One document matched: draft-templin-intarea-seal-51.xml
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<rfc category="info" docName="draft-templin-intarea-seal-51.txt"
ipr="trust200902" obsoletes="rfc5320">
<front>
<title abbrev="SEAL">The Subnetwork Encapsulation and Adaptation Layer
(SEAL)</title>
<author fullname="Fred L. Templin" initials="F. L." role="editor"
surname="Templin">
<organization>Boeing Research & Technology</organization>
<address>
<postal>
<street>P.O. Box 3707</street>
<city>Seattle</city>
<region>WA</region>
<code>98124</code>
<country>USA</country>
</postal>
<email>fltemplin@acm.org</email>
</address>
</author>
<date day="22" month="October" year="2012"/>
<keyword>I-D</keyword>
<keyword>Internet-Draft</keyword>
<abstract>
<t>For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected IP network routing region
and bounded by encapsulating border nodes. These virtual topologies are
manifested by tunnels that may span multiple IP and/or sub-IP layer
forwarding hops, where they may incur packet duplication, packet
reordering, source address spoofing and traversal of links with diverse
Maximum Transmission Units (MTUs). This document specifies a Subnetwork
Encapsulation and Adaptation Layer (SEAL) that addresses these
issues.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (manifested by
tunnels of one form or another) over an actual network that supports the
Internet Protocol (IP) <xref target="RFC0791"/><xref target="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.</t>
<t>The use of IP encapsulation (also known as "tunneling") has long been
considered as the means for creating such virtual topologies. However,
the encapsulation headers often include insufficiently provisioned
per-packet identification values. IP encapsulation also allows an
attacker to produce encapsulated packets with spoofed source addresses
even if the source address in the encapsulating header cannot be
spoofed. A denial-of-service vector that is not possible in non-tunneled
subnetworks is therefore presented.</t>
<t>Additionally, the insertion of an outer IP header reduces the
effective path MTU visible to the inner network layer. When IPv6 is used
as the encapsulation protocol, original sources expect to be informed of
the MTU limitation through IPv6 Path MTU discovery (PMTUD) <xref
target="RFC1981"/>. 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 <xref target="FRAG"/><xref target="FOLK"/><xref
target="RFC4963"/>. Additionally, classical IPv4 PMTUD <xref
target="RFC1191"/> has known operational issues that are exacerbated by
in-the-network tunnels <xref target="RFC2923"/><xref
target="RFC4459"/>.</t>
<t>The following subsections present further details on the motivation
and approach for addressing these issues.</t>
<section title="Motivation">
<t>Before discussing the approach, it is necessary to first understand
the problems. In both the Internet and private-use networks today, IP
is ubiquitously deployed as the Layer 3 protocol. The primary
functions of IP are to provide for routing, addressing, and a
fragmentation and reassembly capability used to accommodate links with
diverse MTUs. While it is well known that the IP address space is
rapidly becoming depleted, there is a lesser-known but growing
consensus that other IP protocol limitations have already or may soon
become problematic.</t>
<t>First, the Internet historically provided no means for discerning
whether the source addresses of IP packets are authentic. This
shortcoming is being addressed more and more through the deployment of
site border router ingress filters <xref target="RFC2827"/>, however
the use of encapsulation provides a vector for an attacker to
circumvent filtering for the encapsulated packet even if filtering is
correctly applied to the encapsulation header. Secondly, the IP header
does not include a well-behaved identification value unless the source
has included a fragment header for IPv6 or unless the source permits
fragmentation for IPv4. These limitations preclude an efficient means
for routers to detect duplicate packets and packets that have been
re-ordered within the subnetwork. Additionally, recent studies have
shown that the arrival of fragments at high data rates can cause
denial-of-service (DoS) attacks on performance-sensitive networking
gear <xref target="I-D.taylor-v6ops-fragdrop"/>, prompting some
administrators to configure their equipment to drop fragments
unconditionally.</t>
<t>For IPv4 encapsulation, when fragmentation is permitted the header
includes a 16-bit Identification field, meaning that at most 2^16
unique packets with the same (source, destination, protocol)-tuple can
be active in the network at the same time <xref
target="I-D.ietf-intarea-ipv4-id-update"/>. (When middleboxes such as
Network Address Translators (NATs) re-write the Identification field
to random values, the number of unique packets is even further
reduced.) Due to the escalating deployment of high-speed links,
however, these numbers have become too small by several orders of
magnitude for high data rate packet sources such as tunnel endpoints
<xref target="RFC4963"/>.</t>
<t>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 (see above). In particular, IPv4 fragmentation raises issues
ranging from minor annoyances (e.g., in-the-network router
fragmentation <xref target="RFC1981"/>) to the potential for major
integrity issues (e.g., mis-association of the fragments of multiple
IP packets during reassembly <xref target="RFC4963"/>).</t>
<t>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 which resulted
in the publication of <xref target="RFC1191"/>. In this negative
feedback-based 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” <xref target="RFC2923"/>.</t>
<t>In particular, site border routers in the Internet have been known
to discard ICMP error messages coming from the outside world. This is
due in large part to the fact that malicious spoofing of error
messages in the Internet is trivial since there is no way to
authenticate the source of the messages <xref target="RFC5927"/>.
Furthermore, when a source node that requires ICMP error message
feedback when a packet is dropped due to an MTU restriction does not
receive the messages, a path MTU-related black hole occurs. This means
that the source will continue to send packets that are too large and
never receive an indication from the network that they are being
discarded. This behavior has been confirmed through documented studies
showing clear evidence of PMTUD failures for both IPv4 and IPv6 in the
Internet today <xref target="TBIT"/><xref target="WAND"/><xref
target="SIGCOMM"/>.</t>
<t>The issues with both IP fragmentation and this
“classical” PMTUD method are exacerbated further when IP
tunneling is used <xref target="RFC4459"/>. For example, an ingress
tunnel endpoint (ITE) may be required to forward encapsulated packets
into the subnetwork on behalf of hundreds, thousands, or even more
original sources. If the ITE allows IP fragmentation on the
encapsulated packets, persistent fragmentation could lead to
undetected data corruption due to Identification field wrapping and/or
reassembly congestion at the ETE. If the ITE instead uses classical IP
PMTUD it must rely on ICMP error messages coming from the subnetwork
that may be suspect, subject to loss due to filtering middleboxes, or
insufficiently provisioned for translation into error messages to be
returned to the original sources.</t>
<t>Although recent works have led to the development of a positive
feedback-based end-to-end MTU determination scheme <xref
target="RFC4821"/>, they do not excuse tunnels from accounting for the
encapsulation overhead they add to packets. Moreover, in current
practice existing tunneling protocols mask the MTU issues by selecting
a "lowest common denominator" MTU that may be much smaller than
necessary for most paths and difficult to change at a later date.
