Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track June 12, 2009
Expires: December 14, 2009
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-00.txt
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Abstract
For the purpose of this document, subnetworks are defined as virtual
topologies that span connected network regions bounded by
encapsulating border nodes. These virtual topologies may span
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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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 6
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 7
4. SEAL Protocol Specification (Version 0) . . . . . . . . . . . 8
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 8
4.2. SEAL Header Format (Version 0) . . . . . . . . . . . . . . 10
4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 11
4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 11
4.3.2. Admitting Packets into the Tunnel Interface . . . . . 12
4.3.3. Inner Fragmentation and Segmentation . . . . . . . . . 12
4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 14
4.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 14
4.3.6. Packet Identification . . . . . . . . . . . . . . . . 15
4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 15
4.3.8. Processing Raw ICMPv4 Messages . . . . . . . . . . . . 15
4.3.9. Processing SEAL Errors . . . . . . . . . . . . . . . . 16
4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 17
4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 17
4.4.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 17
4.4.3. Sending SEAL Fragmentation Reports . . . . . . . . . . 18
4.4.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 18
4.4.5. Decapsulation and Delivery to Upper Layers . . . . . . 19
4.4.6. Generating SEAL Error Messages . . . . . . . . . . . . 19
5. SEAL Protocol Specification (Version 1) . . . . . . . . . . . 21
5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 21
5.2. SEAL Header Format (Version 1) . . . . . . . . . . . . . . 21
5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 22
5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 22
5.3.2. Admitting Packets into the Tunnel Interface . . . . . 22
5.3.3. Inner Fragmentation and Segmentation . . . . . . . . . 23
5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 23
5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 23
5.3.6. Packet Identification . . . . . . . . . . . . . . . . 23
5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 23
5.3.8. Processing Raw ICMPv4 Messages . . . . . . . . . . . . 23
5.3.9. Processing SEAL Errors . . . . . . . . . . . . . . . . 24
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5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 24
5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 24
5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 24
5.4.3. Sending SEAL Fragmentation Reports . . . . . . . . . . 24
5.4.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 24
5.4.5. Decapsulation and Delivery to Upper Layers . . . . . . 24
5.4.6. Sending SEAL Error Messages . . . . . . . . . . . . . 24
6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 24
7. End System Requirements . . . . . . . . . . . . . . . . . . . 25
8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
10. Security Considerations . . . . . . . . . . . . . . . . . . . 25
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 26
12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 26
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 27
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
14.1. Normative References . . . . . . . . . . . . . . . . . . . 28
14.2. Informative References . . . . . . . . . . . . . . . . . . 28
Appendix A. Reliability Extensions . . . . . . . . . . . . . . . 30
Appendix B. Transport Mode . . . . . . . . . . . . . . . . . . . 30
Appendix C. Historic Evolution of PMTUD . . . . . . . . . . . . . 31
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 32
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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]. In the following
subsections, we present further details on the motivation and
approach for addressing these issues.
1.1. Motivation
Before discussing the approach, it is necessary to first understand
the problems. In both the Internet and private-use networks today,
IPv4 is ubiquitously deployed as the Layer 3 protocol. The two
primary functions of IPv4 are to provide for 1) addressing, and 2) a
fragmentation and reassembly capability used to accommodate links
with diverse MTUs. While it is well known that the addressing
properties of IPv4 are limited (hence, the larger address space
provided by IPv6), there is a lesser-known but growing consensus that
other limitations may be unable to sustain continued growth.
First, the IPv4 header Identification field is only 16 bits in
length, meaning that at most 2^16 packets pertaining to the same
(source, destination, protocol, Identification)-tuple may be active
in the Internet at a given time. Due to the escalating deployment of
high-speed links (e.g., 1Gbps Ethernet), however, this number may
soon become too small by several orders of magnitude. Furthermore,
there are many well-known limitations pertaining to IPv4
fragmentation and reassembly - even to the point that it has been
deemed "harmful" in both classic and modern-day studies (cited
above). In particular, IPv4 fragmentation raises issues ranging from
minor annoyances (e.g., in-the-network router fragmentation) to the
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potential for major integrity issues (e.g., mis-association of the
fragments of multiple IP packets during reassembly).
As a result of these perceived limitations, a fragmentation-avoiding
technique for discovering the MTU of the forward path from a source
to a destination node was devised through the deliberations of the
Path MTU Discovery Working Group (PMTUDWG) during the late 1980's
through early 1990's (see Appendix C). In this method, the source
node provides explicit instructions to routers in the path to discard
the packet and return an ICMP error message if an MTU restriction is
encountered. However, this approach has several serious shortcomings
that lead to an overall "brittleness".
In particular, site border routers in the Internet are 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.
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. For example, site border routers that are configured as
ingress tunnel endpoints may be required to forward packets into the
subnetwork on behalf of hundreds, thousands, or even more original
sources located within the site. If IPv4 fragmentation were used,
this would quickly wrap the 16-bit Identification field and could
lead to undetected data corruption. If classical IPv4 path MTU
discovery were used instead, the site border router may be
inconvenienced by excessive ICMP error messages coming from the
subnetwork that may be either untrustworthy or insufficiently
provisioned to allow 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. Due to these many limitations, a new approach
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to accommodate links with diverse MTUs is necessary.
