Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track June 17, 2009
Expires: December 19, 2009
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-01.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 . . . . . . . . . . . . . . . . . . . 8
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. Segmentation . . . . . . . . . . . . . . . . . . . . . 13
4.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 15
4.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 15
4.3.6. Packet Identification . . . . . . . . . . . . . . . . 16
4.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 16
4.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 17
4.3.9. Processing SEAL Control Messages . . . . . . . . . . . 17
4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 19
4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 19
4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 19
4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 20
4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 21
4.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 21
5. SEAL Protocol Specification (Version 1) . . . . . . . . . . . 28
5.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 28
5.2. SEAL Header Format (Version 1) . . . . . . . . . . . . . . 28
5.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 29
5.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 29
5.3.2. Admitting Packets into the Tunnel Interface . . . . . 29
5.3.3. Segmentation . . . . . . . . . . . . . . . . . . . . . 29
5.3.4. Encapsulation . . . . . . . . . . . . . . . . . . . . 30
5.3.5. Probing Strategy . . . . . . . . . . . . . . . . . . . 30
5.3.6. Packet Identification . . . . . . . . . . . . . . . . 30
5.3.7. Sending SEAL Protocol Packets . . . . . . . . . . . . 30
5.3.8. Processing Raw ICMP Messages . . . . . . . . . . . . . 30
5.3.9. Processing SEAL Control Messages . . . . . . . . . . . 30
5.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 30
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5.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 30
5.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 31
5.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 31
5.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 31
5.4.5. Sending SEAL Control Messages . . . . . . . . . . . . 31
6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 31
7. End System Requirements . . . . . . . . . . . . . . . . . . . 31
8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 31
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
10. Security Considerations . . . . . . . . . . . . . . . . . . . 32
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 32
12. SEAL Advantages over Classical Methods . . . . . . . . . . . . 33
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 34
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
14.1. Normative References . . . . . . . . . . . . . . . . . . . 34
14.2. Informative References . . . . . . . . . . . . . . . . . . 35
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Transport Mode . . . . . . . . . . . . . . . . . . . 37
Appendix C. Historic Evolution of PMTUD . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39
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1. Introduction
As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (including
tunnels of one form or another) over an actual network that supports
the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual
topologies have elements that appear as one hop in the virtual
topology, but are actually multiple IP or sub-IP layer hops. These
multiple hops often have quite diverse properties that are often not
even visible to the endpoints of the virtual hop. This introduces
failure modes that are not dealt with well in current approaches.
The use of IP encapsulation has long been considered as the means for
creating such virtual topologies. However, the insertion of an outer
IP header reduces the effective path MTU as-seen by the IP layer.
When IPv4 is used, this reduced MTU can be accommodated through the
use of IPv4 fragmentation, but unmitigated in-the-network
fragmentation has been found to be harmful through operational
experience and studies conducted over the course of many years
[FRAG][FOLK][RFC4963]. Additionally, classical path MTU discovery
[RFC1191] has known operational issues that are exacerbated by in-
the-network tunnels [RFC2923][RFC4459]. The following subsections
present further details on the motivation and approach for addressing
these issues.
1.1. Motivation
Before discussing the approach, it is necessary to first understand
the problems. In both the Internet and private-use networks today,
IPv4 is ubiquitously deployed as the Layer 3 protocol. The two
primary functions of IPv4 are to provide for 1) addressing, and 2) a
fragmentation and reassembly capability used to accommodate links
with diverse MTUs. While it is well known that the 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.
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Due to these many limitations, a new approach 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 denial-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.
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 Reassembly 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 SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size
S_CSS - the SEAL Clamped 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_CPORT - a TCP/UDP service port number used for SEAL control
plane messaging
SEAL_DPORT - a TCP/UDP service port number used for SEAL data
plane messaging
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
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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
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]. 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 "SEAL-MAX". 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.
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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:
+-------------------------+
| |
~ 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/IPvY encapsulations (e.g.,
[RFC2003][RFC2004][RFC2473][RFC4213]), the SEAL header is inserted
between the inner and outer IP headers as: IPvX/SEAL/IPvY.
o For tunnel-mode IPsec encapsulations, [RFC4301], the SEAL header
is inserted between the {AH,ESP} header and outer IP headers as:
IPvX/*/{AH,ESP}/SEAL/IPvY.
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/IPvY.
SEAL-encapsulated packets include a SEAL_ID to uniquely identify each
packet. Routers within the subnetwork use the SEAL_ID for duplicate
packet detection, and ITEs/ETEs use the SEAL_ID for SEAL
segmentation/reassembly and protection against off-path attacks.
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For IPv4, the SEAL_ID is formed from the concatenation of the 16-bit
ID Extension field in the SEAL header as the most-significant bits,
and with the 16-bit Identification value in the outer IPv4 header as
the least-significant bits. For IPv6, the SEAL_ID is written into
the 32-bit Identification field of the fragment header. For tunnels
that traverse middleboxes that might rewrite the IP ID field, e.g., a
Network Address Translator, the SEAL_ID is instead maintained only
within the ID field in the SEAL header.
SEAL enables a multi-level segmentation and reassembly capability.
First, the ITE can use IPv4 fragmentation to fragment inner IPv4
packets before SEAL encapsulation. Secondly, the SEAL layer itself
provides a simple cutting-and-pasting capability for mid-layer
packets to avoid IP fragmentation on the outer packet. Finally,
ordinary IP fragmentation is permitted on the outer packet after SEAL
encapsulation and 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|I|F|M|RSV| NEXTHDR/SEG | 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.