Therefore, a new approach to accommodate tunnels over links with
diverse MTUs is necessary.</t>
</section>
<section title="Approach">
<t>For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected network routing region
and bounded by encapsulating border nodes. Example connected network
routing regions include Mobile Ad hoc Networks (MANETs), enterprise
networks and the global public Internet itself. Subnetwork border
nodes forward unicast and multicast packets over the virtual topology
across multiple IP and/or sub-IP layer forwarding hops that may
introduce packet duplication and/or traverse links with diverse
Maximum Transmission Units (MTUs).</t>
<t>This document introduces a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for tunneling inner network layer protocol packets over
IP subnetworks that connect Ingress and Egress Tunnel Endpoints
(ITEs/ETEs) of border nodes. It provides a modular specification
designed to be tailored to specific associated tunneling protocols. (A
transport-mode of operation is also possible, but out of scope for
this document.)</t>
<t>SEAL provides a mid-layer encapsulation that accommodates links
with diverse MTUs, and allows routers in the subnetwork to perform
efficient duplicate packet and packet reordering detection. The
encapsulation further ensures data origin authentication, packet
header integrity and anti-replay in environments in which these
functions are necessary.</t>
<t>SEAL treats tunnels that traverse the subnetwork as ordinary links
that must support network layer services. Moreover, SEAL provides
dynamic mechanisms to ensure a maximal path MTU over the tunnel. This
is in contrast to static approaches which avoid MTU issues by
selecting a lowest common denominator MTU value that may be overly
conservative for the vast majority of tunnel paths and difficult to
change even when larger MTUs become available.</t>
<t>The following sections provide the SEAL normative specifications,
while the appendices present non-normative additional
considerations.</t>
</section>
<section title="Differences with RFC5320">
<t>This specification of SEAL is descended from an experimental
independent RFC publication of the same name <xref target="RFC5320"/>.
However, this specification introduces a number of important
differences from the earlier publication.</t>
<t>First, this specification includes a protocol version field in the
SEAL header whereas <xref target="RFC5320"/> does not, and therefore
cannot be updated by future revisions. This specification therefore
obsoletes (i.e., and does not update) <xref target="RFC5320"/>.</t>
<t>Secondly, <xref target="RFC5320"/> forms a 32-bit Identification
value by concatenating the 16-bit IPv4 Identification field with a
16-bit Identification "extension" field in the SEAL header. This means
that <xref target="RFC5320"/> can only operate over IPv4 networks
(since IPv6 headers do not include a 16-bit version number) and that
the SEAL Identification value can be corrupted if the Identification
in the outer IPv4 header is rewritten. In contrast, this specification
includes a 32-bit Identification value that is independent of any
identification fields found in the inner or outer IP headers, and is
therefore compatible with any inner and outer IP protocol version
combinations.</t>
<t>Additionally, the SEAL segmentation and reassembly procedures
defined in <xref target="RFC5320"/> differ significantly from those
found in this specification. In particular, this specification defines
a 6-bit Offset field that allows for smaller segment sizes when SEAL
segmentation is necessary (e.g., in order to observe the IPv4 minimum
MTU of 68 bytes). In contrast, <xref target="RFC5320"/> includes a
3-bit Segment field and performs reassembly through concatenation of
consecutive segments.</t>
<t>The SEAL header in this specification also includes an optional
Integrity Check Vector (ICV) that can be used to digitally sign the
SEAL header and the leading portion of the encapsulated inner packet.
This allows for a lightweight integrity check and a loose data origin
authentication capability. The header further includes new control
bits as well as a link identification and encapsulation level field
for additional control capabilities.</t>
<t>Finally, this version of SEAL includes a new messaging protocol
known as the SEAL Control Message Protocol (SCMP), whereas <xref
target="RFC5320"/> performs signalling through the use of
SEAL-encapsulated ICMP messages. The use of SCMP allows SEAL-specific
departures from ICMP, as well as a control messaging capability that
extends to other specifications, including Virtual Enterprise
Traversal (VET) <xref target="I-D.templin-intarea-vet"/>.</t>
</section>
</section>
<section title="Terminology">
<t>The following terms are defined within the scope of this
document:</t>
<t><list style="hanging">
<t hangText="subnetwork"><vspace/>a virtual topology configured over
a connected network routing region and bounded by encapsulating
border nodes.</t>
<t hangText="IP"><vspace/>used to generically refer to either
Internet Protocol (IP) version, i.e., IPv4 or IPv6.</t>
<t hangText="Ingress Tunnel Endpoint (ITE)"><vspace/>a virtual
interface over which an encapsulating border node (host or router)
sends encapsulated packets into the subnetwork.</t>
<t hangText="Egress Tunnel Endpoint (ETE)"><vspace/>a virtual
interface over which an encapsulating border node (host or router)
receives encapsulated packets from the subnetwork.</t>
<t hangText="SEAL Path"><vspace/>a subnetwork path from an ITE to an
ETE beginning with an underlying link of the ITE as the first hop.