1.2. Approach
For the purpose of this document, subnetworks are defined as virtual
topologies that span connected network regions bounded by
encapsulating border nodes. Subnetworks in this sense correspond
exactly to the "enterprise" abstraction defined in Virtual Enterprise
Traversal (VET) [I-D.templin-autoconf-dhcp] and Routing and
Addressing in Next-Generation EnteRprises (RANGER)
[I-D.templin-ranger][I-D.russert-rangers]. 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 tunneling protocols such as VET
[I-D.templin-autoconf-dhcp], the Locator-Identifier Split Protocol
(LISP) [I-D.ietf-lisp] and others. A transport-mode of operation is
also possible, and described in Appendix B. SEAL accommodates links
with diverse MTUs, protects against off-path denail-of-service
attacks, and supports efficient duplicate packet detection through
the use of a minimal mid-layer encapsulation.
SEAL encapsulation introduces an extended Identification field for
packet identification and a mid-layer segmentation and reassembly
capability that allows simplified cutting and pasting of packets.
Moreover, SEAL senses in-the-network IPv4 fragmentation as a "noise"
indication that packet sizing parameters are "out of tune" with
respect to the network path. As a result, SEAL can naturally tune
its packet sizing parameters to eliminate the in-the-network
fragmentation.
SEAL encapsulation additionally includes a 2-bit version number to
accommodate future protocol versions. This document specifies SEAL
protocol versions 0 and 1.
2. Terminology and Requirements
The terms "inner", "mid-layer", and "outer", respectively, refer to
the innermost IP (layer, protocol, header, packet, etc.) before any
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encapsulation, the mid-layer IP (protocol, header, packet, etc.)
after any mid-layer '*' encapsulation, and the outermost IP (layer,
protocol, header, packet etc.) after SEAL/*/IPv4 encapsulation.
The term "IP" used throughout the document refers to either Internet
Protocol version (IPv4 or IPv6). Additionally, the notation IPvX/*/
SEAL/*/IPvY refers to an inner IPvX packet encapsulated in any mid-
layer '*' encapsulations, followed by the SEAL header, followed by
any outer '*' encapsulations, followed by an outer IPvY header, where
the notation "IPvX" means either IP protocol version (IPv4 or IPv6).
The following abbreviations correspond to terms used within this
document and elsewhere in common Internetworking nomenclature:
ITE - Ingress Tunnel Endpoint
ETE - Egress Tunnel Endpoint
PTB - an ICMPv6 "Packet Too Big", an ICMPv4 "Fragmentation Needed"
or a SEAL "Fragmentation Report" message
DF - the IPv4 header "Don't Fragment" flag
MHLEN - the length of any mid-layer '*' headers and trailers
OHLEN - the length of the outer encapsulating SEAL/*/IPv4 headers
HLEN - the sum of MHLEN and OHLEN
S_MRU - the per-ETE SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size
SEAL_ID - a 32-bit Identification value, randomly initialized and
monotonically incremented for each SEAL protocol packet
SEAL_PROTO - an IPv4 protocol number used for SEAL
SEAL_PORT - a TCP/UDP service port number used for SEAL
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
3. Applicability Statement
SEAL was motivated by the specific case of subnetwork abstraction for
Mobile Ad hoc Networks (MANETs); however, the domain of applicability
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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-autoconf-dhcp], the RANGER architecture
[I-D.templin-ranger][I-D.russert-rangers] and the LISP protocol
[I-D.ietf-lisp]. Indeed, the term "subnetwork" within this document
is used synonymously with the term "enterprise" that appears in these
references.
SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation
(e.g., as IPv6/SEAL/IPv4), and appears as a subnetwork encapsulation
as seen by the inner IP layer. SEAL can also be used as a sublayer
for encapsulating inner IP packets within outer UDP/IPv4 headers
(e.g., as IPv6/SEAL/UDP/IPv4) such as for the Teredo domain of
applicability [RFC4380]. When it appears immediately after the outer
IPv4 header, the SEAL header is processed exactly as for IPv6
extension headers.
This document discusses the use of IPv4 as the outer encapsulation
layer; however, the same principles apply when IPv6 is used as the
outer layer.
4. SEAL Protocol Specification (Version 0)
This section specifies the fully-functioned version of SEAL known as
"SEAL Version 0", or "Classical SEAL". A minimal version of SEAL
known as "SEAL Version 1", or "SEAL-lite", is specified in Section 5.
4.1. Model of Operation
SEAL provides an encapsulation sublayer that supports the
transmission of unicast and multicast packets across an underlying IP
network. SEAL-enabled ITEs insert a SEAL header during the
encapsulation of inner IP packets in mid-layer and outer
encapsulating headers/trailers. For example, an inner IPv6 packet
would appear as IPv6/*/SEAL/*/IPv4 after mid-layer and outer
encapsulations, where '*' denotes zero or more additional
encapsulation sublayers.
SEAL-enabled ITEs add mid-layer '*' and outer SEAL/*/IPv4
encapsulations to the inner packets they inject into a subnetwork,
where the outermost IPv4 header contains the source and destination
addresses of the subnetwork entry/exit points (i.e., the ITE/ETE),
respectively. ITEs encapsulate each inner IP packet in mid-layer and
outer encapsulations as shown in Figure 1:
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+-------------------------+
| |
~ Outer */IPv4 headers ~
| |
I +-------------------------+
n | SEAL Header |
n +-------------------------+ +-------------------------+
e ~ Any mid-layer * headers ~ ~ Any mid-layer * headers ~
r +-------------------------+ +-------------------------+
| | | |
I --> ~ Inner IP ~ --> ~ Inner IP ~
P --> ~ Packet ~ --> ~ Packet ~
| | | |
P +-------------------------+ +-------------------------+
a ~ Any mid-layer trailers ~ ~ Any mid-layer trailers ~
c +-------------------------+ +-------------------------+
k ~ Any outer trailers ~
e +-------------------------+
t
(After mid-layer encaps.) (After SEAL/*/IPv4 encaps.)
