Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track November 17, 2011
Expires: May 20, 2012
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-38.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 are manifested by tunnels that 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on May 20, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 6
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 8
4. SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 9
4.1. VET Interface Model . . . . . . . . . . . . . . . . . . . 9
4.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 10
4.3. SEAL Header and Trailer Format . . . . . . . . . . . . . . 11
4.4. ITE Specification . . . . . . . . . . . . . . . . . . . . 14
4.4.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
4.4.2. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 15
4.4.3. Pre-Encapsulation . . . . . . . . . . . . . . . . . . 16
4.4.4. SEAL Encapsulation . . . . . . . . . . . . . . . . . . 17
4.4.5. Outer Encapsulation . . . . . . . . . . . . . . . . . 18
4.4.6. Path Probing and ETE Reachability Verification . . . . 19
4.4.7. Processing ICMP Messages . . . . . . . . . . . . . . . 19
4.4.8. IPv4 Middlebox Reassembly Testing . . . . . . . . . . 20
4.4.9. Stateful MTU Determination . . . . . . . . . . . . . . 22
4.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 22
4.5. ETE Specification . . . . . . . . . . . . . . . . . . . . 23
4.5.1. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 23
4.5.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 23
4.5.3. Decapsulation and Re-Encapsulation . . . . . . . . . . 23
4.6. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 25
4.6.1. Generating SCMP Error Messages . . . . . . . . . . . . 25
4.6.2. Processing SCMP Error Messages . . . . . . . . . . . . 27
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 29
6. End System Requirements . . . . . . . . . . . . . . . . . . . 29
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 29
8. Nested Encapsulation Considerations . . . . . . . . . . . . . 30
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
10. Security Considerations . . . . . . . . . . . . . . . . . . . 30
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 31
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 32
13.1. Normative References . . . . . . . . . . . . . . . . . . . 32
13.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 36
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 37
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 37
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 38
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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 (also known as "tunneling") 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 primary
functions of IPv4 are to provide for routing, addressing, and 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 [I-D.ietf-intarea-ipv4-id-update]. Due to the
escalating deployment of high-speed links, however, this number has
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
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deemed "harmful" in both classic and modern-day studies (see above).
In particular, IPv4 fragmentation raises issues ranging from minor
annoyances (e.g., in-the-network router fragmentation [RFC1981]) 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 have been known 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 trivial since there is no way to
authenticate the source of the messages [RFC5927]. 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][SIGCOMM].
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, an ingress tunnel endpoint (ITE) may be
required to forward encapsulated packets into the subnetwork on
behalf of hundreds, thousands, or even more original sources within
the end site that it serves. 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 must rely
on ICMP error messages coming from the subnetwork that may be
suspect, subject to loss due to filtering middleboxes, or
insufficiently provisioned for translation into error messages to be
returned to the original sources.
Although recent works have led to the development of a robust end-to-
end MTU determination scheme [RFC4821], they do not excuse tunnels
from delivering path MTU discovery feedback when packets are lost due
to size restrictions. Moreover, in current practice existing
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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. Therefore, 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. Example connected network
routing regions include Mobile Ad hoc Networks (MANETs), enterprise
networks and the global public Internet itself. Subnetwork border
nodes forward unicast and multicast 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., IPv4, IPv6,
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
tunneling protocols. A transport-mode of operation is also possible,
and described in Appendix C.
SEAL provides a mid-layer encapsulation that accommodates links with
diverse MTUs and allows routers in the subnetwork to perform
efficient duplicate packet detection. The encapsulation further
ensures data origin authentication, packet header integrity and anti-
replay.
SEAL treats tunnels that traverse the subnetwork as ordinary links
that must support network layer services. Moreover, SEAL provides
dynamic mechanisms to ensure a maximal path MTU over the tunnel.
This is in contrast to static approaches which avoid MTU issues by
selecting a lowest common denominator MTU value that may be overly
conservative for the vast majority of tunnel paths 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:
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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.
ETE Link Path
a subnetwork path from an ITE to an ETE beginning with an
underlying link of the ITE as the first hop.
inner packet
an unencapsulated network layer protocol packet (e.g., IPv6
[RFC2460], IPv4 [RFC0791], OSI/CLNP [RFC1070], etc.) before any
outer encapsulations are added. Internet protocol numbers that
identify inner packets are found in the IANA Internet Protocol
registry [RFC3232]. SEAL protocol packets that incur an
additional layer of SEAL encapsulation are also considered inner
packets.
outer IP packet
a packet resulting from adding an outer IP header (and possibly
other outer headers) to a SEAL-encapsulated inner 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.).
Packet Too Big (PTB)
a control plane message indicating an MTU restriction (e.g., an
ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
"Fragmentation Needed" message [RFC0792], etc.).
IP
used to generically refer to either Internet Protocol (IP)
version, i.e., IPv4 or IPv6.
The following abbreviations correspond to terms used within this
document and/or elsewhere in common Internetworking nomenclature:
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DF - the IPv4 header "Don't Fragment" flag [RFC0791]
ETE - Egress Tunnel Endpoint
HLEN - the length of the SEAL header plus outer headers
ICV - Integrity Check Vector
ITE - Ingress Tunnel Endpoint
MTU - Maximum Transmission Unit
SCMP - the SEAL Control Message Protocol
SDU - SCMP Destination Unreachable message
SNA - SCMP Neighbor Advertisement message
SNS - SCMP Neighbor Solicitation message
SPP - SCMP Parameter Problem message
SPTB - SCMP Packet Too Big message
SEAL - Subnetwork Encapsulation and Adaptation Layer
SEAL_PORT - a transport-layer service port number used for SEAL
SEAL_PROTO - an IP protocol number used for SEAL
TE - Tunnel Endpoint (i.e., either ingress or egress)
VET - Virtual Enterprise Traversal
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" 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 originally motivated by the specific case of subnetwork
abstraction for Mobile Ad hoc Networks (MANETs), however the domain
of applicability also extends to subnetwork abstractions over
enterprise networks, ISP networks, SOHO networks, the global public
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Internet itself, and any other connected network routing region.
SEAL, along with the Virtual Enterprise Traversal (VET)
[I-D.templin-intarea-vet] tunnel virtual interface abstraction, are
the functional building blocks for the Internet Routing Overlay
Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in
Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
architectures.