Note that, if the ITE's interface connection to the underlying link
assigns multiple IP addresses, each address represents a separate
SEAL path.</t>
<t hangText="inner packet"><vspace/>an unencapsulated network layer
protocol packet (e.g., IPv4 <xref target="RFC0791"/>, OSI/CLNP <xref
target="RFC0994"/>, IPv6 <xref target="RFC2460"/>, etc.) before any
outer encapsulations are added. Internet protocol numbers that
identify inner packets are found in the IANA Internet Protocol
registry <xref target="RFC3232"/>. SEAL protocol packets that incur
an additional layer of SEAL encapsulation are also considered inner
packets.</t>
<t hangText="outer IP packet"><vspace/>a packet resulting from
adding an outer IP header (and possibly other outer headers) to a
SEAL-encapsulated inner packet.</t>
<t hangText="packet-in-error"><vspace/>the leading portion of an
invoking data packet encapsulated in the body of an error control
message (e.g., an ICMPv4 <xref target="RFC0792"/> error message, an
ICMPv6 <xref target="RFC4443"/> error message, etc.).</t>
<t hangText="Packet Too Big (PTB) message"><vspace/>a control plane
message indicating an MTU restriction (e.g., an ICMPv6 "Packet Too
Big" message <xref target="RFC4443"/>, an ICMPv4 "Fragmentation
Needed" message <xref target="RFC0792"/>, etc.).</t>
<t hangText="Don't Fragment (DF) bit"><vspace/>a bit that indicates
whether the packet may be fragmented by the network. The DF bit is
explicitly included in the IPv4 header <xref target="RFC0791"/> and
may be set to '0' to allow fragmentation or '1' to disallow further
in-network fragmentation. The bit is absent from the IPv6 header
<xref target="RFC2460"/>, but implicitly set to '1' becauuse
fragmentation can occur only at IPv6 sources.</t>
</list></t>
<t>The following abbreviations correspond to terms used within this
document and/or elsewhere in common Internetworking nomenclature:</t>
<t><list>
<t>HLEN - the length of the SEAL header plus outer
headers<vspace/></t>
<t>ICV - Integrity Check Vector<vspace/></t>
<t>MTU - Maximum Transmission Unit<vspace/></t>
<t>SCMP - the SEAL Control Message Protocol<vspace/></t>
<t>SDU - SCMP Destination Unreachable message<vspace/></t>
<t>SPP - SCMP Parameter Problem message<vspace/></t>
<t>SPTB - SCMP Packet Too Big message<vspace/></t>
<t>SEAL - Subnetwork Encapsulation and Adaptation Layer<vspace/></t>
<t>TE - Tunnel Endpoint (i.e., either ingress or egress)
<vspace/></t>
<t>VET - Virtual Enterprise Traversal<vspace/></t>
</list></t>
</section>
<section title="Requirements">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref target="RFC2119"/>.
When used in lower case (e.g., must, must not, etc.), these words MUST
NOT be interpreted as described in <xref target="RFC2119"/>, but are
rather interpreted as they would be in common English.</t>
</section>
<section title="Applicability Statement">
<t>SEAL was originally motivated by the specific case of subnetwork
abstraction for Mobile Ad hoc Networks (MANETs), however the domain of
applicability also extends to subnetwork abstractions over enterprise
networks, ISP networks, SOHO networks, the global public Internet
itself, and any other connected network routing region.</t>
<t>SEAL provides a network sublayer for encapsulation of an inner
network layer packet within outer encapsulating headers. SEAL can also
be used as a sublayer within a transport layer protocol data payload,
where transport layer encapsulation is typically used for Network
Address Translator (NAT) traversal as well as operation over subnetworks
that give preferential treatment to certain "core" Internet protocols
(e.g., TCP, UDP, etc.). The SEAL header is processed in a similar manner
as for IPv6 extension headers, i.e., it is not part of the outer IP
header but rather allows for the creation of an arbitrarily extensible
chain of headers in the same way that IPv6 does.</t>
<t>To accommodate MTU diversity, the Ingress Tunnel Endpoint (ITE) may
need to perform any necessary segmentation which the Egress Tunnel
Endpoint (ETE) must reassemble. The ETE further acts as a passive
observer that informs the ITE of any packet size limitations. This
allows the ITE to return appropriate PMTUD feedback even if the network
path between the ITE and ETE filters ICMP messages.</t>
<t>SEAL further provides mechanisms to ensure data origin
authentication, packet header integrity, and anti-replay. The SEAL
framework is therefore similar to the IP Security (IPsec) Authentication
Header (AH) <xref target="RFC4301"/><xref target="RFC4302"/>, however it
provides only minimal hop-by-hop authenticating services while leaving
full data integrity, authentication and confidentiality services as an
end-to-end consideration. While SEAL performs data origin
authentication, the origin site must also perform the necessary ingress
filtering in order to provide full source address verification <xref
target="I-D.ietf-savi-framework"/>.</t>
<t>In many aspects, SEAL also very closely resembles the Generic Routing
Encapsulation (GRE) framework <xref target="RFC1701"/>. SEAL can
therefore be applied in the same use cases that are traditionally
addressed by GRE, but goes beyond GRE to also provide additional
capabilities (e.,g., path MTU accommodation, data origin authentication,
etc.) as described in this document.</t>
</section>
<section title="SEAL Specification">
<t>The following sections specify the operation of SEAL:</t>
<section title="SEAL Tunnel Model">
<t>SEAL is an encapsulation sublayer used within point-to-point and
non-broadcast, multiple access (NBMA) tunnels. Each SEAL path is
configured over one or more underlying interfaces attached to
subnetwork links. The SEAL tunnel connects an ITE to one or more ETE
"neighbors" via encapsulation across an underlying subnetwork, where
the tunnel neighbor relationship may be either unidirectional or
bidirectional.</t>
<t>A unidirectional tunnel neighbor relationship allows the near end
ITE to send data packets forward to the far end ETE, while the ETE
only returns control messages when necessary. A bidirectional tunnel
neighbor relationship is one over which both TEs can exchange both
data and control messages.</t>
<t>Implications of the SEAL unidirectional and bidirectional models
are the same as discussed in <xref
target="I-D.templin-intarea-vet"/>.</t>
</section>
<section title="SEAL Model of Operation">
<t>SEAL-enabled ITEs encapsulate each inner packet in a SEAL header
and any outer header encapsulations as shown in <xref
target="encaps1"/>:</t>
<t><figure anchor="encaps1" title="SEAL Encapsulation">
<artwork><![CDATA[ +--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +----------+---------+
]]></artwork>
</figure></t>
<t>The ITE inserts the SEAL header according to the specific tunneling
protocol. For simple encapsulation of an inner network layer packet
within an outer IP header, the ITE inserts the SEAL header following
the outer IP header and before the inner packet as: IP/SEAL/{inner
packet}.</t>
<t>For encapsulations over transports such as UDP, the ITE inserts the
SEAL header following the outer transport layer header and before the
inner packet, e.g., as IP/UDP/SEAL/{inner packet}. In that case, the
UDP header is seen as an "other outer header" as depicted in <xref
target="encaps1"/> and the outer IP and transport layer headers are
together seen as the outer encapsulation headers.</t>
<t>SEAL supports both "nested" tunneling and "re-encapsulating"
tunneling. Nested tunneling occurs when a first tunnel is encapsulated
within a second tunnel, which may then further be encapsulated within
additional tunnels. Nested tunneling can be useful, and stands in
contrast to "recursive" tunneling which is an anomalous condition
incurred due to misconfiguration or a routing loop. Considerations for
nested tunneling and avoiding recursive tunneling are discussed in
Section 4 of <xref target="RFC2473"/>.</t>
<t>Re-encapsulating tunneling occurs when a packet arrives at a first
ETE, which then acts as an ITE to re-encapsulate and forward the
packet to a second ETE connected to the same subnetwork. In that case
each ITE/ETE transition represents a segment of a bridged path between
the ITE nearest the source and the ETE nearest the destination.
