Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track January 08, 2010
Expires: July 12, 2010
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-08.txt
Abstract
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. 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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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 . . . . . . . . . . . . . . . . . . . 8
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. Admitting Packets into the Tunnel Interface . . . . . 15
4.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 16
4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 17
4.3.5. Probing Strategy and Information Exchanges . . . . . . 18
4.3.6. Packet Identification . . . . . . . . . . . . . . . . 18
4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 19
4.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 19
4.3.9. Processing SEAL Control Messages . . . . . . . . . . . 20
4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 22
4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 22
4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 22
4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 23
4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 24
4.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 24
5. SEAL with Fragmentation Sensing (SEAL-FS) Protocol
Specification . . . . . . . . . . . . . . . . . . . . . . . . 31
5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 31
5.2. SEAL-FS Header Format (Version 0) . . . . . . . . . . . . 32
5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 32
5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 33
5.3.2. Admitting Packets into the Tunnel Interface . . . . . 33
5.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 33
5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 34
5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 34
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5.3.6. Packet Identification . . . . . . . . . . . . . . . . 34
5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 34
5.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 34
5.3.9. Processing SEAL Control Messages . . . . . . . . . . . 34
5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 34
5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 34
5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 34
5.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 34
5.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 35
5.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 35
6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 35
7. End System Requirements . . . . . . . . . . . . . . . . . . . 35
8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
10. Security Considerations . . . . . . . . . . . . . . . . . . . 36
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 36
12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 37
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 38
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 38
14.1. Normative References . . . . . . . . . . . . . . . . . . . 38
14.2. Informative References . . . . . . . . . . . . . . . . . . 39
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 41
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 42
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 42
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 43
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 44
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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 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 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.
The issues with both IPv4 fragmentation and this "classical" method
of path MTU discovery are exacerbated further when IP-in-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
ICMP error message into a suitable ICMP error message to return to
the original source.
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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 tunnel-mode operation of IP over subnetworks that
connect Ingress and Egress Tunnel Endpoints (ITEs/ETEs) of border
nodes. It provides a standalone specification designed to be
tailored to specific associated IP in 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 IP services. As for any link,
tunnels that use SEAL must provide suitable IP 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 IP
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 purpose.
SEAL encapsulation introduces an extended Identification field for
packet identification and a mid-layer segmentation and reassembly
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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 (ITE)
a virtual interface over which an encapsulating border node (host
or router) sends encapsulated packets into the subnetwork.
Egress Tunnel Endpoint (ETE)
a virtual interface over which an encapsulating border node (host
or router) receives encapsulated packets from the subnetwork.
inner IP packet
an unencapsulated IP packet before any mid-layer or outer
encapsulations are added.
mid-layer packet
a packet resulting from adding mid-layer encapsulating headers and
trailers to an inner IP packet.
outer IP packet
a packet resulting from adding outer encapsulating headers and
trailers to a mid-layer packet.
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
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document and elsewhere in common Internetworking nomenclature:
PTB - an ICMPv6 "Packet Too Big" [RFC4443]or an ICMPv4
"Fragmentation Needed" [RFC0792] message.
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
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
SEAL_ID - a 32-bit Identification value, randomly initialized and
monotonically incremented for each SEAL protocol packet
SEAL_PROTO - an IPv4 protocol number used for SEAL
SEAL_PORT - a TCP/UDP service port number used for SEAL
SEAL-FS - SEAL with Fragmentation Sensing
SEAL-SR - SEAL with Segmentation and Reassembly
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 may also be useful
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as an adjunct mechanism for other tunneling protocols such as
LISP[I-D.ietf-lisp].
SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation
(e.g., as IPv4/SEAL/IPv6), and appears as a subnetwork encapsulation
as seen by the inner IP layer. SEAL can also be used as a sublayer
for encapsulating inner IPvX packets within outer IPvY/UDP headers
(e.g., as IPv4/UDP/SEAL/IPv6) such as for the Teredo domain of
applicability [RFC4380]. When it appears immediately after the outer
IPv4 header, the SEAL header is processed exactly as for IPv6
extension headers, i.e., it is not part of the outer IPv4 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 fragmentation. 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 over robust links, as well as for other uses in
which packet 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 only
ordinary links, but they may also be configured over paths that
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.