SEAL provides a network sublayer for encapsulation of an inner
network layer packet within outer encapsulating headers. SEAL can
also be used as a sublayer within a transport layer protocol data
payload, where transport layer encapsulation is typically used for
Network Address Translator (NAT) traversal as well as operation over
subnetworks that give preferential treatment to certain "core"
Internet protocols (e.g., TCP, UDP, etc.). 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.
To accommodate MTU diversity, the Egress Tunnel Endpoint (ETE) acts
as a passive observer that simply informs the Ingress Tunnel Endpoint
(ITE) of any packet size limitations. This allows the ITE to return
appropriate path MTU discovery feedback even if the network path
between the ITE and ETE filters ICMP messages.
SEAL further ensures data origin authentication, packet header
integrity, and anti-replay. The SEAL framework is therefore similar
to the IP Security (IPsec) Authentication Header (AH)
[RFC4301][RFC4302], however it provides only minimal hop-by-hop
authenticating services along a path while leaving full data
integrity, authentication and confidentiality services as an end-to-
end consideration. While SEAL performs data origin authentication,
the origin site must also perform the necessary ingress filtering in
order to provide full source address verification
[I-D.ietf-savi-framework].
4. SEAL Specification
The following sections specify the operation of SEAL:
4.1. VET Interface Model
SEAL is an encapsulation sublayer used within VET non-broadcast,
multiple access (NBMA) tunnel virtual interfaces. Each VET interface
is configured over one or more underlying interfaces attached to
subnetwork links. The VET interface connects an ITE to one or more
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ETE "neighbors" via tunneling across an underlying subnetwork, where
tunnel neighbor relationship may be either unidirectional or
bidirectional.
A unidirectional tunnel neighbor relationship allows the near end ITE
to send data packets forward to the far end ETE, while the ETE only
returns control messages when necessary. A bidirectional tunnel
neighbor relationship is one over which both TEs can exchange both
data and control messages.
Implications of the VET unidirectional and bidirectional models are
discussed in [I-D.templin-intarea-vet].
4.2. SEAL Model of Operation
SEAL-enabled ITEs encapsulate each inner packet in a SEAL header, any
outer header encapsulations, and in some instances a SEAL trailer as
shown in Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
| SEAL Trailer |
+--------------------+
Figure 1: SEAL Encapsulation
The ITE inserts the SEAL header according to the specific tunneling
protocol. For simple encapsulation of an inner network layer packet
within an outer IP header (e.g.,
[RFC1070][RFC2003][RFC2473][RFC4213], etc.), the ITE inserts the SEAL
header between the inner packet and outer IP headers as: IP/SEAL/
{inner packet}.
For encapsulations over transports such as UDP, the ITE inserts the
SEAL header between the outer transport layer header and the inner
packet, e.g., as IP/UDP/SEAL/{inner packet} (similar to [RFC4380]).
In that case, the UDP header is seen as an "other outer header" as
depicted in Figure 1.
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When necessary, the ITE also appends a SEAL trailer at the end of the
SEAL packet. In that case, the trailer is added after the final byte
of the encapsulated packet.
SEAL supports both "nested" tunneling and "re-encapsulating"
tunneling. Nested tunneling occurs when a first tunnel is
encapsulated within a second tunnel, which may then further be
encapsulated within additional tunnels. Nested tunneling can be
useful, and stands in contrast to "recursive" tunneling which is an
anomalous condition incurred due to misconfiguration or a routing
loop. Considerations for nested tunneling are discussed in Section 4
of [RFC2473].
Re-encapsulating tunneling occurs when a packet arrives at a first
ETE, which then acts as an ITE to re-encapsulate and forward the
packet to a second ETE connected to the same subnetwork. In that
case each ITE/ETE transition represents a segment of a bridged path
between the ITE nearest the source and the ETE nearest the
destination. Combinations of nested and re-encapsulating tunneling
are also naturally supported by SEAL.
The SEAL ITE considers each {underlying interface, IP address} pair
as the ingress attachment point to a subnetwork link path to the ETE.
The ITE therefore maintains path MTU state on a per ETE link path
basis, although it may instead maintain only the lowest-common-
denominator values for all of the ETE's link paths in order to reduce
state.
Finally, the SEAL ITE ensures that the inner network layer protocol
will see a minimum MTU of 1280 bytes over each ETE link path
regardless of the outer network layer protocol version, i.e., even if
a small amount of fragmentation and reassembly are necessary.
4.3. SEAL Header and Trailer Format
The SEAL header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|P|R|T|U|Z| NEXTHDR | PREFLEN | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ PREFIX (when present) ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 2: SEAL Header Format
VER (2)
a 2-bit version field. This document specifies Version 0 of the
SEAL protocol, i.e., the VER field encodes the value 0.
C (1)
the "Control/Data" bit. Set to 1 by the ITE in SEAL Control
Message Protocol (SCMP) control messages, and set to 0 in ordinary
data packets.
P (1)
the "Prefix Included" bit. Set to 1 if the header includes a
Prefix Field. Used for SCMP messages that do not include a
packet-in-error (see: [I-D.templin-intarea-vet]), and for NULL
SEAL data packets used as probes (see: Section 4.4.6).
R (1)
the "Redirects Permitted" bit. For data packets, set to 1 by the
ITE to inform the ETE that the source is accepting Redirects (see:
[I-D.templin-intarea-vet]).
T (1)
the "Trailer Included" bit. Set to 1 if the ITE was obliged to
include a trailer.
U (1)
the "Unfragmented Packet" bit. Set to 1 by the ITE in SEAL data
packets for which it wishes to receive an explicit acknowledgement
from the ETE if the packet arrives unfragmented.
Z (1)
the "Reserved" bit. Must be set to 0 for this version of the SEAL
specification.
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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.
PREFLEN (8) an 8-bit field that encodes the length of the prefix to
be applied to the source address of the inner packets (when P==0)
or the prefix included in the PREFIX field (when P==1).
LINK_ID (5)
a 5-bit link identification value, set to a unique value by the
ITE for each underlying link as the first hop of a path over which
it will send encapsulated packets to ETEs. Up to 32 ETE link
paths are therefore supported for each ETE.
LEVEL (3)
a 3-bit nesting level; use to limit the number of tunnel nesting
levels. Set to an integer value up to 7 in the innermost SEAL
encapsulation, and decremented by 1 for each successive additional
SEAL encapsulation nesting level. Up to 8 levels of nesting are
therefore supported.
PKT_ID (32)
a 32-bit per-packet identification field. Set to a monotonically-
incrementing 32-bit value for each SEAL packet transmitted to this
ETE, beginning with 0.