Figure 1: SEAL Encapsulation
where the SEAL header is inserted as follows:
o For simple IPvX/IPv4 encapsulations (e.g.,
[RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
the inner IP and outer IPv4 headers as: IPvX/SEAL/IPv4.
o For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
SEAL header is inserted between the {AH,ESP} header and outer IPv4
headers as: IPvX/*/{AH,ESP}/SEAL/IPv4.
o For IP encapsulations over transports such as UDP, the SEAL header
is inserted immediately after the outer transport layer header,
e.g., as IPvX/*/SEAL/UDP/IPv4.
SEAL-encapsulated packets include a 32-bit SEAL_ID formed from the
concatenation of the 16-bit ID Extension field in the SEAL header as
the most-significant bits, and with the 16-bit Identification value
in the outer IPv4 header as the least-significant bits. (For tunnels
that traverse middleboxes that might rewrite the IPv4 ID field, e.g.,
a Network Address Translator, the SEAL_ID is instead maintained only
within the ID field in the SEAL header.) Routers within the
subnetwork use the SEAL_ID for duplicate packet detection, and ITEs/
ETEs use the SEAL_ID for SEAL segmentation/reassembly and protection
against off-path attacks.
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SEAL enables a multi-level segmentation and reassembly capability.
First, the ITE can use IPv4 fragmentation to fragment inner IPv4
packets with DF=0 before SEAL encapsulation. Secondly, the SEAL
layer itself provides a simple cutting-and-pasting capability for
mid-layer packets to avoid IPv4 fragmentation on the outer packet.
Finally, ordinary IPv4 fragmentation is permitted on the outer packet
after SEAL encapsulation and used to detect and dampen any in-the-
network fragmentation as quickly as possible.
The following sections specify the SEAL header format and SEAL-
related operations of the ITE and ETE, respectively.
4.2. SEAL Header Format (Version 0)
The SEAL version 0 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|A|D|M| SEG | Next Header | ID Extension |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SEAL Version 0 Header Format
where the header fields are defined as:
VER (2)
a 2-bit value that encodes the SEAL protocol version number. This
section describes Version 0 of the SEAL protocol, i.e., the VER
field encodes the value '00'.
A (1)
the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes
to receive an explicit acknowledgement from the ETE.
D (1)
the "Don't Fragment" bit. Copied from the D flag in the SEAL
header of the inner packet if the inner packet is itself a SEAL/IP
packet. Otherwise, set to 0 if the inner packet is an IPv4 packet
with DF=0. Otherwise, set to 1.
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.
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R (1)
the "reserved" bit. Set to 0 for the purpose of this
specification.
SEG (2)
a 2-bit segment number. Encodes a segment number between 0 - 3.
Next Header (8) an 8-bit field that encodes an Internet Protocol
number the same as for the IPv4 protocol and IPv6 next header
fields.
ID Extension (16)
a 16-bit extension of the Identification field in the outer IPv4
header; encodes the most-significant 16 bits of a 32 bit SEAL_ID
value.
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 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.
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 an ICMP 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).
4.3.3. Inner Fragmentation and Segmentation
For each ETE, the maintains soft state within the tunnel interface
(e.g., in a destination cache) used to support inner fragmentation
and/or 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., for '*' = AH,
ESP, NULL, etc.).
o an Outer Header Length (OHLEN); set to the length of the outer
SEAL/*/IP encapsulation headers.
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o a total Header Lenght (HLEN); set to MHLEN plus OHLEN.
o a SEAL Maximum Reassembly Unit (S_MRU); initialized to a value no
larger than 2KB and used to determine the maximum-sized packet the
ITE will require the ETE to reassemble.
o a SEAL Maximum Segment Size (S_MSS); initialized to a value that
is no larger than the maximum of (the underlying IPv4 interface
MTU minus OHLEN) and S_MRU/4 bytes. The ITE decreases or
increases S_MSS based on any Fragmentation Report messages
received (see Section 4.3.9).
After an inner packet/fragment has been admitted into the tunnel
interface the ITE first determines whether the packet can be
accommodated and (if so) whether inner IP fragmentation is needed.
The ITE processes each inner packet/fragment as follows:
o if the inner packet is the first IP fragment of a SEAL packet with
D=1, and the packet is larger than (MAX(S_MRU, S_MSS) - HLEN), the
ITE drops the packet and sends a SEAL Fragmentation Report message
to the original source with an MTU value of (MAX(S_MRU, S_MSS) -
HLEN) the same as described in Section 4.4.3; else,
o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1,
and the packet is larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE
drops the packet and sends an ICMP PTB message to the original
source with an MTU value of (MAX(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 (S_MRU - HLEN), the ITE uses inner IP fragmentation to
break the packet into fragments no larger than (S_MRU - HLEN);
else, no inner fragmentation is required.
Note that this final case would constitute a second instance of inner
packet fragmentation, which implementations may elect to combine with
the first instance specified in Section 4.3.2 above.