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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
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 IP packet in mid-layer
headers and trailers, segment the resulting mid-layer packet if
necessary, then append a SEAL header and outer encapsulating headers
and trailers 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:
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+--------------------+
~ outer IPv4 header ~
+--------------------+
~ other outer hdrs ~
I +--------------------+
n | SEAL Header |
n +--------------------+ +--------------------+
e ~ mid-layer headers ~ ~ mid-layer headers ~
r +--------------------+ +--------------------+
| | | |
I --> ~ inner IPv6 ~ --> ~ inner IPv6 ~
P --> ~ Packet ~ --> ~ Packet ~
v | | | |
6 +--------------------+ +--------------------+
~ mid-layer trailers ~ ~ mid-layer trailers ~
P +--------------------+ +--------------------+
a ~ outer trailers ~
c Mid-layer packet +--------------------+
k after mid-layer encaps.
e Outer IPv4 packet
t after SEAL and outer encaps.
Figure 1: SEAL Encapsulation - Single Segment
In a second example, an inner IPv6 packet requiring three SEAL
segments would appear as three separate outer IPv4 packets (each with
its own SEAL header) and with the mid-layer headers only occurring in
segment 0 and the mid-layer trailers only appearing 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 IPv6 ~
| | | | ~ Packet ~
~ inner IPv6 ~ ~ inner IPv6 ~ | (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 IP in IP encapsulations (e.g.,
[RFC2003][RFC2004][RFC2473][RFC4213]), the SEAL header is inserted
between the inner IPvY and outer IPvX headers as: IPvX/SEAL/IPvY.
o For tunnel-mode IPsec encapsulations (e.g., [RFC4301]), the SEAL
header is inserted between the {AH,ESP} header and outer IP
headers as: IPvX/SEAL/{AH,ESP}/IPvY. Here, the {AH, ESP} headers
and trailers are seen as mid-layer encapsulations.
o For IP encapsulations over transports such as UDP (e.g.,
[RFC4380]), the SEAL header is inserted between the outer
transport layer header and the inner IPvY header, e.g., as IPvX/
UDP/SEAL/IPvY. 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 {ITEs; ETEs} use the SEAL_ID for SEAL
segmentation/reassembly and protection against off-path attacks.
For IPv4 as the outer layer of encapsulation, the SEAL_ID is 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
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Identification value in the outer IPv4 header as the least-
significant bits. For IPv6 as the outer layer, the SEAL_ID is
written into the 32-bit Identification field of the IPv6 fragment
header. For tunnels that traverse middleboxes that might rewrite the
IP ID field (e.g., a Network Address Translator) the SEAL_ID is
instead maintained only within the ID extension field in the SEAL
header and/or within additional mid-layer header fields.
The following sections specify the SEAL header format and SEAL-
related operations of the ITE and ETE, respectively.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MOD|C|A|I|F|M|R| NEXTHDR/SEG | ID Extension |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SEAL Mode 1 Header Format
where the header fields are defined as:
MOD (2)
a 2-bit value that encodes the SEAL protocol mode. This section
describes Mode 1 of the SEAL protocol, i.e., the MOD field encodes
the value 1.
C (1)
the "Control" bit. Set to 1 in SEAL control messages, and set to
0 in SEAL data messages.
A (1)
the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes
to receive an explicit acknowledgement from the ETE.
I (1)
the "Information Request Solicit" bit. Set to 1 if the ITE wishes
the ETE to initiate an Information Request.
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.
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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.
R (1)
a Reserved bit. 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.)
ID Extension (16)
a 16-bit Identification extension 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 the inner IP layer
(i.e., Layer 3) as the size for admission of inner IP packets into
the tunnel. Since the tunnel interface may support a potentially
large set of ETEs, however, care must be taken in setting a large-
enough MTU for all ETEs while still upholding end system
expectations.
Due to the ubiquitous deployment of standard Ethernet and similar
networking gear, the nominal Internet cell size has become 1500
bytes; this is the de facto size that end systems have come to expect
will either be delivered by the network without loss due to an MTU
restriction on the path or a suitable ICMP Packet Too Big (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, other SEAL encapsulations,
etc.) such that a 1500 byte packet sent by the original source might
grow to a larger size by the time it reaches the ITE for
encapsulation as an inner IP packet. Similarly, additional
encapsulations on the path from the ITE to the ETE could cause the
encapsulated packet to become larger still and trigger in-the-network
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fragmentation. In order to preserve the end system expectations, the
ITE therefore requires a means for conveying these larger packets to
the ETE even though there may be links within the subnetwork that
configure a smaller MTU.