I (1)
the "Information Request Solicit" bit. Set to 1 if the ITE wishes
the ETE to initiate an Information Request.
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F (1)
the "First Segment" bit. Set to 1 if this SEAL protocol packet
contains the first segment (i.e., Segment #0) of a mid-layer
packet.
M (1)
the "More Segments" bit. Set to 1 if this SEAL protocol packet
contains a non-final segment of a multi-segment mid-layer packet.
RSV (2)
a 2-bit Reserved field. Set to 0 for the purpose of this
specification.
NEXTHDR/SEG (8) an 8-bit field. When 'F'=1, encodes the next header
Internet Protocol number the same as for the IPv4 protocol and
IPv6 next header fields. When 'F'=0, encodes a segment number of
a multi-segment mid-layer packet. (The segment number 0 is
reserved.)
ID Extension (16)
a 16-bit Identification extension field.
4.3. ITE Specification
4.3.1. Tunnel Interface MTU
The ITE configures a tunnel virtual interface over one or more
underlying links that connect the border node to the subnetwork. The
tunnel interface must present a fixed MTU to the inner IP layer
(i.e., Layer 3) as the size for admission of inner IP packets into
the tunnel. Since the tunnel interface may support a potentially
large set of ETEs, however, care must be taken in setting a large-
enough MTU for all ETEs while still upholding end system
expectations.
Due to the ubiquitous deployment of standard Ethernet and similar
networking gear, the nominal Internet cell size has become 1500
bytes; this is the de facto size that end systems have come to expect
will either be delivered by the network without loss due to an MTU
restriction on the path or a suitable 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
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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
fragmentation. In order to preserve the end system expectations, the
ITE therefore requires a means for conveying these larger packets to
the ETE even though there may be links within the subnetwork that
configure a smaller MTU.
The ITE should therefore set a tunnel virtual interface MTU of 1500
bytes plus extra room to accommodate any additional encapsulations
that may occur on the path from the original source (i.e., even if
the path to the ETE does not support an MTU of this size). The ITE
can set larger MTU values still, but should select a value that is
not so large as to cause excessive PTBs coming from within the tunnel
interface (see Sections 4.3.3 and 4.3.8). The ITE can also set
smaller MTU values; however, care must be taken not to set so small a
value that original sources would experience an MTU underflow. In
particular, IPv6 sources must see a minimum path MTU of 1280 bytes,
and IPv4 sources should see a minimum path MTU of 576 bytes.
The ITE can alternatively set an indefinite MTU on the tunnel virtual
interface such that all inner IP packets are admitted into the
interface without regard to size. This option must be carefully
coordinated with the ITE's protocol stack upper layers, since some
upper layer protocols (e.g., TCP) derive their packet sizing
parameters from the MTU of the underlying interface and as such may
select too large an initial size. This is not a problem for upper
layers that use conservative initial estimates, e.g., when mechanisms
such as Packetization Layer Path MTU Discovery [RFC4821] are used.
4.3.2. Admitting Packets into the Tunnel Interface
The inner IP layer consults the tunnel interface MTU when admitting a
packet into the interface. For IPv4 packets with the IPv4 Don't
Fragment (DF) bit set to 0, if the packet is larger than the tunnel
interface MTU the inner IP layer uses IP fragmentation to break the
packet into fragments no larger than the tunnel interface MTU. The
ITE then admits each fragment into the tunnel as an independent
packet.
For all other packets, the ITE admits the packet if it is no larger
than the tunnel interface MTU; otherwise, it drops the packet and
sends 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
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minimum IP MTU (i.e., 576 bytes for IPv4 and 1280 bytes for IPv6).
Note that when the tunnel interface sets an indefinite MTU all
packets are unconditionally admitted into the interface without
fragmentation.
4.3.3. Segmentation
For each ETE, the ITE maintains soft state within the tunnel
interface (e.g., in a destination cache) used to support inner
fragmentation and SEAL segmentation. The soft state includes the
following:
o a Mid-layer Header Length (MHLEN); set to the length of any mid-
layer '*' encapsulation headers and trailers (e.g., for '*' = AH,
ESP, NULL, etc.). MHLEN additionally includes 4 extra bytes for a
trailing mid-layer checksum (see below).
o an Outer Header Length (OHLEN); set to the length of the outer
SEAL/*/IP encapsulation headers and trailers.
o a total Header Length (HLEN); set to MHLEN plus OHLEN.
o a SEAL Maximum Segment Size (S_MSS); initialized to a value that
is no larger than the underlying IP interface MTU. The ITE
decreases or increases S_MSS based on any SEAL Reassembly Report
messages received (see Section 4.3.9).
o a SEAL Clamped Segment Size (S_CSS); a value that is no larger
than S_MSS and that would also be unlikely to incur fragmentation
beyond the tunnel, (e.g., 576 bytes for IPv4 and 1280 bytes for
IPv6). May be set to larger values only if there is high
assurance that all links within the tunnel configure a larger MTU.