ICV1 (32)
a 32-bit header integrity check value that covers the leading 128
bytes of the packet beginning with the SEAL header. The value 128
is chosen so that at least the SEAL header as well as the inner
packet network and transport layer headers are covered by the
integrity check.
PREFIX (variable)
a variable-length string of bytes; present only when P==1. The
field length is determined by calculating Len=(Ceiling(PREFLEN /
32) * 4). For example, if PREFLEN==63, the field is 8 bytes in
length and encodes the leading 63 bits of the inner network layer
prefix beginning with the most significant bit.
When T==1, SEAL encapsulation also includes a trailer 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SEAL Trailer Format
ICV2 (32)
a 32-bit packet integrity check value. Present only when T==1,
and covers the remaining length of the encapsulated packet beyond
the leading 128 bytes (i.e., the remaining portion that was not
covered by ICV1). Added as a trailing 32 bit field following the
final byte of the encapsulated SEAL packet and used to detect
reassembly misassociations. Need not be aligned on an even byte
boundary.
4.4. ITE Specification
4.4.1. Tunnel Interface MTU
The tunnel interface must present a constant MTU value to the inner
network layer as the size for admission of inner packets into the
interface. Since VET NBMA tunnel virtual interfaces may support a
large set of ETE link paths 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 large packets sent by end systems incur
additional encapsulation at an ITE, however, they may be dropped
silently within the tunnel since the network may not always deliver
the necessary PTBs [RFC2923].
The ITE should therefore set a tunnel 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
also set smaller MTU values; however, care must be taken not to set
so small a value that original sources would experience an MTU
underflow. In particular, IPv6 sources must see a minimum path MTU
of 1280 bytes, and IPv4 sources should see a minimum path MTU of 576
bytes.
The inner network layer protocol consults the tunnel interface MTU
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when admitting a packet into the interface. For non-SEAL 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
interface as an independent packet.
For all other inner packets, the inner network layer 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 contains
as much of the invoking packet as possible without the entire message
exceeding the network layer minimum MTU (e.g., 1280 bytes for IPv6,
576 bytes for IPv4, etc.).
The ITE can alternatively set an indefinite MTU on the tunnel
interface such that all inner packets are admitted into the interface
regardless of their size. For ITEs that host applications that use
the tunnel interface directly, 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., by reducing the size advertised in the MSS option of outgoing
TCP messages (sometimes known as "MSS clamping").
In light of the above considerations, the ITE SHOULD configure an
indefinite MTU on tunnel *router* interfaces so that subnetwork
adaptation is handled from within the interface. The ITE MAY instead
set a finite MTU on tunnel *host* interfaces.
4.4.2. Tunnel Neighbor Soft State
Within the tunnel virtual interface, the ITE maintains a per tunnel
neighbor (i.e., a per-ETE) integrity check vector (ICV) calculation
algorithm and (when data origin authentication is required) a
symmetric secret key to calculate the ICV(s) in packets it will send
to this ETE. The ITE also maintains a window of PKT_ID values for
the packets it has recently sent to this ETE.
For each ETE link path, the ITE must account for the lengths of the
headers to be used for encapsulation. The ITE therefore maintains
the per ETE link path constant values "SHLEN" set to length of the
SEAL header, "UHLEN" set to the length of the UDP encapsulating
header (or 0 if UDP encapsulation is not used), "IHLEN" set to the
length of the outer IP layer header, and "HLEN" set to (SHLEN+UHLEN+
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IHLEN). (The ITE must include the length of the uncompressed headers
even if header compression is enabled when calculating these
lengths.) In addition, the ETE maintains a constant value "MIN_MTU"
set to 1280+HLEN as well as a variable "PATH_MTU" initialized to the
MTU of the underlying link.
For IPv4, the ITE also maintains the per ETE link path boolean
variables "USE_DF" (initialized to "FALSE") and "USE_TRAILER"
(initialized to "TRUE" if PATH_MTU is less than MIN_MTU; otherwise
initialized to "FALSE") .
The ITE may instead maintain *HLEN, MIN_MTU, PATH_MTU, USE_DF, and
USE_TRAILER as per ETE (rather than per ETE link path) values. In
that case, the values reflect the lowest-common-denominator MTU
across all of the ETE's link paths.
4.4.3. Pre-Encapsulation
For each inner packet admitted into the tunnel interface, if the
packet is itself a SEAL packet (i.e., one with either SEAL_PROTO in
the IP protocol/next-header field, or with SEAL_PORT in the transport
layer destination port field) and the LEVEL field of the SEAL header
contains the value 0, the ITE silently discards the packet.
Otherwise, for IPv4 inner packets with DF==0 in the IPv4 header, if
the packet is larger than 512 bytes and is not the first fragment of
a SEAL packet (i.e., not a packet that includes a SEAL header) the
ITE fragments the packet into inner fragments no larger than 512
bytes. The ITE then submits each inner fragment for SEAL
encapsulation as specified in Section 4.4.4.
For all other packets, if the packet is no larger than (MAX(PATH_MTU,
MIN_MTU)-HLEN) for the corresponding ETE link path, the ITE submits
it for SEAL encapsulation as specified in Section 4.4.4. Otherwise,
the ITE sends a PTB error message toward the source address of the
inner packet.
To send the PTB message, the ITE first checks its forwarding tables
to discover the previous hop toward the source address of the inner
packet. If the previous hop is reached via the same tunnel
interface, the ITE sends an SCMP PTB (SPTB) message to the previous
hop (see: Section 4.6.1.1) with the MTU field set to (MAX(PATH_MTU,
MIN_MTU)-HLEN). Otherwise, the ITE sends an ordinary PTB message
appropriate to the inner protocol version with the MTU field set to
(MAX(PATH_MTU, MIN_MTU)-HLEN).
After sending the (S)PTB message, the ITE discards the inner packet.
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4.4.4. SEAL Encapsulation
The ITE next encapsulates the inner packet in a SEAL header formatted
as specified in Section 4.3. The ITE sets NEXTHDR to the protocol
number corresponding to the address family of the encapsulated inner
packet. For example, the ITE sets NEXTHDR to the value '4' for
encapsulated IPv4 packets [RFC2003], '41' for encapsulated IPv6
packets [RFC2473][RFC4213], '80' for encapsulated OSI/CLNP packets
[RFC1070], etc.
The ITE then sets R=1 if redirects are permitted (see:
[I-D.templin-intarea-vet]) and sets PREFLEN to the length of the
prefix to be applied to the inner source address. The ITE's claimed
PREFLEN is subject to verification by the ETE; hence, the ITE MUST
set PREFLEN to the exact prefix length that it is authorized to use.