The ITE should therefore set a tunnel virtual interface MTU of 1500
bytes plus extra room to accommodate any additional encapsulations
that may occur on the path from the original source (i.e., even if
the path to the ETE does not support an MTU of this size). The ITE
can set larger MTU values still, but should select a value that is
not so large as to cause excessive PTBs coming from within the tunnel
interface (see Sections 4.3.3 and 4.3.8). 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 IP 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 underlying interface and
as such may select too large an initial size. This is not a problem
for upper layers that use conservative initial estimates, e.g., when
mechanisms such as Packetization Layer Path MTU Discovery [RFC4821]
are used.
4.3.2. Admitting Packets into the Tunnel Interface
The inner IP layer consults the tunnel interface MTU when admitting a
packet into the interface. For 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 IP layer uses IP 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 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
minimum IP MTU (i.e., 576 bytes for IPv4 and 1280 bytes for IPv6).
Note that when the tunnel interface sets an indefinite MTU all
packets are unconditionally admitted into the interface without
fragmentation.
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4.3.3. Segmentation
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. 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.).
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 underlying IP interface MTU. The ITE
decreases or increases S_MSS based on any SEAL Reassembly Report
messages received (see Section 4.3.9).
o a SEAL Maximum Reassembly Unit (S_MRU); initialized to "infinity",
i.e., the largest-possible inner IP packet size. The ITE
decreases or increases S_MRU based on any SEAL Reassembly Report
messages received (see Section 4.3.9). When (S_MRU>(S_MSS*256)),
the ITE uses (S_MSS*256) as the effective S_MRU value.
Note that here as well as in the SEAL control message protocol (see
Section 4.4.5), S_MSS and S_MRU are maintained as 32-bit values
specifically for the purpose of supporting jumbograms.
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 inner IP
fragmentation is needed:
o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1,
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) the same as described in Section 4.3.2;
else,
o if the inner packet is an IPv4 packet with DF=0, and the packet is
larger than (MIN((S_MRU, S_MSS) - HLEN), the ITE uses inner IPv4
fragmentation to break the packet into fragments no larger than
(MIN(S_MRU, S_MSS) - HLEN); else,
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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 on the inner headers. If so,
the ITE must include the length of the uncompressed inner headers
when calculating the total length of the inner packet.)
The ITE next encapsulates each inner packet/fragment in the MHLEN
bytes of mid-layer headers and trailers. 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.
4.3.4. Encapsulation
Following SEAL segmentation, the ITE encapsulates each segment in a
SEAL header formatted as specified in Section 4.3.2 with MOD=1, C=0
and R=0. For the first segment, the ITE sets F=1, then sets NEXTHDR
to the Internet Protocol number of the encapsulated 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.)
The ITE next encapsulates each segment in the requisite outer IP and
other 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
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(e.g., when IP/UDP encapsulation is used). The ITE finally sets the
A and/or I bits as specified in Section 4.3.5, sets the packet
identification values as specified in Section 4.3.6 and sends the
packets as specified in Section 4.3.7.
Note that when IPv6 is used as the outer IP encapsulation layer, the
ITE must insert an IPv6 fragment header with an Identification value
set as described in Section 4.3.6.
4.3.5. Probing Strategy and Information Exchanges
All SEAL packets sent by the ITE are considered implicit probes, and
will elicit "Reassembly Report - Fragmentation Experienced" messages
from the ETE 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 should also send explicit probes, periodically, to verify
that the ETE is still reachable and to manage a window of SEAL_IDs.
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 'Next Header' field to a
value of "No Next Header" (see Section 4.7 of [RFC2460]). The probe
will elicit a "Reassembly Report - Segment Acknowledged" message from
the ETE as an acknowledgement (see Section 4.4.5.1). Finally, the
ITE MAY send "expendable" outer IP probe packets (see Section 4.3.7)
as explicit probes in order to generate PTB messages from routers on
the path to the ETE.
The ITE may in some cases also have out-of-band information to convey
to the ETE. In that case, the ITE can set the I bit (i.e., the
Information Request Solicit bit) in order to prompt the ETE to send
an Information Request message (see: Section 4.4.5.5).
In all cases, the ITE MUST be conservative in its use of the A and I
bits in order to limit the resultant control message overhead.
4.3.6. Packet Identification
For the purpose of packet identification, the ITE maintains a SEAL_ID
value as per-ETE soft state, e.g., in the neighbor cache. The ITE
randomly initializes SEAL_ID when the soft state is created, and
monotonically increments it for each successive SEAL protocol packet
it sends to the ETE.