The ITE decreases S_CSS in conjunction with S_MSS, but only
increases S_CSS to a "safe" value as above if S_MSS is increased.
o a SEAL Maximum Reassembly Unit (S_MRU); initialized to the larger
of S_MSS and the known or estimated Maximum Receive Unit (MRU)
actually configured by the ETE (2KB minimum default). The ITE
decreases or increases S_MRU based on any SEAL Reassembly Report
messages received (see Section 4.3.9). When (S_MRU>(S_MSS*256)),
the ITE uses (S_MSS*256) as the effective S_MRU value.
After an inner packet/fragment has been admitted into the tunnel
interface the ITE uses the following algorithm to determine whether
the packet can be accommodated at all and (if so) whether inner IP
fragmentation is needed:
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o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1,
and the packet is larger than (S_MRU - HLEN), the ITE drops the
packet and sends an ICMP PTB message to the original source with
an MTU value of (S_MRU - HLEN) the same as described in Section
4.3.2; else,
o if the inner packet is a SEAL IPv4 packet with DF=0, and the
packet is larger than (S_MRU - HLEN), the ITE uses inner IPv4
fragmentation to break the packet into fragments no larger than
(S_MRU - HLEN); else,
o if the inner packet is a non-SEAL IPv4 packet with DF=0, the ITE
uses inner IPv4 fragmentation to break the packet into fragments
no larger than (S_CSS - HLEN); else,
o the ITE processes the packet without inner fragmentation.
(Note that in the above the ITE must also track whether the tunnel
interface is using header compression on the inner */IP headers. If
so, the ITE must include the length of the uncompressed */IP inner
header when calculating the total length of the inner packet.)
The ITE next encapsulates each inner packet/fragment in the MHLEN
bytes of mid-layer '*' headers and trailers and reserves 4 bytes for
a trailing checksum at the end of the mid-layer trailers. The ITE
then calculates a checksum of the mid-layer packet using the 16-bit
Fletcher Checksum algorithm specified in [RFC1146], Appendix II, and
writes the 'A' portion as the most significant 16 bits and the 'B'
portion as the least significant 16 bits.
If the length of the resulting mid-layer packet plus OHLEN is greater
than S_MSS, the ITE must additionally perform SEAL segmentation. To
do so, it breaks the mid-layer packet into N segments (N <= 256) that
are no larger than (S_MSS - OHLEN) bytes each. Each segment, except
the final one, MUST be of equal length. The first byte of each
segment MUST begin immediately after the final byte of the previous
segment, i.e., the segments MUST NOT overlap. The ITE SHOULD
generate the smallest number of segments possible, e.g., it SHOULD
NOT generate 6 smaller segments when the packet could be accommodated
with 4 larger segments.
Note that this SEAL segmentation ignores the fact that the mid-layer
packet may be unfragmentable outside of the subnetwork. This
segmentation process is a mid-layer (not an IP layer) operation
employed by the ITE to adapt the mid-layer packet to the subnetwork
path characteristics, and the ETE will restore the packet to its
original form during reassembly. Therefore, the fact that the packet
may have been segmented within the subnetwork is not observable
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outside of the subnetwork.
4.3.4. Encapsulation
Following SEAL segmentation, the ITE encapsulates each segment in a
SEAL header formatted as specified in Section 4.3.2 with VER='00' and
RSV='00'. For the first segment, the ITE sets F=1 and sets NEXTHDR
to the Internet Protocol number of the encapsulated packet; the ITE
next sets M=1 if there are more segments or sets M=0 otherwise. For
each non-initial segment of an N-segment mid-layer packet (N <= 256),
the ITE sets (F=0; M=1; SEG=1) in the SEAL header of the first non-
initial segment, sets (F=0; M=1; SEG=2) in the next non-initial
segment, etc., and sets (F=0; M=0; SEG=N-1) in the final segment.
(Note that the value SEG=0 is not used.)
The ITE next encapsulates each segment in the requisite */IP outer
headers according to the specific encapsulation format (e.g.,
[RFC2003], [RFC2473], [RFC4213], [RFC4380], etc.), except that it
writes 'SEAL_PROTO' in the protocol field of the outer IP header
(when simple IP encapsulation is used) or writes 'SEAL_DPORT' in the
outer destination service port field (e.g., when UDP/IP 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 4.3.6 and sends the packets as specified in
Section 4.3.7.
Note that when IPv6 is used as the outer IP encapsulation layer, the
ITE must insert an IPv6 fragment header with an Identification value
set as described in Section 4.3.6.
4.3.5. Probing Strategy
All SEAL packets sent by the ITE are considered implicit probes, and
will elicit Reassembly Reports from the ETE with a new value for
S_MSS if any IP fragmentation occurs in the path. Thereafter, the
ITE may periodically reset S_MSS to a larger value (e.g., the
underlying IP interface MTU minus OHLEN bytes) to detect path MTU
increases.
The ITE should additionally send explicit probes, periodically, to
verify that the ETE is still reachable and to manage a window of
SEAL_IDs. The ITE sets (A=1; F=1) in the SEAL header of a first-
segment to be used as an explicit probe, where the probe can be
either an ordinary data packet or a NULL packet created by setting
the 'Next Header' field to a value of "No Next Header" (see Section
4.7 of [RFC2460]). The probe will elicit a Reassembly Report from
the ETE as an acknowledgement.
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The ITE can also send probes using non-initial SEAL segments to
determine whether any of the preceding segments of the same SEAL
packet are missing. The probe will elicit a Reassembly Report from
the ETE with a Bitmap of received and missing segments.