The ITE next encapsulates each inner packet/fragment in the MHLEN
bytes of mid-layer '*' headers and trailers. For each such resulting
mid-layer packet of length 'M', if (S_MRU >= (M + OHLEN) > S_MSS),
the ITE must perform SEAL segmentation. To do so, it breaks the mid-
layer packet into N segments (N <= 4) that are no larger than
(MIN(1KB, S_MSS) - OHLEN) bytes each. Each segment, except the final
one, MUST be of equal length, while the final segment includes the
remainder of the packet and MAY be of different 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
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SHOULD generate non-final segments that are as large as possible (see
above) and SHOULD generate the smallest number of segments possible,
e.g., it SHOULD NOT generate 4 smaller segments when the packet could
be accommodated with 2 larger segments.
Note that this SEAL segmentation ignores the fact that the mid-layer
packet may be unfragmentable. This segmentation process is a mid-
layer (not an IP layer) operation employed by the ITE to adapt the
mid-layer packet to the subnetwork path characteristics, and the ETE
will restore the packet to its original form during 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 and sets VER='00'
and R=0. For single-segment packets, the ITE sets (M=0; SEG=0) in
the SEAL header; for N-segment mid-layer packets (N <= 4), the ITE
sets (M=1; SEG=0) for the first segment, (M=1; SEG=1) for the second
segment, etc., with the final segment setting (M=0; SEG=N-1). If the
inner packet (i.e., before mid-layer encapsulation and SEAL
segmentation) was also the first IP fragment of a SEAL packet, the
ITE copies the D value that appeared in the inner SEAL header into
the outer SEAL header of each segment. Otherwise, if the inner
packet was an IPv4 packet with DF=0, the ITE sets D=0; otherwise, it
sets D=1. The ITE also writes the Internet Protocol number
corresponding to the mid-layer packet in the 'Next-Header' field of
each segment.
The ITE next encapsulates each segment in the requisite */IPv4 outer
headers according to the specific encapsulation format (e.g.,
[RFC2003], [RFC4213], [RFC4380], etc.), except that it writes
'SEAL_PROTO' in the protocol field of the outer IPv4 header (when
simple IPv4 encapsulation is used) or writes 'SEAL_PORT' in the outer
destination service port field (e.g., when UDP/IPv4 encapsulation is
used). The ITE finally sets the A bit as specified in Section 4.3.5
(if necessary), sets the packet identification values as specified in
Section 4.3.5 and sends the packets as specified in Section 4.3.6.
4.3.5. Probing Strategy
All SEAL packets sent by the ITE except those with (M=0; SEG!=0) are
used as implicit probes, and will elicit a Fragmentation Report from
an ETE/ITE if an MTU restriction is encountered.
The ITE should additionally send explicit probes, periodically, to
ping the ETE and to manage a window of SEAL_IDs of outstanding
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probes. The ITE sets A=1 in the SEAL header of a packet to be used
as an explicit probe, where the probe can be either an ordinary data
packet or a NULL packet created by setting the 'Next Header' field in
the SEAL header to a value of "No Next Header" (see Section 4.7 of
[RFC2460]).
The ITE should further send probes, periodically, to detect S_MSS
increases by resetting S_MSS to a larger value (e.g., the underlying
IPv4 interface MTU minus OHLEN bytes), and/or by sending explicit
probes that are larger than the current S_MSS.
Finally, the ITE MAY send "expendable" probe packets with DF=1 in the
outer IPv4 header (see Section 4.3.6) in order to generate ICMPv4
Fragmentation Needed messages from routers on the path to the ETE.
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 IPv4 destination 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 packet, the ITE writes the
least-significant 16 bits of the SEAL_ID value in the Identification
field in the outer IPv4 header, and writes the most-significant 16
bits in the ID Extension field in the SEAL header.
For SEAL encapsulations specifically designed for the traversal of
IPv4 Network Address Translators (NATs) and other middleboxes that
may rewrite the outer IPv4 ID field, the ITE instead writes SEAL_ID
in the ID field of the SEAL header and writes a random 16-bit value
in the Identification field in the outer IPv4 header.
4.3.7. Sending SEAL Protocol Packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
the outer IPv4 header of every SEAL packet it sends. For
"expendable" packets (e.g., for NULL packets used as probes -- see
Section 4.3.4), the ITE may instead set DF=1.
The ITE then sends each outer packet that encapsulates a segment of
the same mid-layer packet into the tunnel in canonical order, i.e.,
segment 0 first, followed by segment 1, etc. and finally segment N-1.
4.3.8. Processing Raw ICMPv4 Messages
The ITE may receive "raw" ICMPv4 error messages from either the ETE
or routers within the subnetwork that comprise an outer IPv4 header,
followed by an ICMPv4 header, followed by a portion of the SEAL
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packet that generated the error (also known as the "packet-in-
error"). For such messages, 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. The ITE MAY process raw
ICMPv4 messages as soft errors indicating that the path to the ETE
may be failing.
The ITE should specifically process raw ICMPv4 Protocol Unreachable
messages as a hint that the ETE does not implement the SEAL protocol.
4.3.9. Processing SEAL Errors
In addition to any raw ICMPv4 messages, the ITE may receive SEAL
error messages from either the ETE or an intermediate ITE on the path
to the ETE with 'SEAL_PORT' as the UDP destination port. The ITE
must therefore monitor the 'SEAL_PORT' UDP port and process any
messages that arrive on that port. Each SEAL error message is
formatted as specified in Section 4.4.6.