(Note that if this process is entered via re-encapsulation (see:
Section 4.5.4), PREFLEN and R are instead copied from the SEAL header
of the re-encapsulated packet. This implies that the PREFLEN and R
values are propagated across a re-encapsulating chain of ITE/ETEs
that must all be authorized to represent the prefix.)
Next, the ITE sets (C=0; P=0; Z=0), then sets LINK_ID to the value
assigned to the underlying ETE link path and sets PKT_ID to a
monotonically-increasing integer value for this ETE, beginning with 0
in the first packet transmitted. The ITE also sets U=1 if it needs
to determine whether the ETE will receive the packet without
fragmentation, e.g., for ETE reachability determination (see: Section
4.4.6), to test whether a middlebox on the path is reassembling
fragmented packets before they arrive at the ETE (see: Section
4.4.8), for stateful MTU determination (see Section 4.4.9), etc.
Otherwise, the ITE sets U=0.
Next, if the inner packet is not itself a SEAL packet the ITE sets
LEVEL to an integer value between 0 and 7 as a specification of the
number of additional layers of nested SEAL encapsulations permitted.
If the inner packet is a SEAL packet that is undergoing nested
encapsulation, the ITE instead sets LEVEL to the value that appears
in the inner packet's SEAL header minus 1. If the inner packet is
undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL
value from the SEAL header of the packet to be re-encapsulated.
Next, if this is an IPv4 ETE link path with USE_TRAILER==TRUE, and
the inner packet is larger than (128-SHLEN-UHLEN) bytes but no larger
than 1280 bytes, the ITE sets T=1. Otherwise, the ITE sets T=0. The
ITE then adds the outer encapsulating headers, calculates the ICV(s)
and performs any necessary outer fragmentation as specified in
Section 4.4.5.
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4.4.5. Outer Encapsulation
Following SEAL encapsulation, the ITE next encapsulates the packet in
the requisite outer headers according to the specific encapsulation
format (e.g., [RFC1070], [RFC2003], [RFC2473], [RFC4213], 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 transport service port field
(e.g., when IP/UDP encapsulation is used).
When UDP encapsulation is used, the ITE sets the UDP header fields as
specified in Section 5.5.4 of [I-D.templin-intarea-vet] (where the
UDP header length field includes the length of the SEAL trailer, if
present). The ITE then performs outer IP header encapsulation as
specified in Section 5.5.5 of [I-D.templin-intarea-vet]. If this
process is entered via re-encapsulation (see: Section 4.5.4), the ITE
instead follows the outer IP/UDP re-encapsulation procedures
specified in Section 5.5.6 of [I-D.templin-intarea-vet].
When IPv4 is used as the outer encapsulation layer, if USE_DF==FALSE
the ITE sets DF=0 in the IPv4 header to allow the packet to be
fragmented within the subnetwork if it encounters a restricting link.
Otherwise, the ITE sets DF=1.
When IPv6 is used as the outer encapsulation layer, the "DF" flag is
absent but implicitly set to 1. The packet therefore will not be
fragmented within the subnetwork, since IPv6 deprecates in-the-
network fragmentation.
The ITE next sets ICV1=0 in the SEAL header and calculates the packet
ICVs. The ICVs are calculated using an algorithm agreed on by the
ITE and ETE. When data origin authentication is required, the
algorithm uses a symmetric secret key so that the ETE can verify that
the ICVs were generated by the ITE.
The ITE first calculates the ICV over the leading 128 bytes of the
packet (or up to the end of the packet if there are fewer than 128
bytes) beginning with the UDP header (if present) then places result
in the ICV1 field in the header. If T==1, the ITE next calculates
the ICV over the remainder of the packet and places the result in the
ICV2 field in the SEAL trailer. The ITE then submits the packet for
outer encapsulation.
Next, the ITE uses IP fragmentation if necessary to fragment the
encapsulated packet into outer IP fragments that are no larger than
PATH_MTU. By virtue of the pre-encapsulation packet size
calculations specified in Section 4.4.3, fragmentation will therefore
only occur for outer packets that are larger than PATH_MTU but no
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larger than MIN_MTU. (Note that, for IPv6, fragmentation must be
performed by the ITE itself, while for IPv4 the fragmentation could
instead be performed by a router in the ETE link path.)
The ITE then sends each outer packet/fragment via the underlying link
corresponding to LINK_ID.
4.4.6. Path Probing and ETE Reachability Verification
All SEAL data packets sent by the ITE are considered implicit probes.
SEAL data packets will elicit an SCMP message from the ETE if it
needs to acknowledge a probe and/or report an error condition. SEAL
data packets may also be dropped by either the ETE or a router on the
path, which will return an ICMP message.
The ITE can also send an SCMP Router/Neighbor Solicitation message to
elicit an SCMP Router/Neighbor Advertisement response (see:
[I-D.templin-intarea-vet]) as verification that the ETE is still
reachable via a specific link path.
The ITE processes ICMP messages as specified in Section 4.4.7.
The ITE processes SCMP messages as specified in Section 4.6.2.
4.4.7. Processing ICMP Messages
When the ITE sends SEAL packets, it may receive ICMP error
messages[RFC0792][RFC4443] from another ITE on the path to the ETE
(i.e., in case of nested encapsulations) or from an ordinary router
within the subnetwork. Each ICMP message includes an outer IP
header, followed by an ICMP header, followed by a portion of the SEAL
data packet that generated the error (also known as the "packet-in-
error") beginning with the outer IP header.
The ITE should process 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. The ITE can also process other ICMP messages that do not
include sufficient information in the packet-in-error as a hint that
the ETE link path may be failing. Specific actions that the ITE may
take in these cases are out of scope.
For other ICMP messages, the should use any outer header information
available as a first-pass authentication filter (e.g., to determine
if the source of the message is within the same administrative domain
as the ITE) and discards the message if first pass filtering fails.
Next, the ITE examines the packet-in-error beginning with the SEAL
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header. If the value in the PKT_ID field is not within the window of
packets the ITE has recently sent to this ETE, or if the value in the
SEAL header ICV1 field is incorrect, the ITE discards the message.