Finally, the ITE MAY send "expendable" probe packets (see Section
4.3.7) in order to generate ICMP PTB 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 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 outer IPv4 packet, the ITE writes the least-significant 16
bits of the SEAL_ID value into 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 each outer IPv6 packet, the
ITE writes the entire SEAL_ID value into the Identification field in
the IPv6 fragment header.
For ITE->ETR tunnels specifically designed for the traversal of
Network Address Translators (NATs) and other middleboxes that may
rewrite the outer IP ID field, the ITE instead writes least
significant bits of the SEAL_ID in the ID field of the SEAL header
and writes a random value in the Identification field in the outer IP
header. Since the ID field in the SEAL header is only 16 bits,
however, the ITE must limit the rate at which it sends packets to
avoid wrapping the ID field. Alternatively, the ITE and ETE can use
SEAL-LITE to obtain a larger ID field in the SEAL header (see Section
5.3.6).
4.3.7. Sending SEAL Protocol Packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 for
ordinary SEAL/*/IPv4 packets, but may set DF=1 for "expendable" SEAL/
*/IPv4 packets (e.g., for NULL packets used as probes -- see Section
4.3.5). For SEAL/*/IPv6 packets, the "DF" bit is always implicitly
set to 1, but when a fragment header is included a translating router
on the path may still fragment the packet.
The ITE sends each outer packet that encapsulates a segment of the
same mid-layer packet into the tunnel in canonical order, i.e.,
segment 0 first, followed by segment 1, etc., and finally segment
N-1.
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4.3.8. Processing Raw ICMP Messages
The ITE may receive "raw" ICMP error messages [RFC0792][RFC4443] from
either the ETE or routers within the subnetwork that comprise an
outer IP header, followed by an ICMP header, followed by a portion of
the SEAL packet that generated the error (also known as the "packet-
in-error"). The ITE can use the SEAL ID encoded in the packet-in-
error as a nonce to confirm that the ICMP message came from either
the ETE or an on-path router, and can use any additional information
to determine whether to accept or discard the message.
The ITE should specifically process raw ICMPv4 Protocol Unreachable
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.
4.3.9. Processing SEAL Control Messages
In addition to any raw ICMP messages, the ITE may receive UDP/IP SEAL
control messages from the ETE formatted as specified in Section 4.4.5
and with 'SEAL_CPORT' as the UDP destination port. The ITE must
therefore monitor the 'SEAL_CPORT' UDP port and process any messages
that arrive on that port.
For each control message, the ITE verifies the UDP checksum and
discards the message if the checksum is incorrect. The ITE can then
verify that the SEAL_ID is within the current window of transmitted
SEAL_IDs for this ETE. If the SEAL_ID is outside of the window, the
ITE discards the message; otherwise, it advances the window and
processes the message. The ITE processes SEAL control messages as
follows:
4.3.9.1. Reassembly Report (Type=0)
When the ITE receives a Reassembly Report formatted as specified in
Section 4.4.5.1, it processes the message according to the Code value
as follows:
4.3.9.1.1. IP Fragmentation Experienced (Code=0)
The ITE records the value in the S_MRU field in its soft state for
this ETE and adjusts the S_MSS value in its soft state. If the S_MSS
value in the Reassembly Report is greater than 576 (i.e., the nominal
minimum MTU for IPv4 links), the ITE records this new value in its
soft state. If the S_MSS value in the report is less than the
current soft state value and also less than 576, the can discern that
IP fragmentation is occurring but it cannot determine the true MTU of
the restricting link due to a router on the path generating runt
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first-fragments.
The ITE should therefore search for a reduced S_MSS value through an
iterative searching strategy that parallels (Section 5 of [RFC1191]).
This searching strategy may require multiple iterations of sending
SEAL packets using a reduced S_MSS and receiving additional
Reassembly Report messages, but it will soon converge to a stable
value. During this process, it is essential that the ITE reduce
S_MSS based on the first Reassembly Report message received, and
refrain from further reducing S_MSS until SEAL Reassembly Report
messages pertaining to packets sent under the new S_MSS are received.
4.3.9.1.2. Segment Acknowledged (Code=1)
The ITE records the value in the S_MRU field in its soft state for
this ETE. If the S_MSS value in the report is non-zero, the ITE also
adjusts its S_MSS value the same as for an IP Fragmentation
Experienced message (see Section 4.3.9.1.1).
The ITE next examines the Bitmap field to determine which segments of
this SEAL packet were received. The map is arranged with segment 0
represented as the most significant bit, segment 1 represented as the
next most significant bit, etc., up to the segment that triggered the
reassembly report (i.e., segment N) as the final bit. For example,
when N=6 the bit map '0110011' means that segments 1, 2, 5 and 6 were
received but segments 0, 3 and 4 were missing from the ETE's
reassembly buffer. The ITE can then retransmit segments 0, 3 and 4
if it still has them in its cache, and if there is reason to believe
the retransmissions may satisfy the pending reassembly.
4.3.9.1.3. Packet Too Big (Code=2)
The ITE records the value in the S_MRU field in its soft state for
this ETE.
4.3.9.1.4. Time Exceeded (Code=3)
The ITE examines the time encoded in the Data field, and reduces its
S_MRU estimate for this ETE if it there is significant evidence that
large packets are timing out prior to SEAL reassembly completion.
The ITE may log the event for network management purposes.