For each error message, the ITE can use the SEAL_ID as well as
addresses, etc. encoded in the packet-in-error as nonces to confirm
that the message came from a legitimate on-path source. The ITE can
then verify that the SEAL_ID encoded in the packet-in-error is within
the current window of transmitted SEAL_IDs for this ETE. If the
SEAL_ID is outside of the window, the ITE discards the message;
otherwise, it advances the window and processes the message.
The ITE processes SEAL error messages other than IPv4 Fragmentation
Reports according to [RFC0792] and [RFC4443]. (Processing
considerations for additional error types may be specified in a
future document.)
For IPv4 Fragmentation Report messages, the ITE sets 'L' to the value
encoded in the MTU field minus OHLEN. If (L > S_MSS), or if the
packet-in-error is an IPv4 first-fragment (i.e., with MF=1; Offset=0)
and (L >= (576 - OHLEN)), the ITE sets (S_MSS = L).
Note that 576 in the above corresponds to the nominal minimum MTU for
IPv4 links. When an ITE instead receives an IPv4 first-fragment
packet-in-error with (L < (576 - OHLEN)), it discovers that IPv4
fragmentation is occurring in the network 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
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Fragmentation 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 Fragmentation Report message received, and
refrain from further reducing S_MSS until Fragmentation Report
messages pertaining to packets sent under the new S_MSS are received.
4.4. ETE Specification
4.4.1. Reassembly Buffer Requirements
ETEs must be capable of performing IPv4-layer reassembly for SEAL
protocol outer IPv4 packets up to 2KB in length, and must also be
capable of performing SEAL-layer reassembly for mid-layer packets up
to (2KB - OHLEN).
Note that the ETE must retain the SEAL/*/IPv4 header during both
IPv4-layer and SEAL-layer reassembly for the purpose of associating
the fragments/segments of the same packet.
4.4.2. IPv4-Layer Reassembly
ETEs perform IPv4 reassembly as normal, and should maintain a
conservative high- and low-water mark for the number of outstanding
reassemblies pending for each ITE. When the size of the reassembly
buffer exceeds this high-water mark, the ETE actively discards
incomplete reassemblies (e.g., using an Active Queue Management (AQM)
strategy) until the size falls below the low-water mark. The ETE
should also use a reduced IPv4 maximum segment lifetime value (e.g.,
15 seconds) as the time after which it will discard an incomplete
IPv4 reassembly for a SEAL protocol packet. Finally, the ETE should
also actively discard any pending reassemblies that clearly have no
opportunity for completion, e.g., when a considerable number of new
IPv4 fragments have been received before a fragment that completes a
pending reassembly has arrived.
After reassembly, the ETE either accepts or discards the reassembled
packet based on the current status of the IPv4 reassembly cache
(congested versus uncongested). The SEAL_ID included in the IPv4
first-fragment provides an additional level of reassembly assurance,
since it can record a distinct arrival timestamp useful for
associating the first-fragment with its corresponding non-initial
fragments. The choice of accepting/discarding a reassembly may also
depend on the strength of the upper-layer integrity check if known
(e.g., IPSec/ESP provides a strong upper-layer integrity check)
and/or the corruption tolerance of the data (e.g., multicast
streaming audio/video may be more corruption-tolerant than file
transfer, etc.). In the limiting case, the ETE may choose to discard
all IPv4 reassemblies and process only the IPv4 first-fragment for
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SEAL-encapsulated error generation purposes (see the following
sections).
4.4.3. Sending SEAL Fragmentation Reports
When the ETE processes the IP first-fragment (i.e, one with MF=1 and
Offset=0 in the IP header) of a fragmented SEAL packet that does not
have (M=0; SEG!=0), it sends a Fragmentation Report message back to
the ITE with the MTU field set to the length of the first-fragment.
When an intermediate ITE on the path to the ETE is unable to
accommodate a SEAL packet with D=1 (see Section 4.3.3), it drops the
packet and also sends a Fragmentation Report back to the original
ITE.
Additionally, when the ETE processes a SEAL protocol packet with A=1
in the SEAL header following IP reassembly, it sends a Fragmentation
Report message back to the ITE with the MTU value set to the IP
length of the packet. Note therefore that when A=1, and IP
reassembly was required, the ETE only sends a single Fragmentation
Report message, i.e., it does not send two separate messages (one for
the first-fragment and a second for the reassembled whole SEAL
packet).
The Fragmentation Report message is formatted as either an ICMPv4
Fragmentation Needed or an ICMPv6 Packet Too Big message, as
specified in Section 4.4.6.
4.4.4. SEAL-Layer Reassembly
Following IP reassembly of a SEAL packet with VER set to an
unrecognized value or with R=1, the ETE generates an Parameter
Problem message (with pointer set to the flags field in the SEAL
header) as specified in Section 4.4.6, and discards the packet
following SEAL reassembly. For all other SEAL packets, the ETE adds
the packet to a SEAL-Layer pending-reassembly queue.
The ETE performs SEAL-layer reassembly through simple in-order
concatenation of the encapsulated segments from N consecutive SEAL
protocol packets from the same mid-layer packet. SEAL-layer
reassembly requires the ETE to maintain a cache of recently received
segments for a hold time that would allow for reasonable inter-
segment delays. The ETE uses a SEAL maximum segment lifetime of 15
seconds for this purpose, i.e., the time after which it will discard
an incomplete reassembly. However, the ETE should also actively
discard any pending reassemblies that clearly have no opportunity for
completion, e.g., when a considerable number of new SEAL packets have
been received before a packet that completes a pending reassembly has
arrived.