Combinations of nested and re-encapsulating tunneling are also
naturally supported by SEAL.</t>
<t>The SEAL ITE considers each underlying interface as the ingress
attachment point to a SEAL path to the ETE. The ITE therefore may
experience different path MTUs on different SEAL paths.</t>
<t>Finally, the SEAL ITE ensures that the inner network layer protocol
will see a minimum MTU of 1500 bytes over each SEAL path regardless of
the outer network layer protocol version, i.e., even if a small amount
of fragmentation and reassembly are necessary. This is necessary to
avoid path MTU "black holes" for the minimum MTU configured by the
vast majority of links in the Internet.</t>
</section>
<section title="SEAL Header and Trailer Format">
<t>The SEAL header is formatted as follows:</t>
<t><figure anchor="minimal" title="SEAL Header Format ">
<artwork><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|A|R|L|I|V|X|M| Offset | NEXTHDR | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Vector (ICV) (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list style="hanging">
<t hangText="VER (2)"><vspace/>a 2-bit version field. This
document specifies Version 0 of the SEAL protocol, i.e., the VER
field encodes the value 0.</t>
<t hangText="C (1)"><vspace/>the "Control/Data" bit. Set to 1 by
the ITE in SEAL Control Message Protocol (SCMP) control messages,
and set to 0 in ordinary data packets.</t>
<t hangText="A (1)"><vspace/>the "Acknowledgement Requested" bit.
Set to 1 by the ITE in SEAL data packets for which it wishes to
receive an explicit acknowledgement from the ETE.</t>
<t hangText="R (1)"><vspace/>the "Redirects Permitted" bit
(reserved for use by VET:<xref target="I-D.templin-intarea-vet">
</xref>).</t>
<t hangText="L (1)"><vspace/>the "Rate Limit" bit.</t>
<t hangText="I (1)"><vspace/>the "Identification Included"
bit.</t>
<t hangText="V (1)"><vspace/>the "ICV included" bit.</t>
<t hangText="X (1)">a 1-bit reserved field.</t>
<t hangText="M (1)">the "More Segments" bit. Set to 1 in a
non-final segment and set to 0 in the final segment of the SEAL
packet.</t>
<t hangText="Offset (6)">a 6-bit Offset field. Set to 0 in the
first segment of a segmented SEAL packet. Set to an integral
number of 32 byte blocks in subsequent segments (e.g., an Offset
of 10 indicates a block that begins at the 320th byte in the
packet).</t>
<t hangText="NEXTHDR (8)">an 8-bit field that encodes the next
header Internet Protocol number the same as for the IPv4 protocol
and IPv6 next header fields.</t>
<t hangText="LINK_ID (5)"><vspace/>a 5-bit link identification
value, set to a unique value by the ITE for each SEAL path over
which it will send encapsulated packets to the ETE (up to 32 SEAL
paths per ETE are therefore supported). Note that, if the ITE's
interface connection to the underlying link assigns multiple IP
addresses, each address represents a separate SEAL path that must
be assigned a separate LINK_ID.</t>
<t hangText="LEVEL (3)"><vspace/>a 3-bit nesting level; use to
limit the number of tunnel nesting levels. Set to an integer value
up to 7 in the innermost SEAL encapsulation, and decremented by 1
for each successive additional SEAL encapsulation nesting level.
Up to 8 levels of nesting are therefore supported.</t>
<t hangText="Identification (32)"><vspace/>an optional 32-bit
per-packet identification field; present when I==1. Set to a
32-bit value (beginning with 0) that is monotonically-incremented
for each SEAL packet transmitted to this ETE.</t>
<t hangText="Integrity Check Vector (ICV) (32)"><vspace/>an
optional 32-bit header integrity check value; present when V==1.