4.3.9.1.5. Checksum Incorrect (Code=4)
The ITE may log the event for network management purposes. When
sustained Checksum Incorrect messages are received from this ETE, the
ITE may also benefit by adjusting its packet sizing parameters.
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4.3.9.2. Parameter Problem (Type=1)
When the ITE receives a Parameter Problem message formatted as
specified in Section 4.4.5.2, it examines the encapsulated SEAL
header in the message to determine whether the header was corrupted
or whether the header specified features that the ETE did not
recognize. The ITE MAY log the event for network management
purposes. For uncorrupted headers, the SHOULD adjust its SEAL header
parameters in subsequent SEAL packets.
4.3.9.3. Information Request (Type=2)
When the ITE receives an Information Request message formatted as
specified in Section 4.4.5.5 and with a SEAL_ID that corresponds to a
SEAL packet that it sent earlier with I=1, it sends an Information
Reply as specified in Section 4.4.5.6.
4.4. ETE Specification
4.4.1. Reassembly Buffer Requirements
ETEs must be capable of performing IP-layer reassembly for SEAL
protocol IP packets up to 2KB in length, and must also be capable of
performing SEAL-layer reassembly for mid-layer packets up to (2KB -
OHLEN). Hence, ETEs:
o MUST configure a reassembly buffer of at least 2KB
o MAY configure a larger reassembly buffer
o MUST be capable of discarding SEAL packets that are too large to
reassemble
Note that the ETE must retain the SEAL/*/IP header during both IP-
layer and SEAL-layer reassembly for the purpose of associating the
fragments/segments of the same packet.
4.4.2. IP-Layer Reassembly
ETEs perform standard IP-layer reassembly for SEAL protocol IP
fragments, and should maintain a conservative reassembly cache high-
and low-water mark . When the size of the reassembly cache exceeds
this high-water mark, the ETE should actively discard incomplete
reassemblies (e.g., using an Active Queue Management (AQM) strategy)
until the size falls below the low-water mark. The ETE should also
actively discard any pending reassemblies that clearly have no
opportunity for completion, e.g., when a considerable number of new
fragments have been received before a fragment that completes a
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pending reassembly has arrived.
When the ETE processes the IP first-fragment (i.e, one with MF=1 and
Offset=0 in the IP header) of a fragmented SEAL packet, it sends a
SEAL Reassembly Report message back to the ITE with the S_MSS field
set to the length of the first-fragment and with the S_MRU field set
to the size of the ETE's reassembly buffer (see Section 4.4.5).
4.4.3. SEAL-Layer Reassembly
Following IP reassembly of a SEAL segment, the ETE adds the segment
to a SEAL-Layer pending-reassembly queue according to the (Source,
Destination, SEAL_ID)-tuple found in the outer SEAL/*/IP headers.
The ETE performs SEAL-layer reassembly through simple in-order
concatenation of the encapsulated segments of the same mid-layer
packet from N consecutive SEAL packets. SEAL-layer reassembly
requires the ETE to maintain a cache of recently received segments
for a hold time that would allow for reasonable inter-segment delays
(e.g., 15 seconds). When a SEAL reassembly times out, the ETE
discards the incomplete reassembly and returns a Reassembly Report -
Time Exceeded message to the ITE (see Section 4.4.5). As for IP-
layer reassembly, the ETE should also maintain a conservative
reassembly cache high- and low-water mark and should actively discard
any pending reassemblies that clearly have no opportunity for
completion, e.g., when a considerable number of new SEAL packets have
been received before a packet that completes a pending reassembly has
arrived.
When the ETE receives a SEAL packet with an incorrect value in the
SEAL header, it discards the packet and returns a Parameter Problem
message (see Section 4.4.5). If the ETE receives a SEAL packet for
which a segment with the same (Source, Destination, SEAL_ID)-tuple is
already in the queue, it must determine whether to accept the new
segment and release the old, or drop the new segment. If accepting
the new segment would cause an inconsistency with other segments
already in the queue (e.g., differing segment lengths), the ETE drops
the segment that is least likely to complete the reassembly.
When the ETE adds a SEAL packet with A=1 and with SEG=N to a
reassembly queue, it sends a Reassembly Report - Segment Acknowledged
message back to the ITE as specified in Section 4.4.6 that contains a
bitmask of this and all prior segments already in the queue beginning
with segment 0 in the most-significant bit, segment 1 in the next
bit, etc.,up to segment N in the final bit. For example, when N=4,
the bitmask '01101' indicates that segments 1, 2 and 4 are in the
queue while segments 0 and 3 are missing.
After all segments are gathered, the ETE reassembles the mid-layer
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packet by concatenating the segments encapsulated in N consecutive
SEAL packets beginning with the initial segment (i.e., SEG=0) and
followed by any non-initial segments 1 through N-1. That is, for an
N-segment mid-layer packet, reassembly entails the concatenation of
the SEAL-encapsulated mid-layer packet segments with (F=1, M=1,
SEAL_ID=j) in the first SEAL header, followed by (F=0, M=1, SEG=1,
SEAL_ID=(j+1)) in the next SEAL header, followed by (F=0, M=1, SEG=2,
SEAL_ID=(j+2)), etc., up to (F=0, M=0, SEG=(N-1), SEAL_ID=(j + N-1))
in the final SEAL header. (Note that modulo arithmetic based on the
length of the SEAL_ID field is used).