For each outer IPv4 packet, the ITE writes the least-significant 16
bits of the SEAL_ID value into the Identification field in the outer
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IPv4 header, and writes the most-significant 16 bits in the ID
Extension field in the SEAL header. For each outer IPv6 packet, the
ITE writes the entire SEAL_ID value into the Identification field in
the IPv6 fragment header.
For tunnels specifically designed for the traversal of Network
Address Translators (NATs) (e.g., Teredo [RFC4380]) and other
middleboxes that might rewrite the outer IP ID field, the ITE instead
writes the least significant bits of the SEAL_ID in the ID field of
the SEAL header and writes a random value in the Identification field
in the outer IP header. The ITE can additionally write the high-
order bits of the SEAL_ID (or, alternatively, the entire SEAL_ID) in
a mid-layer header field, but in any case both the ITE and ETE must
be aware of the manner in which the SEAL_ID is inserted.
If only the least-significant bits of the SEAL_ID are included, the
ITE must limit the rate at which it sends packets to avoid wrapping
the ID field.
4.3.7. 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
4.3.5), but these may be lost due to an MTU restriction). For outer
IPv6 packets, the "DF" bit is always implicitly set to 1, but when a
fragment header is included a translating router on the path may
still fragment the packet.
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.
4.3.8. 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
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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.
4.3.9. Processing SEAL Control Messages
In addition to any raw ICMP messages, the ITE may receive SEAL
control messages from the ETE which have the C bit set to 1 in the
SEAL header and are formatted as specified in Section 4.4.5. For
each control message, the ITE verifies the checksum and discards the
message if the checksum is incorrect. The ITE can then verify that
the SEAL_ID in the invoking packet is within the current window of
transmitted SEAL_IDs for this ETE. If the SEAL_ID is outside of the
window, the ITE discards the message; otherwise, it advances the
window and processes the message. The ITE processes SEAL control
messages as follows:
4.3.9.1. Reassembly Report (Type=0)
When the ITE receives a Reassembly Report formatted as specified in
Section 4.4.5.1, it processes the message according to the Code value
as follows:
4.3.9.1.1. Segment Acknowledged (Code=0)
If the value in the S_MRU field is non-zero, the ITE records the
value in its soft state for this ETE.
4.3.9.1.2. Fragmentation Experienced (Code=1)
If the value in the S_MRU field is non-zero, the ITE records the
value in its soft state for this ETE. The ITE then adjusts the S_MSS
value in its soft state. If the S_MSS value in the Reassembly Report
is greater than 576 (i.e., the nominal minimum MTU for IPv4 links),
the ITE records this new value in its soft state. If the S_MSS value
in the report is less than the current soft state 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.
The ITE should therefore 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 of sending
SEAL packets using a reduced S_MSS and receiving additional
Reassembly Report messages, but it will soon converge to a stable
value. During this process, it is essential that the ITE reduce
S_MSS based on the first Reassembly Report message received, and
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refrain from further reducing S_MSS until SEAL Reassembly Report
messages pertaining to packets sent under the new S_MSS are received.
4.3.9.1.3. Packet Too Big (Code=2)
If the value in the S_MRU field is non-zero, the ITE records the
value in its soft state for this ETE. The ITE can then translate the
message into a PTB message to return to the original source, where
the translation is based on the encapsulated portion of the invoking
packet at the end of the reassembly report message.
4.3.9.1.4. Time Exceeded (Code=3)
If the value in the S_MRU field is non-zero, the ITE records the
value in its soft state for this ETE. The ITE MAY then log the event
for network management purposes. When excessive Time Exceeded
messages are received from this ETE, the ITE should also reduce its
S_MRU and/or S_MSS estimates.
Unlike other SEAL control messages, the ETE does not necessarily
generate the Time Exceeded message in synchronous response to the
receipt of an invoking SEAL packet. The ITE must therefore consider
as suspect any Time Exceeded messages that cannot be correlated with
a recently sent SEAL packet.
4.3.9.2. Parameter Problem (Type=1)
When the ITE receives a Parameter Problem message formatted as
specified in Section 4.4.5.2, it examines the encapsulated SEAL
header in the message to determine whether the header was corrupted
or whether the header specified features that the ETE did not
recognize. The ITE MAY log the event for network management
purposes, and SHOULD adjust its SEAL header parameters in subsequent
SEAL packets.