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The ETE reassembles the mid-layer packet segments in SEAL protocol
packets that contain segment numbers 0 through N-1, with M=1/0 in
non-final/final segments, respectively, and with consecutive SEAL_ID
values. That is, for an N-segment mid-layer packet, reassembly
entails the concatenation of the SEAL-encapsulated segments with
(SEG=0, SEAL_ID=i), followed by (SEG=1, SEAL_ID=((i + 1) mod 2^32)),
etc. up to (SEG=(N-1), SEAL_ID=((i + N-1) mod 2^32)). (For SEAL
encapsulations that use only an M-bit SEAL_ID value, the ETE instead
uses mod 2^M arithmetic to associate the segments of the same
packet.)
4.4.5. Decapsulation and Delivery to Upper Layers
Following SEAL-layer reassembly, the ETE silently discards the
reassembled packet if it was a NULL packet (see Section 4.3.4). In
the same manner, the ETE silently discards any (reassembled) mid-
layer packet larger than (2KB - OHLEN) that either experienced IPv4
fragmentation or did not arrive as a single SEAL segment.
Next, the ETE begins the decapsulation process. During this process,
if the ETE determines that the inner packet would cause an error
message to be generated it prepares an error message sends it back to
the ITE as specified in Section 4.4.6. The ETE then either accepts
or drops the packet according to the type of error.
Note that errors can occur through any stage of inner packet
decapsulation, i.e., before, during or after decapsulation. For
example, if IPv4 and IPv6 are used as the outer and inner IP
protocols, respectively, the ETE may generate ICMPv4-formatted error
messages before and during decapsulation, and it may generate ICMPv6-
formatted error messages during and after decapsulation. This can be
understood as a continuum along which the ETE transforms an IPv4
packet into an IPv6 packet, where the ETE must generate an error
message that is appropriate for the particular point in the continuum
at which the error occurs.
In all cases, the packet-in-error includes all IP/*/SEAL/*IPv4
headers, i.e., even if the error occurred at the very last stage of
decapsulation.
Finally, if the packet is accepted, the ETE discards the outer
*/SEAL/*/IPv4 headers and delivers the inner packet to the upper-
layer protocol indicated in the SEAL Next Header field.
4.4.6. Generating SEAL Error Messages
The ETE or intermediate ITE reporting the error prepares the message
as shown in Figure 3:
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+-------------------------+ -
| | \
~ Outer UDP/IP hdrs ~ |
| (dport='SEAL_PORT') | |
+--------+----------------+ |
| Nxthdr | Reserved | |
+--------+----------------+ |
| ICMP Header | |
+-------------------------+ > Up to 576 bytes for IPv4,
| | > or 1280 bytes for IPv6
~ IP/*/SEAL/*/IP ~ |
~ hdrs of packet/fragment ~ |
| | |
+-------------------------+ |
| | |
~ Data of packet/fragment ~ |
| | /
+-------------------------+ -
Figure 3: SEAL Error Message Format
The error message consists of outer UDP/IP headers followed by a 32
bit shim header. The shim header includes an 8-bit "Next Header"
field in bits 0 thru 7 and a 24-bit Reserved field in bits 8 thru 31.
The shim header is followed by the body of an ICMP error message
formatted exactly as specified for ICMPv4 [RFC0792] or [RFC4443].
The ETE/ITE reporting the error sets the outer IP destination and
source addresses of the error message to the source and destination
addresses (respectively) of the SEAL packet. If the destination
address in the SEAL packet was multicast, the ETE/ITE instead sets
the outer IP source address to an address assigned to the underlying
IP interface.
The ETE/ITE next sets the UDP destination port to 'SEAL_PORT'' and
sets the UDP source port to a constant value of its choosing. It
then sets the "Next Header" field to the IP protocol type of the
header that follows (e.g., to the value '1' for an ICMPv4 message,
the value '58' for an ICMPv6 message, etc.) and sets the Reserved
field to 0. Associated tunneling mechanisms may instead set the
Next-Header field to a different value (e.g., '59' for No-Next-
Header) and define their own protocol specific coding in the Reserved
field.
The shim header is followed by an ICMP header of the correct IP
protocol version and with fields filled out as specified in [RFC0792]
or [RFC4443]. The ICMP header is followed by as much of the invoking
packet as possible without the entire message exceeding the minimum
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IP MTU (i.e., 576 bytes for IPv4 and 1280 bytes for IPv6) .
The ETE/ITE finally sends the error message to the original ITE.
When the A bit in the packet/fragment is not set, the message is sent
subject to rate limiting.
5. SEAL Protocol Specification (Version 1)
This section specifies a minimal version of SEAL known as "SEAL
Version 1", or "SEAL-lite". SEAL-lite observes the same protocol
specifications as for Classical SEAL (see Section 4) with the
exception that the ITE/ETE do not perform segmentation and
reassembly. In particular, the ETE unilaterally drops any SEAL-lite
packets that arrive as multiple IPv4 fragments and/or multiple SEAL
segments.
SEAL-lite can be considered for use by associated tunneling protocol
specifications when it highly unlikely that "marginal" links will
occur in any path, e.g., when it is known that the vast majority of
links configure MTUs that are appreciably larger than 1500 bytes.