Covers the leading 128 bytes of the packet beginning with the SEAL
header. The value 128 is chosen so that at least the SEAL header
as well as the inner packet network and transport layer headers
are covered by the integrity check.</t>
</list></t>
</section>
<section title="ITE Specification">
<section title="Tunnel Interface MTU">
<t>The tunnel interface must present a constant MTU value to the
inner network layer as the size for admission of inner packets into
the interface. Since NBMA tunnel virtual interfaces may support a
large set of SEAL paths that accept widely varying maximum packet
sizes, however, a number of factors should be taken into
consideration when selecting a tunnel interface MTU.</t>
<t>Due to the ubiquitous deployment of standard Ethernet and similar
networking gear, the nominal Internet cell size has become 1500
bytes; this is the de facto size that end systems have come to
expect will either be delivered by the network without loss due to
an MTU restriction on the path or a suitable ICMP Packet Too Big
(PTB) message returned. When large packets sent by end systems incur
additional encapsulation at an ITE, however, they may be dropped
silently within the tunnel since the network may not always deliver
the necessary PTBs <xref target="RFC2923"/>. The ITE SHOULD
therefore set a tunnel interface MTU of at least 1500 bytes.</t>
<t>The inner network layer protocol consults the tunnel interface
MTU when admitting a packet into the interface. For non-SEAL inner
IPv4 packets with the IPv4 Don't Fragment (DF) cleared (i.e, DF==0),
if the packet is larger than the tunnel interface MTU the inner IPv4
layer uses IPv4 fragmentation to break the packet into fragments no
larger than the tunnel interface MTU. The ITE then admits each
fragment into the interface as an independent packet.</t>
<t>For all other inner packets, the inner network layer admits the
packet if it is no larger than the tunnel interface MTU; otherwise,
it drops the packet and sends a PTB error message to the source with
the MTU value set to the tunnel interface MTU. The message contains
as much of the invoking packet as possible without the entire
message exceeding the network layer MINMTU size.</t>
<t>The ITE can alternatively set an indefinite MTU on the tunnel
interface such that all inner packets are admitted into the
interface regardless of their size. For ITEs that host applications
that use the tunnel interface directly, this option must be
carefully coordinated with protocol stack upper layers since some
upper layer protocols (e.g., TCP) derive their packet sizing
parameters from the MTU of the outgoing interface and as such may
select too large an initial size. This is not a problem for upper
layers that use conservative initial maximum segment size estimates
and/or when the tunnel interface can reduce the upper layer's
maximum segment size, e.g., by reducing the size advertised in the
MSS option of outgoing TCP messages (sometimes known as "MSS
clamping").</t>
<t>In light of the above considerations, the ITE should configure an
indefinite MTU on tunnel *router* interfaces so that SEAL performs
all subnetwork adaptation from within the interface as specified in
Section 5.4.3. The ITE can instead set a smaller MTU on tunnel
*host* interfaces (e.g., the smallest MTU among all of the
underlying links minus the size of the encapsulation headers) but
SHOULD NOT set an MTU smaller than 1500 bytes.</t>
</section>
<section title="Tunnel Neighbor Soft State">
<t>The tunnel virtual interface maintains a number of soft state
variables for each ETE and for each SEAL path.</t>
<t>When per-packet identification is required, the ITE maintains a
per ETE window of Identification values for the packets it has
recently sent to this ETE. The ITE then sets a variable "USE_ID" to
TRUE, and includes an Identification in each packet it sends to this
ETE; otherwise, it sets USE_ID to FALSE.</t>
<t>When data origin authentication and integrity checking is
required, the ITE also maintains a per ETE integrity check vector
(ICV) calculation algorithm and a symmetric secret key to calculate
the ICV in each packet it will send to this ETE. The ITE then sets a
variable "USE_ICV" to TRUE, and includes an ICV in each packet it
sends to this ETE; otherwise, it sets USE_ICV to FALSE.</t>
<t>The ITE further maintains a variable "RATE_LIMIT" initialized to
FALSE. If the SEAL path subsequently exhibits unavoidable IP
fragmentation the ETE sets RATE_LIMIT to TRUE.</t>
<t>For each SEAL path, the ITE must also account for encapsulation
header lengths. The ITE therefore maintains the per SEAL path
constant values "SHLEN" set to the length of the SEAL header,
"THLEN" set to the length of the outer encapsulating transport layer
headers (or 0 if outer transport layer encapsulation is not used),
"IHLEN" set to the length of the outer IP layer header, and "HLEN"
set to (SHLEN+THLEN+IHLEN). (The ITE must include the length of the
uncompressed headers even if header compression is enabled when
calculating these lengths.) In addition, the ITE maintains a per
SEAL path variable "MAXMTU" initialized to the maximum of 1500 bytes
and the MTU of the underlying link minus HLEN. (Thereafter, the ITE
must not reduce MAXMTU to a value smaller than 1500 bytes.)</t>
<t>The ITE further sets a variable 'MINMTU' to the minimum MTU for
the SEAL path over which encapsulated packets will travel. For IPv6
paths the ITE sets MINMTU=1280 (see: <xref target="RFC2460"/>) and
for IPv4 paths the ITE sets MINMTU=576 even though the true MINMTU
for IPv4 is only 68 bytes (see: <xref target="RFC0791"/>). The ITE
can also set MINMTU to a larger value (but no larger than 1500
bytes) if there is reason to believe that the path MTU is larger. If
this value proves too large, the ITE will receive PTB message
feedback either from the ETE or from a router on the path and will
be able to reduce its MINMTU to a smaller value.</t>
<t>The ITE may instead maintain the packet sizing variables and
constants as per ETE (rather than per SEAL path) values. In that
case, the values reflect the lowest-common-denominator size across
all of the SEAL paths associated with this ETE.</t>
</section>
<section title="SEAL Layer Pre-Processing">
<t>The SEAL layer is logically positioned between the inner and
outer network protocol layers, where the inner layer is seen as the
(true) network layer and the outer layer is seen as the (virtual)
data link layer. Each packet to be processed by the SEAL layer is
either admitted into the tunnel interface by the inner network layer
protocol as described in Section 5.4.1 or is undergoing
re-encapsulation from within the tunnel interface. The SEAL layer
sees the former class of packets as inner packets that include inner
network and transport layer headers, and sees the latter class of
packets as transitional SEAL packets that include the outer and SEAL
layer headers that were inserted by the previous hop SEAL ITE. For
these transitional packets, the SEAL layer re-encapsulates the
packet with new outer and SEAL layer headers when it forwards the
packet to the next hop SEAL ITE.</t>
<t>We now discuss the SEAL layer pre-processing actions for these
two classes of packets.</t>
<section title="Inner Packet Pre-Processing">
<t>For each inner packet admitted into the tunnel interface, if
the packet is itself a SEAL packet (i.e., one with the port number
for SEAL in the transport layer header or one with the protocol
number for SEAL in the IP layer header) and the LEVEL field of the
SEAL header contains the value 0, the ITE silently discards the