4.3.9.3. Information Request Solicit (Type=2)
When the ITE receives an Information Request Solicit message
formatted as specified in Section 4.4.5.3 and with a SEAL_ID that
corresponds to a SEAL packet that it sent earlier, it sends an
Information Request as specified in Section 4.4.5.4.
4.3.9.4. Information Request (Type=3)
When the ITE receives an Information Request message formatted as
specified in Section 4.4.5.4 and with a SEAL_ID that corresponds to a
SEAL packet that it sent earlier with I=1, it sends an Information
Reply as specified in Section 4.4.5.5.
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4.3.9.5. Information Reply (Type=4)
When the ITE receives an Information Reply message formatted as
specified in Section 4.4.5.5 and with a SEAL_ID that corresponds to a
SEAL packet that it sent earlier, it processes any out-of-band data
included in the reply.
4.4. ETE Specification
4.4.1. Reassembly Buffer Requirements
ETEs must be capable of performing IP-layer reassembly for SEAL
protocol IP packets up to 2KB in length, and must also be capable of
performing SEAL-layer reassembly for mid-layer packets up to (2KB -
OHLEN). Hence, ETEs:
o MUST configure a reassembly buffer size (i.e., a SEAL Maximum
Reassembly Unit (S_MRU)) of at least 2KB
o MAY configure a larger S_MRU
o MUST be capable of discarding SEAL packets that are too large to
reassemble
The ETE can also maintain S_MRU as a per-ITE value that can be
reduced if the current value becomes to too large, e.g., based on
excessive reassembly timeouts. If so, the ETE SHOULD ensure that the
per-ITE S_MRU converges to a stable value as quickly as possible.
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.
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
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Offset=0 in the IP header) of a fragmented SEAL packet, it sends a
"Reassembly Report - Fragmentation Experienced" message back to the
ITE with the S_MSS field set to the length of the first-fragment and
with the S_MRU field set to no more than the size of the reassembly
buffer (see Section 4.4.5).
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 a "Reassembly Report - Time Exceeded" message
to the ITE (see Section 4.4.5). 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 a "Parameter Problem"
message (see Section 4.4.5). 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.
After all segments are gathered, the ETE reassembles the mid-layer
packet by discarding the outer headers and 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 mid-
layer 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).
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4.4.4. Decapsulation and Delivery to Upper Layers
Following IP- and SEAL-layer reassembly, if the reassembled mid-layer
packet is larger than (S_MRU-OHLEN), the ETE discards the packet and
sends a "Reassembly Report - Packet Too Big" message with the S_MRU
field set to the maximum-sized packet it is willing to accept from
this ITE (see Section 4.4.5).
Next, the ETE discards the outer and mid-layer headers and trailers,
and delivers the inner packet to the upper-layer protocol indicated
in the SEAL Next Header field. The ETE instead silently discards the
inner packet if it was a NULL packet (see Section 4.3.4).
4.4.5. Sending SEAL Control Messages
An ETE sends SEAL control messages in response to certain SEAL data
packets and control messages received from the ITE. An ITE can also
send SEAL control messages during an information exchange with an
ETE.
SEAL control messages are formatted much the same as for ICMPv4
[RFC0792] and ICMPv6 [RFC4443] messages, and are used for very
similar purposes. The messages are formatted as shown in Figure 4:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Outer, SEAL and Mid-Layer Headers ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Control Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Mid-Layer and Outer Trailers |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: SEAL Control Message Format
As for ICPMv4 and ICMPv6 messages, the {ITE, ETE} prepares the
message body beginning with 8-bit Type and Code fields followed by a
16-bit Checksum field. The Checksum field is followed by a variable-
length control data field which is followed by as much of the
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invoking packet as possible without the entire message (including
encapsulating headers and trailers) exceeding 576 bytes. As for
ICMPv4 messages, the Checksum is the 16-bit ones's complement of the
one's complement sum of the SEAL control message body starting with
the Type field and ending with the final byte of the encapsulated
invoking packet. For computing the checksum , the Checksum field is
set to zero.
After the {ITE, ETE} prepares the control message body, it
encapsulates the body in outer, SEAL and mid-layer headers and
trailers the same as for the encapsulation of an ordinary inner IP
packet (see Section 4.3). During encapsulation, the {ITE, ETE} sets
the outer IP destination and source addresses of the message to the
source and destination addresses (respectively) of the invoking
packet. If the destination address in the packet was multicast, the
{ITE, ETE} instead sets the outer IP source address to an address
assigned to the underlying IP interface.