SEAL-lite can also be used in instances when it is acceptable for the
ITE to return ICMP PTB messages for packet sizes smaller than 1500
bytes. Finally, the use of SEAL-lite requires that the associated
tunneling protocol specification either defines a next header field
or ensures that the data immediately following the SEAL header is an
IP header (i.e., either IPv4 or IPv6). The use of SEAL-lite must
therefore be carefully examined in relation to the particular use
case.
With respect to Section 4, the SEAL-lite protocol corresponds to
Classical SEAL as follows:
5.1. Model of Operation
SEAL-lite follows the same model of operation as for Classical SEAL
as described in Section 4.1 except as noted in the following
sections.
5.2. SEAL Header Format (Version 1)
The SEAL-lite 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|A|D| Reserved / Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: SEAL Version 1 Header Format
where the header fields are defined as:
VER (2)
a 2-bit value that encodes the SEAL protocol version number. This
section describes Version 1 of the SEAL protocol, i.e., the VER
field encodes the value '01'.
A (1)
the "Acknowledgement Requested" bit. Set to 1 if the ITE wishes
to receive an explicit acknowledgement from the ETE.
D (1)
the "Don't Fragment" bit. Set to 1 if the inner packet is an IPv6
packet, an IPv4 packet with DF=1, or a SEAL packet with D=1. Set
to 0 otherwise.
Reserved (12 or fewer)
a reserved field; used in a manner defined in the associated
tunneling protocol specification.
Identification (16 or more)
an identification field; used either as an extension to the IPv4
ID field or as an independent Identification field as defined in
the associated tunneling protocol specification.
5.3. ITE Specification
5.3.1. Tunnel Interface MTU
SEAL-lite observes the Classical SEAL specification found in Section
4.3.1.
5.3.2. Admitting Packets into the Tunnel Interface
SEAL-lite observes the Classical SEAL specification found in Section
4.3.2.
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5.3.3. Inner Fragmentation and Segmentation
SEAL-lite observes the Classical SEAL specification found in Section
4.3.3, except that S_MRU is set to 0. The ITE must therefore break
inner IP packets that are to undergo inner fragmentation into
fragments that are no larger than would both provide a reasonably-
large fragment size 'S' and avoid further fragmentation in the
network. In that case, it is recommended that the ITE select an
initial value for S between 1280 and (1500 - HLEN) unless it is known
that all links in the path to the ETE configure an MTU that is
significantly larger than this.
5.3.4. Encapsulation
SEAL-lite observes the Classical SEAL specification found in Section
4.3.4, except that it uses the header format defined in this section
and with the VER field set to '01'. SEAL-lite further uses the A and
D bits the same as specified for Classical SEAL, but the Reserved and
Identification fields are used in the manner specified by the
associated tunneling protocol.
5.3.5. Probing Strategy
SEAL-lite observes the Classical SEAL specification found in Section
4.3.5.
5.3.6. Packet Identification
SEAL-lite observes the Classical SEAL soft state specifications found
in Section 4.3.6, but configures and sets the Identification field in
a manner specified by the associated tunneling protocol.
As for the Classical SEAL specification in Section 4.3.6, SEAL-lite
increments the Identification field modulo the field length for each
consecutive SEAL packet.
5.3.7. Sending SEAL Protocol Packets
SEAL-lite observes the Classical SEAL specification found in Section
4.3.7.
5.3.8. Processing Raw ICMPv4 Messages
SEAL-lite observes the Classical SEAL specification found in Section
4.3.8.
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5.3.9. Processing SEAL Errors
SEAL-lite observes the Classical SEAL specification found in Section
4.3.9.
5.4. ETE Specification
5.4.1. Reassembly Buffer Requirements
SEAL-lite *does not* observe the Classical SEAL specification found
in Section 4.4.1, i.e., it does not maintain a reassembly buffer for
SEAL reassembly.
5.4.2. IP-Layer Reassembly
SEAL-lite uses SEAL-protocol IP first-fragments solely for the
purpose of generating fragmentation reports as specified in Section
4.4.2, but thereafter discards all SEAL-protocol IP fragments.
5.4.3. Sending SEAL Fragmentation Reports
SEAL-lite observes the Classical SEAL specification found in Section
4.4.3.
5.4.4. SEAL-Layer Reassembly
SEAL-lite observes the Classical SEAL error checking procedures in
Section 4.4.4, i.e., SEAL-lite returns a Parameter Problem for SEAL
packets with an unrecognized VER value.
SEAL-lite *does not* observe the Classical SEAL reassembly procedures
in Section 4.4.4; Instead, SEAL-lite discards all SEAL packets with
(M!=0 || SEG!=0) following IP layer reassembly.
5.4.5. Decapsulation and Delivery to Upper Layers
SEAL-lite observes the Classical SEAL specification found in Section
4.4.5.
5.4.6. Sending SEAL Error Messages
SEAL-lite observes the Classical SEAL specification found in Section
4.4.6.
6. Link Requirements
Subnetwork designers are expected to follow the recommendations in
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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.
10. Security Considerations
Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-
overlapping.
An amplification/reflection attack is possible when an attacker sends
IPv4 first-fragments with spoofed source addresses to an ETE,
resulting in a stream of ICMPv4 Fragmentation Needed messages
returned to a victim ITE. The encapsulated segment of the spoofed
IPv4 first-fragment provides mitigation for the ITE to detect and
discard spurious ICMPv4 Fragmentation Needed messages.
The SEAL header is sent in-the-clear (outside of any IPsec/ESP
encapsulations) the same as for the outer */IPv4 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.