Next, if the received ICMP message is a PTB the ITE sets the
temporary variable "PMTU" for this ETE link path to the MTU value in
the PTB message. If PMTU==0, the ITE consults a plateau table (e.g.,
as described in [RFC1191]) to determine PMTU based on the length
field in the outer IP header of the packet-in-error. (For example,
if the ITE receives a PTB message with MTU==0 and length 1500, it can
set PMTU=1450. If the ITE subsequently receives a PTB message with
MTU==0 and length 1450, it can set PMTU=1400, etc.) If the ITE is
performing stateful MTU determination for this ETE link path (see
Section 4.4.9), the ITE next sets PATH_MTU=PMTU. If PMTU is less
than MIN_MTU, the ITE sets PATH_MTU=PMTU (and for IPv4 also sets
USE_TRAILER=TRUE), then discards the message.
If the ICMP message was not discarded, the ITE then transcribes it
into a message to return to the previous hop. If the previous hop
toward the inner source address within the packet-in-error is reached
via the same tunnel interface the SEAL data packet was sent on, the
ITE transcribes the ICMP message into an SCMP message. Otherwise,
the ITE transcribes the ICMP message into a message appropriate for
the inner protocol version.
To transcribe the message, the ITE extracts the inner packet from
within the ICMP message packet-in-error field and uses it to generate
a new message corresponding to the type of the received ICMP message.
For SCMP messages, the ITE generates the message the same as
described for ETE generation of SCMP messages in Section 4.6.1. For
(S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field.
The ITE finally forwards the transcribed message to the previous hop
toward the inner source address.
4.4.8. IPv4 Middlebox Reassembly Testing
For IPv4, the ITE can perform a qualification exchange over an ETE
link path to ensure that the subnetwork correctly delivers fragments
to the ETE. This procedure can be used, e.g., to determine whether
there are middleboxes on the path that violate the [RFC1812], Section
5.2.6 requirement that: "A router MUST NOT reassemble any datagram
before forwarding it".
When possible, the ITE should use knowledge of its topological
arrangement as an aid in determining when middlebox reassembly
testing is necessary. For example, if the ITE is aware that the ETE
is located somewhere in the public Internet, middlebox reassembly
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testing is unnecessary. If the ITE is aware that the ETE is located
behind a NAT or a firewall, however, then middlebox reassembly
testing is recommended.
The ITE can perform a middlebox reassembly test by setting U=1 in the
header of a SEAL data packet to be used as a probe. Next, the ITE
encapsulates the packet in the appropriate outer headers, splits it
into two outer IPv4 fragments, then sends both fragments over the
same ETE link path.
While performing the test, the ITE should select only inner packets
that are no larger than 1280 bytes for testing purposes in order to
avoid reassembly buffer overruns. The ITE can also construct a NULL
test packet instead of using ordinary SEAL data packets for testing.
To create the NULL packet, the ITE prepares a data packet with (C=0;
P=1; R=0; T=0; U=1; Z=0) in the SEAL header, writes the length of the
ITE's claimed prefix in the PREFLEN field, and writes the ITE's
claimed prefix in the PREFIX field. The ITE then sets NEXTHDR
according to the address family of the PREFIX, i.e., it sets NEXTHDR
to the value '4' for an IPv4 prefix, '41' for an IPv6 prefix , '80'
for an OSI/CLNP prefix, etc.
The ITE can further add padding following the PREFIX field to a
length that would not cause the size of the NULL packet to exceed
1280 bytes before encapsulation. The ITE then sets LINK_ID, LEVEL
and PKT_ID to the appropriate values for this ETE link path and
calculates ICV1 the same as for an ordinary SEAL data packet.
The ITE should send a series of test packets (e.g., 3-5 tests with
1sec intervals between tests) instead of a single isolated test in
case of packet loss, and will eventually receive an SPTB message from
the ITE (see: Section 4.6.2.1). If the ETE returns an SCMP PTB
message with MTU != 0, then the ETE link path correctly supports
fragmentation.
If the ETE returns an SCMP PTB message with MTU==0, however, then a
middlebox in the subnetwork is reassembling the fragments before
forwarding them to the ETE. In that case, the ITE sets
PATH_MTU=MIN_MTU and sets (USE_TRAILER=TRUE; USE_DF=FALSE). The ITE
may instead enable stateful MTU determination for this ETE link path
as specified in Section 4.4.9 to attempt to discover larger MTUs.
NB: Examples of middleboxes that may perform reassembly include
stateful NATs and firewalls. Such devices could still allow for
stateless MTU determination if they gather the fragments of a
fragmented IPv4 SEAL data packet for packet analysis purposes but
then forward the fragments on to the final destination rather than
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forwarding the reassembled packet.
4.4.9. Stateful MTU Determination
SEAL supports a stateless MTU determination capability, however the
ITE may in some instances wish to impose a stateful MTU limit on a
particular ETE link path. For example, when the ETE is situated
behind a middlebox that performs IPv4 reassembly (see: Section 4.4.8)
it is imperative that fragmentation of large packets be avoided on
the path to the middlebox. In other instances (e.g., when the ETE
link path includes performance-constrained links), the ITE may deem
it necessary to cache a conservative static MTU in order to avoid
sending large packets that would only be dropped due to an MTU
restriction somewhere on the path.
To determine a static MTU value, the ITE can send a series of probe
packets of various sizes to the ETE with U=1 in the SEAL header and
DF=1 in the outer IP header. The ITE can then cache the size of the
largest packet for which it receives a probe reply from the ETE as
the PATH_MTU value this ETE link path.
For example, the ITE could send NULL probe packets of 1500 bytes,
followed by 1450 bytes, followed by 1400 bytes, etc. then set
PATH_MTU for this ETE link path to the size of the largest probe
packet for which it receives an SPTB reply message. While probing
with NULL probe packets, the ITE processes any ICMP PTB message it
receives as a potential indication of probe failure then discards the
message.
For IPv4, if the largest successful probe is larger than MIN_MTU the
ITE then sets (USE_TRAILER=FALSE; USE_DF=TRUE) for this ETE link
path; otherwise, the ITE sets (USE_TRAILER=TRUE; USE_DF=FALSE).
4.4.10. Detecting Path MTU Changes
For IPv6, the ITE can periodically reset PATH_MTU to the MTU of the
underlying link to determine whether the ETE link path now supports
larger packet sizes. If the path still has a too-small MTU, the ITE
will receive a PTB message that reports a smaller size.
For IPv4, when USE_TRAILER==TRUE and PATH_MTU is larger than MIN_MTU
the ITE can periodically reset USE_TRAILER=FALSE to determine whether
the ETE link path still requires trailers. If the ITE receives an
SPTB message for an inner packet that is no larger than 1280 bytes
(see: Section 4.6.1.1), the ITE should again set USE_TRAILER=TRUE.