packet.</t>
<t>Otherwise, for non-SEAL IPv4 inner packets with DF==0 in the IP
header and IPv6 inner packets with a fragment header and with
(MF=0; Offset=0), if the packet is larger than (MINMTU-HLEN) the
ITE uses IP fragmentation to fragment the packet into N roughly
equal-length pieces, where N is minimized and each fragment is
significantly smaller than (MINMTU-HLEN) to allow for additional
encapsulations in the path. The ITE then submits each fragment for
SEAL encapsulation as specified in Section 5.4.4.</t>
<t>For all other inner packets:</t>
<t><list style="symbols">
<t>if the packet is no larger than larger than (MINMTU-HLEN),
the ITE submits it for SEAL encapsulation as specified in
Section 5.4.4</t>
<t>if the packet is larger than (MINMTU-HLEN) but no larger
than 1500 bytes, the ITE submits the packet for SEAL
segmentation as specifed in Section 5.4.4. For IPv6 packets,
the ITE also sends an ICMPv6 PTB message with the MTU field
set to (MIN(MINMTU,1280)-HLEN) and the Code field set to 1
toward the original source subject to rate limiting (see:
<xref target="I-D.generic-6man-tunfrag"/>). This PTB message
is advisory in nature and does not represent a packet loss;
instead, it instructs the original IPv6 host to begin
fragmenting future packets that it will send to this
destination.</t>
<t>if the packet is larger than 1500 bytes but no larger than
MAXMTU for the corresponding SEAL path the ITE submits it for
SEAL encapsulation as specified in Section 5.4.4.</t>
<t>if the packet is larger than MAXMTU, the ITE drops the
packet and sends an ordinary PTB message appropriate to the
inner protocol version with the MTU field set to MAXMTU. (For
IPv4 SEAL packets with DF==0, the ITE should set DF=1 and
re-calculate the IPv4 header checksum before generating the
PTB message in order to avoid bogon filters.) After sending
the PTB message, the ITE discards the inner packet.</t>
</list></t>
</section>
<section title="Transitional SEAL Packet Pre-Processing">
<t>For each transitional packet that is processed by the SEAL
layer from within the tunnel interface, the ITE sets aside the
SEAL encapsulation headers that were added by the previous hop and
examines the inner packet as follows:</t>
<t><list style="symbols">
<t>if the packet is no larger than larger than (MINMTU-HLEN),
the ITE submits it for SEAL encapsulation as specified in
Section 5.4.4.</t>
<t>if the packet is larger than (MINMTU-HLEN) but no larger
than 1500 bytes, the ITE submits the packet for SEAL
segmentation as specifed in Section 5.4.4. The ITE also sends
an SCMP PTB (SPTB) message toward the previous hop SEAL ITE
with the MTU field set to (MINMTU-HLEN) subject to rate
limiting (see: Section 5.6.1.1). This SPTB message is advisory
in nature and does not represent a packet loss; instead, it
instructs the previous hop ITE to begin sending smaller SEAL
segments.</t>
<t>if the packet is larger than 1500 bytes but no larger than
MAXMTU for the corresponding SEAL path the ITE submits it for
SEAL encapsulation as specified in Section 5.4.4.</t>
<t>if the packet is larger than MAXMTU, the ITE drops the
packet and sends an SPTB message to the previous hop subject
to rate limiting (see: Section 5.6.1.1) with the MTU field set
to MAXMTU. After sending the SPTB message, the ITE discards
the packet.</t>
</list></t>
</section>
</section>
<section title="SEAL Encapsulation and Segmentation">
<t>For each inner packet/fragment submitted for SEAL encapsulation,
the ITE next encapsulates the packet in a SEAL header formatted as
specified in Section 5.3. The SEAL header includes an Identification
field when USE_ID is TRUE, followed by an ICV field when USE_ICV is
TRUE.</t>
<t>The ITE next sets C=0 in the SEAL header. The ITE also sets A=1
if necessary for ETE reachability determination (see: Section 5.4.6)
or for stateful MTU determination (see Section 5.4.9). Otherwise,
the ITE sets A=0. Next, when RATE_LIMIT is TRUE the ITE sets L=1;
otherwise, it sets L=0. The ITE also sets X=0.</t>
<t>The ITE then sets LINK_ID to the value assigned to the underlying
SEAL path, and sets NEXTHDR to the protocol number corresponding to
the address family of the encapsulated inner packet. For example,
the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets
<xref target="RFC2003"/>, '41' for encapsulated IPv6 packets <xref
target="RFC2473"/><xref target="RFC4213"/>, '80' for encapsulated
OSI/CLNP packets <xref target="RFC1070"/>, etc.</t>
<t>Next, if the inner packet is not itself a SEAL packet the ITE
sets LEVEL to an integer value between 0 and 7 as a specification of
the number of additional layers of nested SEAL encapsulations
permitted. If the inner packet is a SEAL packet that is undergoing
nested encapsulation, the ITE instead sets LEVEL to the value that
appears in the inner packet's SEAL header minus 1. If the inner
packet is undergoing SEAL re-encapsulation, the ITE instead copies
the LEVEL value from the SEAL header of the packet to be
re-encapsulated.</t>
<t>Next, if the inner packet is no larger than (MINMTU-HLEN) or
larger than 1500, the ITE sets (M=0; Offset=0). Otherwise, the ITE
breaks the inner packet into a N roughly equal-length
non-overlapping segments (where N is minimized and each fragment is
significantly smaller than (MINMTU-HLEN) to allow for additional
encapsulations in the path) then appends a clone of the SEAL header
from the first segment onto the head of each additional segment. The
ITE then sets (M=1; Offset=0) in the first segment, sets (M=0/1;
Offset=i) in the second segment, sets (M=0/1; Offset=j) in the third
segment (if needed), etc., then finally sets (M=0; Offset=k) in the
final segment (where i, j, k, etc. are the number of 32 byte blocks
that preceded this segment).</t>
<t>When USE_ID is FALSE, the ITE next sets I=0. Otherwise, the ITE
sets I=1 and writes a monotonically-incrementing integer value for
this ETE in the Identification field beginning with 0 in the first
packet transmitted. (For SEAL packets that have been split into
multiple pieces, the ITE writes the same Identification value in
each piece.) The monotonically-incrementing requirement is to
satisfy ETEs that use this value for anti-replay purposes. The value
is incremented modulo 2^32, i.e., it wraps back to 0 when the
previous value was (2^32 - 1).</t>
<t>When USE_ICV is FALSE, the ITE next sets V=0. Otherwise, the ITE
sets V=1 and calculates the packet header ICV value using an
algorithm agreed on by the ITE and ETE. When data origin
authentication is required, the algorithm uses a symmetric secret
key so that the ETE can verify that the ICV was generated by the
ITE. Beginning with the SEAL header, the ITE calculates the ICV over
the leading 128 bytes of the packet (or up to the end of the packet
if there are fewer than 128 bytes) and places result in the ICV
field. (For SEAL packets that have been split into multiple pieces,
each piece calculates its own ICV value.)</t>
<t>The ITE then adds the outer encapsulating headers as specified in
Section 5.4.5.</t>
</section>
<section title="Outer Encapsulation">
<t>Following SEAL encapsulation, the ITE next encapsulates each
segment in the requisite outer transport (when necessary) and IP
layer headers. When a transport layer header such as UDP or TCP is
included, the ITE writes the port number for SEAL in the transport
destination service port field.</t>
<t>When UDP encapsulation is used, the ITE sets the UDP checksum
field to zero for IPv4 packets and also sets the UDP checksum field
to zero for IPv6 packets if the encapsulation will include an ICV.
