-segment SEAL packet with N
segments is reassembled, its segments will have been inspected by N
independent link layer integrity check streams instead of a single
stream that a single segment SEAL packet of the same size would have
received. Intuitively, a reassembled packet subjected to N
independent integrity check streams of shorter-length segments would
seem to have integrity assurance that is no worse than a single-
segment packet subjected to only a single integrity check steam,
since the integrity check strength diminishes in inverse proportion
with segment length. In any case, the link-layer integrity assurance
for a multi-segment SEAL packet is no different than for a multi-
fragment IPv6 packet.
Fragmentation and reassembly schemes must also consider packet-
splicing errors, e.g., when two segments from the same packet are
concatenated incorrectly, when a segment from packet X is reassembled
with segments from packet Y, etc. The primary sources of such errors
include implementation bugs and wrapping IP ID fields. In terms of
implementation bugs, the SEAL segmentation and reassembly algorithm
is much simpler than IP fragmentation resulting in simplified
implementations. In terms of wrapping ID fields, when IPv4 is used
as the outer IP protocol, the 16-bit IP ID field can wrap with only
64K packets with the same (src, dst, protocol)-tuple alive in the
system at a given time [RFC4963] increasing the likelihood of
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reassembly mis-associations. However, SEAL ensures that any outer
IPv4 fragmentation and reassembly will be short-lived and tuned out
as soon as the ITE receives a Reassembly Repot, and SEAL segmentation
and reassembly uses a much longer ID field. Therefore, reassembly
mis-associations of IP fragments nor of SEAL segments should be
prohibitively rare.
Appendix C. Transport Mode
SEAL can also be used in "transport-mode", e.g., when the inner layer
comprises 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 IPv4/SEAL/TCP. In this
sense, the "subnetwork" becomes the entire end-to-end path between
the TCP peers and may potentially span the entire Internet.
Sections 4 and 5 specify the operation of SEAL in "tunnel mode",
i.e., when there are both an inner and outer IP layer with a SEAL
encapsulation layer between. However, the SEAL protocol can also be
used in a "transport mode" of operation within a subnetwork region 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 carried as TCP/SEAL/IPv4 and the connection
will be serviced by the SEAL protocol using TCP (instead of an
encapsulating tunnel endpoint) as the transport layer protocol. The
SEAL protocol for transport mode otherwise observes the same
specifications as for Sections 4 and 5.
Appendix D. Historic Evolution of PMTUD
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:
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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 (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
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The first four assertions, although perhaps valid at the time, have
been overcome by historical events. The final assertion is addressed
by the mechanisms specified in SEAL.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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