When the ETE determines that a mid-layer packet is too large to
reassemble, it releases the reassembly queue resources and sends a
Reassembly Report - Packet Too Big message back to the ITE with the
S_MRU field set to the size of the ETE's reassembly buffer (see
Section 4.4.5).
4.4.4. Decapsulation and Delivery to Upper Layers
Following SEAL-layer reassembly, the ETE verifies the trailing
checksum of the mid-layer packet using the algorithm in Section
4.3.3. If the checksum is incorrect, the ETE discards the packet and
sends a Reassembly Report - Checksum Incorrect message to the ITE
(see Section 4.4.5). If the reassembled mid-layer packet is larger
than (S_MRU-OHLEN), the ETE discards the packet and sends a
Reassembly Report - Packet Too Big message to the ITE (see Section
4.4.5).
Otherwise, the ETE discards the outer and mid-layer headers and
trailers, and delivers the inner packet to the upper-layer protocol
indicated in the SEAL Next Header field. (If the reassembled packet
if it was a NULL packet (see Section 4.3.4), the ETE instead silently
discards the packet).
4.4.5. Sending SEAL Control Messages
The ETE generates SEAL control messages in response to certain SEAL
packets. SEAL control messages are formated much the same as for
ICMPv4 [RFC0792] and ICMPv6 [RFC4443] messages, and are used for very
similar purposes. The ETE prepares each control message as a UDP/IP
packet as shown in Figure 3:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ UDP/IP Headers (dport=SEAL_CPORT) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Message Body +
| |
+-+-+ ...
Figure 3: SEAL Control Message Format
The control message consists of outer UDP/IP headers followed by a
32-bit SEAL_ID followed by a 32-bit control field followed by the
message body. When the ETE/ITE prepares a control message, it sets
the outer IP destination and source addresses of the message to the
source and destination addresses (respectively) of the SEAL packet
that triggered the message. If the destination address in the 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_CPORT'' and sets the UDP
source port to a constant value of its choosing. It then sets the
SEAL_ID to the Identification value encoded in the SEAL packet that
triggered the message.
As for ICPMv4 and ICMPv6 messages, the SEAL control header includes
an 8-bit Type field in bits 0 thru 7 and an 8-bit Code field in bits
8 thru 15. Unlike ICMPv4 and ICMPv6 messages, however, the control
header does not include a checksum field (since the UDP header
already contains a checksum) but instead includes a 16-bit Data field
in bits 16 thru 31. The ETE/ITE sets the Type, Code, Data and
Message body fields according to the specific SEAL control message
type, then sends the message. The following types are currently
defined; other values for Type will be recorded in the IANA registry
for SEAL:
4.4.5.1. Reassembly Report (Type=0)
The ETE generates a Reassembly Report to inform the ITE of various
conditions encountered during SEAL-layer reassembly. The following
values for Code are defined:
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o Code = 0 : IP Fragmentation Experienced
o Code = 1 : Segment Acknowledged
o Code = 2 : Packet Too Big
o Code = 3 : Time Exceeded
o Code = 4 : Checksum Incorrect
(Other values for Code will be recorded in the IANA registry for
SEAL.)
The ETE prepares the Reassembly Report according to the Code as
follows:
4.4.5.1.1. IP Fragmentation Experienced (Code=0)
When the ITE receives an IP first-fragment of a SEAL packet that
experienced outer IP fragmentation, it examines SEAL header and
examines the IP reassembly buffer to assess the likelihood that
reassembly will complete. If the 'A" bit is not set in the SEAL
header, or if IP reassembly completion appears unlikely, the ETE uses
the IP first-fragment to prepare a "Reassembly Report - IP
Fragmentation Experienced" message with Type=0, Code=0, and Data=0.
The message is formatted as follows
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=0 | Data=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL Header of Packet that Triggered the Report |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MSS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: IP Fragmentation Experienced Message Format
The ETE unconditionally writes the size of its reassembly buffer (see
Section 4.4.1) in the S_MRU field and writes the length of the first
IP fragment in the S_MSS field.
If the 'A' bit is set in the SEAL header and IP reassembly completion
appears likely, the ETE should refrain from sending this message if
possible and instead send a Segment Acknowledged message according to
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the next section. (Note that it is not an error for the ETE to
generate both the IP Fragmentation Experienced and Segment
Acknowledged messages for the same SEAL packet, however this may be
inefficient in some instances.)
4.4.5.1.2. Segment Acknowledged (Code=1)
When the ITE receives a SEAL segment following IP reassembly that has
the 'A' bit set in the SEAL header, it prepares a "Reassembly Report
- Segment Acknowledged" message with Type=0, Code=1, and Data=0. The
message is formatted as follows
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=1 | Data=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL Header of Packet that Triggered the Report |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MSS |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Bitmap |
+-+-+ ...
Figure 5: Segment Acknolwedged Message Format
The ETE unconditionally writes the size of its reassembly buffer in
the S_MRU field. If the Segment arrived as multiple IP fragments,
the ETE also writes the length of the IP first-fragment in the S_MSS
field; otherwise, it writes the value 0 in that field.
The ETE next includes a Bitmap recording this and all preceding
segments of the same SEAL packet that are already in its SEAL
reassembly buffer. For each segment, the Bitmap records the value 1
if the segment is present and 0 if the segment is absent. The most
significant bit of the Bitmap corresponds to segment 0, the next-most
significant bit corresponds to segment 1, etc., and the least
significant bit corresponds to the final segment. For example, when
SEG=6 in the SEAL header, the bit map '0110011' means that segments
1, 2, 5 and 6 were received but segments 0, 3 and 4 were missing from
the ETE's reassembly buffer. The Bitmap must include at least (SEG+
1)-many bits, where SEG is at most 255.