Further considerations for setting the UDP checksum field for IPv6
packets are discussed in <xref target="I-D.ietf-6man-udpzero"/>.</t>
<t>The ITE then sets the outer IP layer headers the same as
specified for ordinary IP encapsulation (e.g., <xref
target="RFC1070"/><xref target="RFC2003"/><xref
target="RFC2473">,</xref><xref target="RFC4213">,</xref>, etc.)
except that for ordinary SEAL packets the ITE copies the "TTL/Hop
Limit", "Type of Service/Traffic Class" and "Congestion Experienced"
values in the inner network layer header into the corresponding
fields in the outer IP header. For transitional SEAL packets
undergoing re-encapsulation, the ITE instead copies the "TTL/Hop
Limit", "Type of Service/Traffic Class" and "Congestion Experienced"
values in the outer IP header of the received packet into the
corresponding fields in the outer IP header of the packet to be
forwarded (i.e., the values are transferred between outer headers
and *not* copied from the inner network layer header).</t>
<t>The ITE also sets the IP protocol number to the appropriate value
for the first protocol layer within the encapsulation (e.g., UDP,
TCP, SEAL, etc.). When IPv6 is used as the outer IP protocol, the
ITE then sets the flow label value in the outer IPv6 header the same
as described in <xref target="RFC6438"/>. When IPv4 is used as the
outer IP protocol, the ITE instead sets DF=0 in the IPv4 header to
allow the packet to be fragmented if it encounters a restricting
link (for IPv6 SEAL paths, the DF bit is implicitly set to 1).</t>
<t>The ITE finally sends each outer packet via the underlying link
corresponding to LINK_ID. For SEAL paths with RATE_LIMIT=TRUE, the
ITE sends the packet subject to rate limiting so that the
Identification value is not repeated within the IP maximum datagram
lifetime (i.e., 120 seconds for IPv4 <xref target="RFC1122"/> and 60
seconds for IPv6 <xref target="RFC2460"/>).</t>
</section>
<section title="Path Probing and ETE Reachability Verification">
<t>All SEAL data packets sent by the ITE are considered implicit
probes. SEAL data packets will elicit an SCMP message from the ETE
if it needs to acknowledge a probe and/or report an error condition.
SEAL data packets may also be dropped by either the ETE or a router
on the path, which may or may not result in an ICMP message being
returned to the ITE.</t>
<t>The ITE processes ICMP messages as specified in Section
5.4.7.</t>
<t>The ITE processes SCMP messages as specified in Section
5.6.2.</t>
</section>
<section title="Processing ICMP Messages">
<t>When the ITE sends SEAL packets, it may receive ICMP error
messages <xref target="RFC0792"/><xref target="RFC4443"/> from an
ordinary router within the subnetwork. Each ICMP message includes an
outer IP header, followed by an ICMP header, followed by a portion
of the SEAL data packet that generated the error (also known as the
"packet-in-error") beginning with the outer IP header.</t>
<t>The ITE should process ICMPv4 Protocol Unreachable messages and
ICMPv6 Parameter Problem messages with Code "Unrecognized Next
Header type encountered" as a hint that the IP destination address
does not implement SEAL. The ITE can optionally ignore ICMP messages
that do not include sufficient information in the packet-in-error,
or process them as a hint that the SEAL path may be failing.</t>
<t>For other ICMP messages, the ITE should use any outer header
information available as a first-pass authentication filter (e.g.,
to determine if the source of the message is within the same
administrative domain as the ITE) and discards the message if first
pass filtering fails.</t>
<t>Next, the ITE examines the packet-in-error beginning with the
SEAL header. If the value in the Identification field (if present)
is not within the window of packets the ITE has recently sent to
this ETE, or if the value in the SEAL header ICV field (if present)
is incorrect, the ITE discards the message.</t>
<t>Next, if the received ICMP message is a PTB the ITE sets the
temporary variable "PMTU" for this SEAL path to the MTU value in the
PTB message. If PMTU==0, the ITE consults a plateau table (e.g., as
described in <xref target="RFC1191"/>) to determine PMTU based on
the length field in the outer IP header of the packet-in-error. For
example, if the ITE receives a PTB message with MTU==0 and length
4KB, it can set PMTU=2KB. If the ITE subsequently receives a PTB
message with MTU==0 and length 2KB, it can set PMTU=1792, etc. to a
minimum value of PMTU=(1500+HLEN). If the ITE is performing stateful
MTU determination for this SEAL path (see Section 5.4.9), the ITE
next sets MAXMTU=MAX((PMTU-HLEN), 1500).</t>
<t>If the ICMP message was not discarded, the ITE then transcribes
it into a message to return to the previous hop. If the inner packet
was a SEAL data packet, the ITE transcribes the ICMP message into an
SCMP message. Otherwise, the ITE transcribes the ICMP message into a
message appropriate for the inner protocol version.</t>
<t>To transcribe the message, the ITE extracts the inner packet from
within the ICMP message packet-in-error field and uses it to
generate a new message corresponding to the type of the received
ICMP message. For SCMP messages, the ITE generates the message the
same as described for ETE generation of SCMP messages in Section
5.6.1. For (S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU
field.</t>
<t>The ITE finally forwards the transcribed message to the previous
hop toward the inner source address.</t>
</section>
<section title="IPv4 Middlebox Reassembly Testing">
<t>The ITE can perform a qualification exchange to ensure that the
subnetwork correctly delivers fragments to the ETE. This procedure
can be used, e.g., to determine whether there are middleboxes on the
path that violate the <xref target="RFC1812"/>, Section 5.2.6
requirement that: "A router MUST NOT reassemble any datagram before
forwarding it".</t>
<t>The ITE should use knowledge of its topological arrangement as an
aid in determining when middlebox reassembly testing is necessary.