The following SEAL control message types are currently defined; other
values for Type will be recorded in the IANA registry for SEAL:
4.4.5.1. Reassembly Report (Type=0)
An ETE generates a Reassembly Report to inform the ITE of various
conditions encountered during outer IP and SEAL-layer reassembly.
The following values for Code are currently defined (other values for
Code will be recorded in the IANA registry for SEAL):
o Code = 0 : Segment Acknowledged
o Code = 1 : Fragmentation Experienced
o Code = 2 : Packet Too Big
o Code = 3 : Time Exceeded
The ETE prepares the Reassembly Report according to the Code. In
each case, the Reassembly Report includes an S_MRU value that denotes
the maximum-sized packet the ETE is willing to receive from the ITE
(normally set to the ETE's reassembly buffer size - see Section
4.4.1). The ETE MAY advertise different S_MRU values to different
ITEs, but it SHOULD maintain a persistent value for each ITE that
changes only very rarely (if at all). Reassembly Report formats for
each Code are specified in the following sections:
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4.4.5.1.1. Segment Acknowledged (Code=0)
When an ETE receives a SEAL segment following IP reassembly that has
the 'A' bit set in the SEAL header, it prepares a "Reassembly Report
- Segment Acknowledged" message with Type=0 and Code=0. The message
body 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: Segment Acknowledged Message Format
The ETE writes the maximum-sized packet it is willing to receive from
this ITE in a 32-bit S_MRU field ( a value of zero in this field
means that S_MRU is not specified in this message). The ETE then
writes as much of the invoking packet in the reassembly buffer as
possible at the end of the message body, adds the encapsulating
headers and trailers, and sends the message to the ITE.
4.4.5.1.2. Fragmentation Experienced (Code=1)
When an ETE receives an IP first-fragment of a SEAL packet that
experienced outer IP fragmentation, it uses the IP first-fragment to
prepare a "Reassembly Report - Fragmentation Experienced" message
with Type=0 and Code=1. The message body is formatted as follows:
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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=0 | Code=1 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MSS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: IP Fragmentation Experienced Message Format
The ETE writes the maximum-sized packet it is willing to receive from
this ITE in the S_MRU field (a value of zero in this field means that
S_MRU is not specified in this message) then writes the length of the
first IP fragment in the S_MSS field. The ETE then writes as much of
the invoking packet as possible at the end of the message body, adds
the encapsulating headers and trailers, and sends the message to the
ITE.
4.4.5.1.3. Packet Too Big (Code=2)
An ETE generates a "Reassembly Report - Packet Too Big" message when
it discards a (reassembled) SEAL data packet that is larger than it
is willing to receive from this ITE. The ETE sets Type=0 and Code=2.
The message body 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=2 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Packet Too Big Message Format
The ETE writes the maximum-sized packet it is willing to receive from
this ITE in the S_MRU field (a value of zero in this field means that
S_MRU is not specified in this message). The ETE then writes as much
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of the invoking packet as possible at the end of the message body,
adds the encapsulating headers and trailers, and sends the message to
the ITE.
4.4.5.1.4. Time Exceeded (Code=3)
An ETE generates a "Reassembly Report - Time Exceeded" message when
it discards an incomplete SEAL reassembly buffer due to a reassembly
timeout. The ETE sets Type=0 and Code=3. The message body 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=3 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Time (in milliseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of most recent packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Time Exceeded Message Format
The ETE writes the maximum-sized packet it is willing to receive from
this ITE in the S_MRU field (a value of zero in this field means that
S_MRU is not specified in this message) then writes the Time (in
milliseconds) from when the first SEAL segment arrived until the SEAL
reassembly timeout expired in the Time field. The ETE finally writes
as much of the most recently received packet in the reassembly buffer
as possible at the end of the message body, adds the encapsulating
headers and trailers, and sends the message to the ITE.
4.4.5.2. Parameter Problem (Type=1)
An ETE generates a "Parameter Problem" message when it receives a
SEAL packet with an invalid value in the SEAL header. The ETE sets
Type=1 and Code=0; other values for Code will be recorded in the IANA
registry for SEAL. The message body is formatted as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=1 | Code=0 | Reserved=0 | Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Pointer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Parameter Problem Message Format
The ETE writes the bit number of the SEAL header field that triggered
the error in a 32-bit Pointer field. (For example, when the
parameter problem is specific to the NEXTHDR/SEG field the ETE writes
the value 8 in this field.) The ETE finally writes as much of the
invoking packet as possible at the end of the message body, adds the
encapsulating headers and trailers, and sends the message to the ITE.