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4.4.5.1.3. Packet Too Big (Code=2)
The ETE generates a "Reassembly Report - Packet Too Big" message when
it discards a SEAL packet that is too large for it to receive. The
ETE sets Type=0, Code=2, and Data to 0 . The ETE then writes the
SEAL header of segment 0 of the packet that generated the error into
the first four bytes of the message body, then writes the size of its
reassembly buffer in the S_MRU field. The message is formatted as
follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=2 | Data=(Time, in Seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL Header of First SEAL Segment of the Too-big Packet |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| S_MRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: Segment Acknolwedged Message Format
4.4.5.1.4. Time Exceeded (Code=3)
The ETE generates a "Reassembly Report - Time Exceeded" message when
it discards an incomplete SEAL reassembly buffer due to a reassembly
timeout. The ETE sets Type=0, Code=3, and sets Data to the time in
seconds from when the initial SEAL segment arrived until the
reassembly time expired. The ETE finally writes the SEAL header of
segment 0 of the packet that generated the error into the first four
bytes of the message body. If segment 0 is unavailable, the ETE
instead writes the SEAL header of the first available segment. The
message is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=3 | Data=(Time, in Seconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL Header of First SEAL Segment in the Reassembly Buffer |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Segment Acknolwedged Message Format
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4.4.5.1.5. Checksum Incorrect (Code=4)
The ETE generates a "Reassembly Report - Checksum Incorrect" message
when it reassembles a SEAL packet with an invalid checksum. The ETE
sets Type=0, Code=4 and Data=0. The ETE finally writes the SEAL
header of the first segment of the packet into the first four bytes
of the message body. The message is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=0 | Code=4 | Data=0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL Header of First SEAL Segment |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: Reassembly Checksum Failure Message Format
4.4.5.2. Parameter Problem (Type=1)
The ETE generates a Parameter Problem message when it receives a SEAL
packet with an invalid value in one of the SEAL header fields. The
ETE sets Type=1 and Code=0, then sets data to the bit number of the
SEAL header field that triggered the error (e.g., when Data=8, the
parameter problem is specific to the NEXTHDR/SEG field). The ETE
finally writes the SEAL header of the packet that generated the error
into the first four bytes of the message body.
The Parameter Problem message is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=1 | Code=0 | Data=SEAL Header bit # |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL Header of Packet that Triggered the Parameter Problem |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: Parameter Problem Message Format
Other values for Code will be recorded in the IANA registry for SEAL.
4.4.5.3. Information Request (Type=2)
The ETE generates an Information Request message when it receives a
SEAL packet with I=1 in the SEAL header. The ETE sets Type=2 and
sets Code/Data to values that are specific to the associated
tunneling protocol (for example, the LISP protocol can use the
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Information Request message to request mapping updates). The message
body further contains opaque data that is interpreted according to
the Code/Data values.
When Code=0, both Data and the Opaque Data are discarded upon receipt
by the ETE. The information request message is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=2 | Code=0 | Data=X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Data |
+-+-+-+ ...
Figure 10: Information Request Message Format
Other values for Code will be recoded in the IANA registry for SEAL.
After the ETE sends an Information Request message, it must retry
until it receives a corresponding Information Reply (see Section
4.4.5.4). Note that while in this loop the ETE may receive further
SEAL packets with I=1. In that case, the ETE should begin sending
Information Requests specific to the SEAL_ID of the new packet and
should not dwell on the old SEAL_ID.
4.4.5.4. Information Reply (Type=3)
When the ETE sends an Information Request message to the ITE, the ITE
responds by sending an Information Reply message back to the ETE with
the IP source and destination address set to the destination and
source address, the UDP destination port set to the UDP source port,
and the SEAL-ID set to the same value that was present in the
Information Request message.
The ITE sets Type=3 and sets Code/Data to values that are specific to
the associated tunneling protocol (for example, the LISP protocol can
use the Information Reply message to encode mapping updates). The
message body further contains opaque data that is interpreted
according to the Code/Data values.
When Code=0, both Data and the Opaque Data are discarded upon receipt
by the ETE. The information reply message is formatted as follows:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type=3 | Code=0 | Data=X |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Opaque Data |
+-+-+-+ ...
Figure 11: Information Reply Message Format
Other values for Code will be recoded in the IANA registry for SEAL.
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 SEAL-MAX (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 IP 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
SEAL-MAX as follows:
5.1. Model of Operation
SEAL-LITE follows the same model of operation as for SEAL-MAX 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|I| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: 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.
I (1)
the "Information Request Solicit" bit. Set to 1 if the ITE wishes
the ETE to initiate an Information Request.
Identification (28)
a 28-bit identification field.
5.3. ITE Specification
5.3.1. Tunnel Interface MTU
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.1.
5.3.2. Admitting Packets into the Tunnel Interface
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.2.