When stateful MTU determination is used, the ITE should periodically
re-probe the path as described in Section 4.4.9 to determine whether
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routing changes have resulted in a reduced or increased PATH_MTU.
4.5. ETE Specification
4.5.1. Tunnel Neighbor Soft State
The ETE maintains a per-ITE ICV calculation algorithm and (when data
origin authentication is required) a symmetric secret key to verify
the ICV(s) in the SEAL header and trailer. The ETE also maintains a
window of PKT_ID values for the packets it has recently received from
this ITE.
4.5.2. IP-Layer Reassembly
The ETE must maintain a minimum IP-layer reassembly buffer size of
1500 bytes for both IPv4 [RFC0791] and IPv6 [RFC2460].
The ETE should maintain conservative reassembly cache high- and low-
water marks. When the size of the reassembly cache exceeds this
high-water mark, the ETE should actively discard stale 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 arrived before a fragment that completes a pending
reassembly arrives.
The ETE processes non-SEAL IP packets as specified in the normative
references, i.e., it performs any necessary IP reassembly then
discards the packet if it is larger than the reassembly buffer size
or delivers the (fully-reassembled) packet to the appropriate upper
layer protocol module.
For SEAL packets, the ITE performs any necessary IP reassembly until
it has received at least the first 1280 bytes beyond the SEAL header
or up to the end of the packet. For IPv4, the ETE then submits the
(fully- or partially-reassembled) packet for decapsulation as
specified in Section 4.5.3. For IPv6, the ETE only submits the
packet if it was fully-reassembled and no larger than the reassembly
buffer size.
4.5.3. Decapsulation and Re-Encapsulation
For each SEAL packet submitted for decapsulation, the ETE first
examines the PKT_ID and ICV1 fields. If the PKT_ID is not within the
window of acceptable values for this ITE, or if the ICV1 field
includes an incorrect value, the ETE silently discards the packet.
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Next, if the SEAL header has T==1 and the inner packet is larger than
1280 bytes the ETE silently discards the packet. If the SEAL header
has T==1 and the inner packet is no larger than 1280 bytes, the ETE
instead verifies the ICV2 value and silently discards the packet if
the value is incorrect.
Next, if the SEAL header has C==0 and there is an incorrect value in
a SEAL header field (e.g., an incorrect "VER" field value), the ETE
returns an SCMP "Parameter Problem" (SPP) message (see Section
4.6.1.2) and discards the packet.
Next, if the packet arrived as multiple IPv4 fragments and the inner
packet is larger than 1280 bytes, the ETE sends an SPTB message back
to the ITE with MTU set to the size of the largest fragment received
minus HLEN (see: Section 4.6.1.1) then discards the packet. If the
packet arrived as multiple IPv6 fragments and the inner packet is
larger than 1280 bytes, the ETE instead silently discards the packet.
Next, if the packet arrived as multiple IPv4 fragments, the SEAL
header has (C==0; T==0), and the inner packet is larger than (128-
SHLEN-UHLEN) bytes, the ETE sends an SPTB message back to the ITE
with MTU set to the size of the largest fragment received minus HLEN
(see: Section 4.6.1.1) then continues to process the packet.
Next, if the SEAL header has C==1, the ETE processes the packet as an
SCMP packet as specified in Section 4.6.2. Otherwise, the ETE
continues to process the packet as a SEAL data packet.
Next, if the packet arrived unfragmented and the SEAL header has
U==1, the ETE sends an SPTB message back to the ITE with MTU=0 (see:
Section 4.6.1.1).
Next, if the SEAL header has P==1 the ETE discards the (NULL) packet.
Finally, the ETE discards the outer headers and processes the inner
packet according to the header type indicated in the SEAL NEXTHDR
field. If the next hop toward the inner destination address is via a
different interface than the SEAL packet arrived on, the ETE discards
the SEAL header and delivers the inner packet either to the local
host or to the next hop interface if the packet is not destined to
the local host.
If the next hop is on the same interface the SEAL packet arrived on,
however, the ETE submits the packet for SEAL re-encapsulation
beginning with the specification in Section 4.4.3 above. In this
process, the packet remains within the tunnel interface (i.e., it
does not exit and then re-enter the interface); hence, the packet is
not discarded if the LEVEL field in the SEAL header contains the
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value 0.
4.6. The SEAL Control Message Protocol (SCMP)
SEAL provides a companion SEAL Control Message Protocol (SCMP) that
uses the same message types and formats as for the Internet Control
Message Protocol for IPv6 (ICMPv6) [RFC4443]. As for ICMPv6, each
SCMP message includes a 4-byte header and a variable-length body.
The TE encapsulates the SCMP message in a SEAL header and outer
headers as shown in Figure 4:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| SCMP message header| --> | SCMP message header|
+--------------------+ +--------------------+
| | --> | |
~ SCMP message body ~ --> ~ SCMP message body ~
| | --> | |
+--------------------+ +--------------------+
SCMP Message SCMP Packet
before encapsulation after encapsulation
Figure 4: SCMP Message Encapsulation
The following sections specify the generation, processing and
relaying of SCMP messages.
4.6.1. Generating SCMP Error Messages
ETEs generate SCMP error messages in response to receiving certain
SEAL data packets using the format shown in Figure 5:
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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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-Specific Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of the inner packet within the invoking |
~ SEAL data packet as possible without the SCMP ~
| packet exceeding 576 bytes (*) |
(*) also known as the "packet-in-error"
Figure 5: SCMP Error Message Format
The error message includes the 4 byte SCMP message header, followed
by a 4 byte Type-Specific Data field, followed by the leading portion
of the inner packet within the invoking SEAL data packet (i.e.,
beginning immediately after the SEAL header) as the "packet-in-
error". The packet-in-error includes as much of the inner packet as
possible extending to a length that would not cause the entire SCMP
packet following outer encapsulation to exceed 576 bytes.
When the ETE processes a SEAL data packet for which the ICVs are
correct but an error must be returned, it prepares an SCMP error
message as shown in Figure 5. The ETE sets the Type and Code fields
to the same values that would appear in the corresponding ICMPv6
message and calculates the Checksum beginning with the SCMP message
header and continuing to the end of the message. (When calculating
the Checksum, the TE sets the Checksum field itself to 0.)