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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
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 A 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, IPv4
fragmentation provides a short-term interim mechanism for
ensuring that packets are delivered while SEAL adjusts its packet
sizing parameters.
2. Classical path MTU discovery requires that routers generate an
ICMP PTB message for *all* packets lost due to an MTU
restriction; this situation is exacerbated at high data rates and
becomes severe for in-the-network tunnels that service many
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communicating end systems. Since SEAL ensures that packets no
larger than S_MRU are delivered, however, it is sufficient for
the ETE to return ICMP PTB messages subject to rate limiting and
not for every packet-in-error.
3. Classical path MTU may require several iterations of dropping
packets and returning ICMP PTB messages until an acceptable path
MTU value is determined. Under normal circumstances, SEAL
determines the correct packet sizing parameters in a single
iteration.
4. 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.
5. Using SEAL, ETEs encapsulate ICMP 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.
6. Most importantly, all SEAL packets have a 32-bit Identification
value that can be used for duplicate packet detection purposes
and to match ICMP error messages with actual packets sent without
requiring per-packet state; hence, certain denial-of-service
attack vectors open to the classical methods are eliminated.
In summary, the SEAL approach represents an architecturally superior
method for ensuring that packets of various sizes 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, 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.
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Extending the IP identification field was first proposed by Steve
Deering on the MTUDWG mailing list in 1989.
14. References
14.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[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.
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-01 (work in progress), May 2009.
[I-D.russert-rangers]
Russert, S., Fleischman, E., and F. Templin, "RANGER
Scenarios", draft-russert-rangers-00 (work in progress),
May 2009.
[I-D.templin-autoconf-dhcp]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-autoconf-dhcp-38 (work in progress),
April 2009.
[I-D.templin-ranger]
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Templin, F., "Routing and Addressing in Next-Generation
EnteRprises (RANGER)", draft-templin-ranger-07 (work in
progress), February 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.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, February 2000.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[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.
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[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[TCP-IP] "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 Extensions
Future updates to this specification will examine improved
reliability in the face of loss due to congestion, signal
intermittence, etc. Automatic Repeat-ReQuest (ARQ) mechanisms are
used to ensure reliable delivery between the endpoints of links
[RFC3366] (e.g., on-link neighbors in an IEEE 802.11 network) as well
as between the endpoints of an end-to-end transport (e.g., the
endpoints of a TCP connection). However, ARQ mechanisms may not be
ideally sutiable for all SEAL use cases, since retransmission of lost
segments may require considerable state maintenance at the ITE and
would result in considerable delay variance and packet reordering
within the subnetwork.
Alternate reliability mechanisms such as Forward Error Correction
(FEC) may also be examined in future updates to this specification
for the purpose of improved reliability. Such mechanisms may entail
the ITE performing proactive transmissions of redundant data, e.g.,
by sending multiple copies of the same data. Signaling from the ETE
(e.g., by sending Source Quench messages) may also be considered as a
means for the ETE to dynamically inform the ITE of changing FEC
conditions.
Appendix B. Transport Mode
SEAL can also be used in "transport-mode", e.g., when the inner layer
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includes upper-layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL for
the carriage of protocol data encapsulated as TCP/SEAL/IPv4. In this
sense, the "subnetwork" becomes the entire end-to-end path between
the TCP peers and may potentially span the entire Internet.
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 C. Historic Evolution of PMTUD
(Taken from "Neighbor Affiliation Protocol for IPv6-over-(foo)-over-
IPv4"; written 10/30/2002):
The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
and numerous proposals in the late 1980's through early 1990. The
initial problem was posed by Art Berggreen on May 22, 1987 in a
message to the TCP-IP discussion group [TCP-IP]. The discussion that
followed provided significant reference material for [FRAG]. An IETF
Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
with charter to produce an RFC. Several variations on a very few
basic proposals were entertained, including:
1. Routers record the PMTUD estimate in ICMP-like path probe
messages (proposed in [FRAG] and later [RFC1063])
2. The destination reports any fragmentation that occurs for packets
received with the "RF" (Report Fragmentation) bit set (Steve
Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal)
3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw
RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)
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4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
1990)
5. Fragmentation avoidance by setting "IP_DF" flag on all packets
and retransmitting if ICMPv4 "fragmentation needed" messages
occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191]
by Mogul and Deering).
Option 1) seemed attractive to the group at the time, since it was
believed that routers would migrate more quickly than hosts. Option
2) was a strong contender, but repeated attempts to secure an "RF"
bit in the IPv4 header from the IESG failed and the proponents became
discouraged. 3) was abandoned because it was perceived as too
complicated, and 4) never received any apparent serious
consideration. Proposal 5) was a late entry into the discussion from
Steve Deering on Feb. 24th, 1990. The discussion group soon
thereafter seemingly lost track of all other proposals and adopted
5), which eventually evolved into [RFC1191] and later [RFC1981].
In retrospect, the "RF" bit postulated in 2) is not needed if a
"contract" is first established between the peers, as in proposal 4)
and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
Feb 19. 1990. These proposals saw little discussion or rebuttal, and
were dismissed based on the following the assertions:
o routers upgrade their software faster than hosts
o PCs could not reassemble fragmented packets
o Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets
o Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)
o the 16-bit IP_ID field could wrap around and disrupt reassembly at
high packet arrival rates
The first four assertions, although perhaps valid at the time, have
been overcome by historical events. The final assertion is addressed
by the mechanisms specified in SEAL.
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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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