4.4.5.3. Information Request Solicit (Type=2)
An ETE generates an "Information Request Solicit" message when it
receives a SEAL data packet with stale information and wishes to
inform the ITE of new information. The ETE sets Type=2 and sets Code
to a value specific to the associated tunneling protocol (for
example, the tunneling protocol can use the Information Request
Solicit message to initiate mapping updates). When the ETE sets
Code=0 the Control Data field is NULL; other values for Code will be
recorded in the IANA registry for SEAL, and other Control Data field
formats will be specified by the associated tunneling protocol.
The Information Request Solicit message 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=2 | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Information Request Solicit Message Format
The ETE writes as much of the invoking packet as possible at the end
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of the message body, adds the encapsulating headers and trailers,
then sends the message to the ITE and listens for a corresponding
Information Request (see Section 4.4.5.4).
4.4.5.4. Information Request (Type=3)
An ITE generates an "Information Request" message when it receives an
Information Request Solicit control message from an ETE. An ETE
generates an Information Request message when it receives a SEAL data
packet with I=1 in the SEAL header from an ITE.
When an {ITE, ETE} generates an Information Request message, it sets
Type=3 and sets Code to a value specific to the associated tunneling
protocol (for example, the tunneling protocol can use the Information
Request message to request mapping updates). When the {ITE, ETE}
sets Code=0 the Control Data field is NULL; other values for Code
will be recorded in the IANA registry for SEAL, and other Control
Data field formats will be specified by the associated tunneling
protocol.
The Information Request message 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=3 | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11: Information Request Message Format
The {ITE, ETE} writes as much of the invoking packet as possible at
the end of the message body, adds the encapsulating headers and
trailers, then sends the Information Request message. The {ITE, ETE}
MAY cache the SEAL_ID in the control message SEAL header so that it
can be matched against a corresponding Information Reply (see Section
4.4.5.5).
4.4.5.5. Information Reply (Type=4)
When an {ITE, ETE} receives an Information Request message, it
responds by sending an "Information Reply" message. The {ITE, ETE}
sets Type=4 and sets Code to a value specific to the associated
tunneling protocol (for example, the tunneling protocol can use the
Information Reply message to encode mapping updates). When Code=0
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the Control Data field is NULL; other values for Code will be
recorded in the IANA registry for SEAL, and other Control Data field
formats will be specified by the associated tunneling protocol.
The information reply message 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=4 | Code=0 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking packet |
~ as possible without the message ~
| exceeding 576 bytes |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: Information Reply Message Format
The {ITE, ETE} writes as much of the invoking packet as possible at
the end of the message body, adds the encapsulating headers and
trailers, then sends the Information Reply message.
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:
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.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|MOD|C|A|I| RSV | NEXTHDR | Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 13: SEAL Version 1 Header Format
where the header fields are defined as:
MOD (2)
a 2-bit value that encodes the SEAL protocol mode. This section
describes Mode 0 of the SEAL protocol, i.e., the MOD field encodes
the value '0'.
C (1)
the "Control" bit. Set to 1 in SEAL control messages, and set to
0 in SEAL data messages.
A (1)
the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes
to receive an explicit acknowledgement from the ETE.
I (1)
the "Information Request Solicit" bit. Set to 1 if the ITE wishes
the ETE to initiate an Information Request.
RSV (3)
a 3-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.
ID Extension (16)
a 16-bit Identification extension field.
5.3. ITE Specification
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5.3.1. Tunnel Interface MTU
SEAL-FS observes the SEAL-SR specification found in Section 4.3.1.
5.3.2. Admitting Packets into the Tunnel Interface
SEAL-FS observes the SEAL-SR specification found in Section 4.3.2.
5.3.3. Segmentation
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. The
ITE then 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 an IPv6 packet or an IPv4 packet with DF=1,
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 an IPv4 packet with DF=0, and the packet is
larger than (MIN(S_MRU, S_MSS) - HLEN), the ITE uses inner IPv4
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 an IPv6 packet or an IPv4 packet with DF=1,
the ITE can instead employ a stateless strategy by simply
encapsulating and sending the packet as specified in Section 4.3.4
through 4.3.7. The ITE then translates any "Reassembly Report -
Fragmentation Needed" and "Reassembly Report - Packet Too big"
messages into PTB messages to return to the original source (where
the translation is based on the encapsulated portion of the invoking
packet at the end of the reassembly report message). In this method,
the ITE need not retain per-ETE S_MRU and S_MSS state.