The ETE next encapsulates the SCMP message in the requisite SEAL
header, outer headers and SEAL trailer as shown in Figure 4. During
encapsulation, the ETE sets the outer destination address/port
numbers of the SCMP packet to the outer source address/port numbers
of the original SEAL data packet and sets the outer source address/
port numbers to its own outer address/port numbers.
The ETE then sets (C=1; R=0; T=0; U=0; Z=0) in the SEAL header, then
sets NEXTHDR, PREFLEN, LINK_ID, LEVEL, and PKT_ID to the same values
that appeared in the SEAL header of the data packet. If the SEAL
data packet header had P==1, the ETE also copies the PREFIX field
from the data packet into the SEAL header and sets P=1; otherwise, it
sets P=0.
The ETE then calculates and sets the ICV1 field the same as specified
for SEAL data packet encapsulation in Section 4.4.4. Next, the ETE
encapsulates the SCMP message in the requisite outer encapsulations
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and sends the resulting SCMP packet to the ITE the same as specified
for SEAL data packets in Section 4.4.5.
The following sections describe additional considerations for various
SCMP error messages:
4.6.1.1. Generating SCMP Packet Too Big (SPTB) Messages
An ETE generates an SCMP "Packet Too Big" (SPTB) message when it
receives a SEAL data packet that arrived as multiple outer IPv4
fragments and for which the reassembled inner packet would be larger
than 1280 bytes. The ETE also generates an SPTB when it receives the
fragments of a fragmented IPv4-encapsulated SEAL data packet with
T==0 in the SEAL header but that following reassembly would be larger
than (128-SHLEN-UHLEN) bytes but no larger than 1280 bytes. The ETE
prepares the SPTB message the same as for the corresponding ICMPv6
PTB message, and writes the length of the largest outer IP fragment
received minus HLEN in the MTU field of the message.
The ETE also generates an SPTB message when it accepts a SEAL
protocol data packet which did not undergo IP fragmentation and with
U==1 in the SEAL header. The ETE prepares the SPTB message the same
as above, except that it writes the value 0 in the MTU field.
4.6.1.2. Generating Other SCMP Error Messages
An ETE generates an SCMP "Destination Unreachable" (SDU) message
under the same circumstances that an IPv6 system would generate an
ICMPv6 Destination Unreachable message.
An ETE generates an SCMP "Parameter Problem" (SPP) message when it
receives a SEAL packet with an incorrect value in the SEAL header.
IN THIS CASE ALONE, the ETE prepares the packet-in-error beginning
with the SEAL header instead of beginning immediately after the SEAL
header.
TEs generate other SCMP message types using methods and procedures
specified in other documents. For example, SCMP message types used
for tunnel neighbor coordinations are specified in VET
[I-D.templin-intarea-vet].
4.6.2. Processing SCMP Error Messages
An ITE may receive SCMP messages after sending packets to an ETE.
The ITE first verifies that the outer addresses of the SCMP packet
are correct, and that the PKT_ID is within its window of values for
this ETE. The ITE next verifies that the SEAL header fields are set
correctly as specified in Section 4.6.1. The ITE then verifies the
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ICV1 value. If the outer addresses, SEAL header information and/or
ICV1 value are incorrect, the ITE silently discards the message;
otherwise, it processes the message as follows:
4.6.2.1. Processing SCMP PTB Messages
After an ITE sends a SEAL data packet to an ETE, it may receive an
SPTB message with a packet-in-error containing the leading portion of
the inner packet (see: Section 4.6.1.1). For IP SPTB messages with
MTU==0, the ITE processes the message as confirmation that the ETE
received an unfragmented SEAL data packet with U==1 in the SEAL
header. The ITE then discards the message.
For IPv4 SPTB messages with MTU != 0, the ITE instead processes the
message as an indication of a packet size limitation as follows. The
ITE first determines the inner packet length by subtracting SHLEN
from the length field in the UDP header within the packet-in-error
(and also subtracting the length of the SEAL trailer when T=1). If
the inner packet is no larger than 1280 bytes, the ITE sets
USE_TRAILER=TRUE. If the inner packet is larger than 1280 bytes, the
ITE instead examines the SPTB message MTU field. If the MTU value is
not substantially less than (1500-HLEN), the value is likely to
reflect the true MTU of the restricting link on the path to the ETE;
otherwise, a router on the path may be generating runt fragments.
In that case, the ITE can consult a plateau table (e.g., as described
in [RFC1191]) to rewrite the MTU value to a reduced size. For
example, if the ITE receives an IPv4 SPTB message with MTU==256 and
inner packet length 1500, it can rewrite the MTU to 1450. If the ITE
subsequently receives an IPv4 SPTB message with MTU==256 and inner
packet length 1450, it can rewrite the MTU to 1400, etc. If the ITE
is performing stateful MTU determination for this ETE link path, it
then writes the new MTU value in PATH_MTU.
The ITE then checks its forwarding tables to discover the previous
hop toward the source address of the inner packet. If the previous
hop is reached via the same tunnel interface the SPTB message arrived
on, the ITE relays the message to the previous hop. In order to
relay the message, the ITE rewrites the SEAL header fields with
values corresponding to the previous hop and recalculates the ICV1
values using the ICV calculation parameters associated with the
previous hop. Next, the ITE replaces the SPTB's outer headers with
headers of the appropriate protocol version and fills in the header
fields as specified in Sections 5.5.4-5.5.6 of
[I-D.templin-intarea-vet], where the destination address/port
correspond to the previous hop and the source address/port correspond
to the ITE. The ITE then sends the message to the previous hop the
same as if it were issuing a new SPTB message.
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If the previous hop is not reached via the same tunnel interface, the
ITE instead transcribes the message into a format appropriate for the
inner packet (i.e., the same as described for transcribing ICMP
messages in Section 4.4.7) and sends the resulting transcribed
message to the original source. The ITE then discards the SPTB
message.
4.6.2.2. Processing Other SCMP Error Messages
An ITE may receive an SDU message with an appropriate code under the
same circumstances that an IPv6 node would receive an ICMPv6
Destination Unreachable message. The ITE either transcribes or
relays the message toward the source address of the inner packet
within the packet-in-error the same as specified for SPTB messages in
Section 4.6.2.1.
An ITE may receive an SPP message when the ETE receives a SEAL packet
with an incorrect value in the SEAL header. The ITE should examine
the SEAL header within the packet-in-error to determine whether a
different setting should be used in subsequent packets, but does not
relay the message further.