5.3.3. Segmentation
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.3,
except that the inner fragmentation algorithm is adjusted to avoid
all fragmentation and/or segmentation either within or beyond the
tunnel as follows:
o if the inner packet is an IPv6 packet or an IPv4 packet with DF=1,
and the packet is larger than (MIN(S_MRU, S_MSS) - HLEN), the ITE
drops the packet and sends an ICMP PTB message to the original
source with an MTU value of (MIN(S_MRU, S_MSS) - HLEN) the same as
described in Section 4.3.2; else,
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o if the inner packet is an IPv4 packet with DF=0, and the packet is
larger than (S_CSS - HLEN), the ITE uses inner IPv4 fragmentation
to break the packet into fragments no larger than (S_CSS - HLEN);
else,
o the ITE processes the packet without inner fragmentation.
5.3.4. Encapsulation
SEAL-LITE observes the SEAL-MAX 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'. In particular, SEAL-LITE uses the A
and I bits the same as specified for SEAL-MAX.
5.3.5. Probing Strategy
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.5.
5.3.6. Packet Identification
SEAL-LITE observes the SEAL-MAX soft state specifications found in
Section 4.3.6, but the SEAL_ID is treated as a 28-bit value that is
written into the Identification field in the SEAL header.
As for the SEAL-MAX specification in Section 4.3.6, SEAL-LITE
increments the Identification field (modulo 28) for each consecutive
SEAL packet.
5.3.7. Sending SEAL Protocol Packets
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.7.
5.3.8. Processing Raw ICMP Messages
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.8.
5.3.9. Processing SEAL Control Messages
SEAL-LITE observes the SEAL-MAX specification found in Section 4.3.9.
5.4. ETE Specification
5.4.1. Reassembly Buffer Requirements
SEAL-LITE does not maintain a reassembly buffer for SEAL reassembly.
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5.4.2. IP-Layer Reassembly
SEAL-LITE uses SEAL-protocol IP first-fragments solely for the
purpose of generating SEAL Reassembly Reports as specified in Section
4.4.2, but thereafter discards all SEAL-protocol IP fragments.
5.4.3. SEAL-Layer Reassembly
SEAL-LITE does not observe the SEAL-MAX reassembly procedures in
Section 4.4.3; Instead, the SEAL-LITE ETE discards all SEAL packets
with F=0 following IP layer reassembly, and may also return
Reassembly Report - Packet Too Big messages when a packet that is too
large to receive is discarded.
As for SEAL-MAX, SEAL-LITE returns a Parameter Problem for SEAL
packets with unrecognized values in the SEAL header.
5.4.4. Decapsulation and Delivery to Upper Layers
SEAL-LITE observes the SEAL-MAX specification found in Section 4.4.4.
5.4.5. Sending SEAL Control Messages
SEAL-LITE observes the SEAL-MAX specification found in Section 4.4.5.
6. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
7. End System Requirements
SEAL provides robust mechanisms for returning PTB messages; however,
end systems that send unfragmentable IP packets larger than 1500
bytes are strongly encouraged to use Packetization Layer Path MTU
Discovery per [RFC4821].
8. Router Requirements
IPv4 routers within the subnetwork are strongly encouraged to
implement IPv4 fragmentation such that the first-fragment is the
largest and approximately the size of the underlying link MTU, i.e.,
they should avoid generating runt first-fragments.
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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 both
'SEAL_CPORT' and 'SEAL_DPORT' in the 'port-numbers' registry.
The IANA is instructed to establish a "SEAL Control Protocol"
registry to record SEAL control message Code and Type values. This
registry should be initialized to include the Code and Type values
defined in Section 4.4.5.
10. Security Considerations
Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-
overlapping.
An amplification/reflection attack is possible when an attacker sends
IP first-fragments with spoofed source addresses to an ETE, resulting
in a stream of Reassembly Report messages returned to a victim ITE.
The SEAL_ID in the encapsulated segment of the spoofed IP first-
fragment provides mitigation for the ITE to detect and discard
spurious Reassembly Reports.
The SEAL header is sent in-the-clear (outside of any IPsec/ESP
encapsulations) the same as for the outer */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.
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.
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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, IP fragmentation
provides a short-term interim mechanism for ensuring that packets
are delivered while SEAL adjusts its packet sizing parameters.
2. Classical path MTU 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
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.
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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, Pascal Thubert, Margaret Wasserman, Magnus
Westerlund, Robin Whittle, James Woodyatt, and members of the Boeing
Research & Technology NST DC&NT group.
Path MTU determination through the report of fragmentation was first
proposed by Charles Lynn on the TCP-IP mailing list in 1987.
Extending the IP identification field was first proposed by Steve
Deering on the MTUDWG mailing list in 1989.
14. References
14.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
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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]
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.
[RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum
options", RFC 1146, March 1990.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
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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.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, February 2000.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 2006.
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[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
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).
Implementations of this specification can use the Bitmap feature of
Reassembly Report control messages as a hint of segments for
retransmission, however this may not be ideally suitable for all SEAL
use cases since retransmission of lost segments may require
considerable state maintenance at the ITE and may also result in
considerable delay variance and packet reordering within the
subnetwork.
Alternate reliability mechanisms such as Forward Error Correction
(FEC) may also be used 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 may also be considered as a means for the ETE
to dynamically inform the ITE of changing FEC conditions.
Future studies should examine the use of ARQ and FEC mechanisms for
improved reliability in the face of loss due to congestion, signal
intermittence, etc.
Appendix B. Transport Mode
SEAL can also be used in "transport-mode", e.g., when the inner layer
includes upper-layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL for
the carriage of protocol data encapsulated as 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
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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)
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).
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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.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
Templin Expires December 19, 2009 [Page 39]