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5.3.4. Encapsulation
SEAL-FS observes the SEAL-SR specification found in Section 4.3.4,
except that it uses the header format defined in this section and
with the MOD field set to '0'. SEAL-FS uses the C, A and I bits the
same as specified for SEAL-SR.
5.3.5. Probing Strategy
SEAL-FS observes the SEAL-SR specification found in Section 4.3.5.
5.3.6. Packet Identification
SEAL-FS observes the SEAL-SR soft state specifications found in
Section 4.3.6.
5.3.7. Sending SEAL Protocol Packets
SEAL-FS observes the SEAL-SR specification found in Section 4.3.7.
5.3.8. Processing Raw ICMP Messages
SEAL-FS observes the SEAL-SR specification found in Section 4.3.8.
5.3.9. Processing SEAL Control Messages
SEAL-FS observes the SEAL-SR specification found in Section 4.3.9.
5.4. ETE Specification
5.4.1. Reassembly Buffer Requirements
SEAL-FS does not maintain a reassembly buffer for SEAL reassembly,
but still maintains a value for S_MRU as the largest packet size the
ETE is willing to receive.
5.4.2. IP-Layer Reassembly
SEAL-FS uses SEAL-protocol IP first-fragments solely for the purpose
of generating SEAL Reassembly Reports as specified in Section 4.4.2,
but otherwise discards all SEAL-protocol packets that arrived 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 SEAL-FS headers contain no segmentation and reassembly
information.
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As for SEAL-SR, SEAL-FS returns a Parameter Problem for SEAL packets
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.
5.4.5. Sending SEAL Control Messages
SEAL-FS observes the SEAL-SR specification found in Section 4.4.5.
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 use 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.
The IANA is instructed to establish a "SEAL Protocol" registry to
record SEAL Mode values and SEAL control message Code and Type
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.
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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 wrt 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 Reassembly Report 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 Reassembly Reports.
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
and trailers. 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.
SEAL control messages carry the SEAL_ID of the packet-in-error.
Therefore, when an ITE receives a SEAL control message it can
unambiguously associate the message with the 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 tunnel-mode 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
fragmentation.
Section 4 of [RFC2460] specifies a method for inserting and
processing extension headers between the base IPv6 header and
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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 error messages in an outer UDP/IP
header 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. Most importantly, all SEAL packets have an Identification field
that is sufficiently long to be used for duplicate packet
detection purposes and to match 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.
In summary, the SEAL approach ensures that packets of various sizes
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are either delivered or deterministically dropped. When end systems
use their own end-to-end MTU determination mechanisms [RFC4821], the
SEAL advantages are further enhanced.
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.
[RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum
options", RFC 1146, March 1990.
[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.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
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14.2. 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-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-05 (work in progress), September 2009.
[I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-07 (work in progress),
December 2009.
[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-05 (work in progress),
December 2009.
[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.".
[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.
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[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.
[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.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[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.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
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ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
[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.".
Appendix A. Reliability
Although a SEAL-SR 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
be inefficient to also require the tunnel to perform ARQ in the
general sense.
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 Reassembly 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].
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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 first-pass 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
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
includes upper-layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL for
the carriage of protocol data encapsulated as 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.
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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:
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
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believed that routers would migrate more quickly than hosts. Option
2) was a strong contender, but repeated attempts to secure an "RF"
bit in the IPv4 header from the IESG failed and the proponents became
discouraged. 3) was abandoned because it was perceived as too
complicated, and 4) never received any apparent serious
consideration. Proposal 5) was a late entry into the discussion from
Steve Deering on Feb. 24th, 1990. The discussion group soon
thereafter seemingly lost track of all other proposals and adopted
5), which eventually evolved into [RFC1191] and later [RFC1981].
In retrospect, the "RF" bit postulated in 2) is not needed if a
"contract" is first established between the peers, as in proposal 4)
and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
Feb 19. 1990. These proposals saw little discussion or rebuttal, and
were dismissed based on the following the assertions:
o routers upgrade their software faster than hosts
o PCs could not reassemble fragmented packets
o Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets
o Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)
o the 16-bit IP_ID field could wrap around and disrupt reassembly at
high packet arrival rates
The first four assertions, although perhaps valid at the time, have
been overcome by historical events. 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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