TEs process other SCMP message types using methods and procedures
specified in other documents. For example, SCMP message types used
for tunnel neighbor coordinations are specified in VET
[I-D.templin-intarea-vet].
5. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
6. End System Requirements
End systems are encouraged to implement end-to-end MTU assurance
(e.g., using Packetization Layer Path MTU Discovery per [RFC4821])
even if the subnetwork is using SEAL.
7. Router Requirements
Routers within the subnetwork are expected to observe the router
requirements found in the normative references, including the
implementation of IP fragmentation and reassembly [RFC1812][RFC2460]
as well as the generation of ICMP messages [RFC0792][RFC4443].
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8. Nested Encapsulation Considerations
SEAL supports nested tunneling for up to 8 layers of encapsulation.
In this model, the SEAL ITE has a tunnel neighbor relationship only
with ETEs at its own nesting level, i.e., it does not have a tunnel
neighbor relationship with any ITEs/ETEs at other nesting levels.
Therefore, when an ITE 'A' within an inner nesting level needs to
return an error message to an ITE 'B' within an outer nesting level,
it generates an ordinary ICMP error message the same as if it were an
ordinary router within the subnetwork. 'B' can then perform message
validation as specified in Section 4.4.7, but full message origin
authentication is not possible.
Since ordinary ICMP messages are used for coordinations between ITEs
at different nesting levels, nested SEAL encapsulations should only
be used when the ITEs are within a common administrative domain
and/or when there is no ICMP filtering middlebox such as a firewall
or NAT between them. An example would be a recursive nesting of
mobile networks, where the first network receives service from an
ISP, the second network receives service from the first network, the
third network receives service from the second network, etc.
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 Version values. This registry should be initialized to
include the initial SEAL Version number, i.e., Version 0.
10. Security Considerations
SEAL provides a segment-by-segment data origin authentication and
anti-replay service across the (potentially) multiple segments of a
re-encapsulating tunnel. It further provides a segment-by-segment
integrity check of the headers of encapsulated packets, but does not
verify the integrity of the rest of the packet beyond the headers
unless fragmentation is unavoidable. SEAL therefore considers full
message integrity checking, authentication and confidentiality as
end-to-end considerations in a manner that is compatible with
securing mechanisms such as TLS/SSL [RFC5246].
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An amplification/reflection/buffer overflow attack is possible when
an attacker sends IP fragments with spoofed source addresses to an
ETE in an attempt to clog the ETE's reassembly buffer and/or cause
the ETE to generate a stream of SCMP messages returned to a victim
ITE. The SCMP message ICVs, PKT_ID, as well as the inner headers of
the packet-in-error, provide mitigation for the ETE to detect and
discard SEAL segments with spoofed source addresses.
The SEAL header is sent in-the-clear 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. Unlike IPv6 extension
headers, however, the SEAL header is protected by an integrity check
that also covers the inner packet headers.
Security issues that apply to tunneling in general are discussed in
[RFC6169].
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. This
capability was implemented in the first edition of SEAL, but is now
deprecated.
Section 3 of [RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU.
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.
IPsec/AH is [RFC4301][RFC4301] is used for full message integrity
verification between tunnel endpoints, whereas SEAL only ensures
integrity for the inner packet headers. The AYIYA proposal
[I-D.massar-v6ops-ayiya] uses similar means for providing full
message authentication and integrity.
The concepts of path MTU determination through the report of
fragmentation and extending the IPv4 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. An historical analysis of the evolution of these concepts,
as well as the development of the eventual path MTU discovery
mechanism, appears in Appendix D of this document.
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12. 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, Remi Despres, Ralph
Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, 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, Mark
Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin
Whittle, James Woodyatt, and members of the Boeing Research &
Technology NST DC&NT group.
Discussions with colleagues following the publication of RFC5320 have
provided useful insights that have resulted in significant
improvements to this, the Second Edition of SEAL.
Path MTU determination through the report of fragmentation was first
proposed by Charles Lynn on the TCP-IP mailing list in 1987.
Extending the IP identification field was first proposed by Steve
Deering on the MTUDWG mailing list in 1989.
13. References
13.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[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
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
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"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
13.2. 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-intarea-ipv4-id-update]
Touch, J., "Updated Specification of the IPv4 ID Field",
draft-ietf-intarea-ipv4-id-update-04 (work in progress),
September 2011.
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-05 (work in progress),
July 2011.
[I-D.massar-v6ops-ayiya]
Massar, J., "AYIYA: Anything In Anything",
draft-massar-v6ops-ayiya-02 (work in progress), July 2004.
[I-D.templin-aero]
Templin, F., "Asymmetric Extended Route Optimization
(AERO)", draft-templin-aero-04 (work in progress),
October 2011.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-29 (work in progress),
November 2011.
[I-D.templin-ironbis]
Templin, F., "The Internet Routing Overlay Network
(IRON)", draft-templin-ironbis-07 (work in progress),
October 2011.
[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.
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[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.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[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.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
[RFC5720] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720,
February 2010.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and
Addressing in Networks with Global Enterprise Recursion
(RANGER) Scenarios", RFC 6139, February 2011.
[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169, April 2011.
[SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU
Discovery Behavior", November 2010.
[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,
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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. 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 tunnels may traverse
arbitrarily-long paths over links of various types that are already
either performing or omitting ARQ as appropriate, it would therefore
often be inefficient to also require the tunnel endpoints to also
perform ARQ.
Appendix B. Integrity
The SEAL header includes an ICV field that covers the SEAL header and
at least the inner packet headers. This provides for header
integrity verification on a segment-by-segment basis for a segmented
re-encapsulating tunnel path. When IPv4 fragmentation is needed, the
SEAL packet also contains a trailer with a secondary ICV that covers
the remainder of the packet.
Fragmentation and reassembly schemes must consider packet-splicing
errors, e.g., when two fragments from the same packet are
concatenated incorrectly, when a fragment from packet X is
reassembled with fragments from packet Y, etc. The primary sources
of such errors include implementation bugs and wrapping IPv4 ID
fields.
In terms of wrapping ID fields, the IPv4 16-bit 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
When reassembly is unavoidable, SEAL provides an extended ICV to
detect reassembly mis-associations for packets no larger than 1280
bytes and also discards any reassembled packets larger than 1280
bytes.
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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 (e.g.,
by inserting a 'SEAL_OPTION' TCP option during connection
establishment) for the carriage of protocol data encapsulated as IP/
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.
If both TCPs agree on the use of SEAL, their protocol messages will
be carried as IP/SEAL/TCP 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 Section 4.
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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