For example, if the ITE is aware that the ETE is located somewhere
in the public Internet, middlebox reassembly testing should not be
necessary. If the ITE is aware that the ETE is located behind a NAT
or a firewall, however, then middlebox reassembly testing is
recommended.</t>
<t>The ITE can perform a middlebox reassembly test by selecting a
data packet to be used as a probe. While performing the test with
real data packets, the ITE should select only inner packets that are
no larger than (1500-HLEN) bytes for testing purposes. The ITE can
also construct a dummy probe packet instead of using ordinary SEAL
data packets.</t>
<t>To generate a dummy probe packet, the ITE creates a packet buffer
beginning with the same outer headers, SEAL header and inner network
layer header that would appear in an ordinary data packet, then pads
the packet with random data to a length that is at least 128 bytes
but no longer than (1500-HLEN) bytes. The ITE then writes the value
'0' in the inner network layer TTL (for IPv4) or Hop Limit (for
IPv6) field.</t>
<t>The ITE then sets C=0 in the SEAL header of the probe packet and
sets the NEXTHDR field to the inner network layer protocol type.
(The ITE may also set A=1 if it requires a positive acknowledgement;
otherwise, it sets A=0.) Next, the ITE sets LINK_ID and LEVEL to the
appropriate values for this SEAL path, sets Identification and I=1
(when USE_ID is TRUE), then finally calculates the ICV and sets
V=1(when USE_ICV is TRUE).</t>
<t>The ITE then encapsulates the probe packet in the appropriate
outer headers, splits it into two outer IPv4 fragments, then sends
both fragments over the same SEAL path.</t>
<t>The ITE should send a series of probe packets (e.g., 3-5 probes
with 1sec intervals between tests) instead of a single isolated
probe in case of packet loss. If the ETE returns an SCMP PTB message
with MTU != 0, then the SEAL path correctly supports fragmentation;
otherwise, the ITE enables stateful MTU determination for this SEAL
path as specified in Section 5.4.9.</t>
<t>(Examples of middleboxes that may perform reassembly include
stateful NATs and firewalls. Such devices could still allow for
stateless MTU determination if they gather the fragments of a
fragmented IPv4 SEAL data packet for packet analysis purposes but
then forward the fragments on to the final destination rather than
forwarding the reassembled packet.)</t>
</section>
<section title="Stateful MTU Determination">
<t>SEAL supports a stateless MTU determination capability, however
the ITE may in some instances wish to impose a stateful MTU limit on
a particular SEAL path. For example, when the ETE is situated behind
a middlebox that performs IPv4 reassembly (see: Section 5.4.8) it is
imperative that fragmentation be avoided. In other instances (e.g.,
when the SEAL path includes performance-constrained links), the ITE
may deem it necessary to cache a conservative static MTU in order to
avoid sending large packets that would only be dropped due to an MTU
restriction somewhere on the path.</t>
<t>To determine a static MTU value, the ITE sends a series of dummy
probe packets of various sizes to the ETE with A=1 in the SEAL
header and DF=1 in the outer IP header. The ITE then caches the size
'S' of the largest packet for which it receives a probe reply from
the ETE by setting MAXMTU=MAX((S-HLEN), 1500) for this SEAL
path.</t>
<t>For example, the ITE could send probe packets of 4KB, followed by
2KB, followed by 1792 bytes, etc. While probing, the ITE processes
any ICMP PTB message it receives as a potential indication of probe
failure then discards the message.</t>
</section>
<section title="Detecting Path MTU Changes">
<t>When stateful MTU determination is used, the ITE SHOULD
periodically reset MAXMTU and/or re-probe the path to determine
whether MAXMTU has increased. If the path still has a too-small MTU,
the ITE will receive a PTB message that reports a smaller size.</t>
<t>For IPv4 SEAL paths, when the path correctly implements
fragmentation and RATE_LIMIT is TRUE, the ITE can periodically reset
RATE_LIMIT=FALSE to determine whether the path still requires rate
limiting. If the ITE receives an SPTB message it should again set
RATE_LIMIT=TRUE.</t>
</section>
</section>
<section title="ETE Specification">
<section title="Minimum Reassembly Buffer Requirements">
<t>For IPv6, the ETE must configure a minimum reassembly buffer size
of 1500 bytes for the reassembly of outer IPv6 packets (see: <xref
target="RFC2460"/>. For IPv4, the ETE must also configure a minimum
reassembly buffer size of 1500 bytes for the reassembly of outer
IPv4 packets, i.e., even though the true minimum reassembly size for
IPv4 is only 576 bytes <xref target="RFC1122"/>.</t>
<t>In addition to this outer reassembly buffer requirement, the ETE
must further configure a minimum SEAL reassembly buffer size of
(1500 + HLEN) bytes for the reassembly of segmented SEAL packets
(see: Section 5.5.4).</t>
</section>
<section title="Tunnel Neighbor Soft State">
<t>When data origin authentication and integrity checking is
required, the ETE maintains a per-ITE ICV calculation algorithm and
a symmetric secret key to verify the ICV. When per-packet
identification is required, the ETE also maintains a window of
Identification values for the packets it has recently received from
this ITE.</t>
<t>When the tunnel neighbor relationship is bidirectional, the ETE
further maintains a per SEAL path mapping of outer IP and transport
layer addresses to the LINK_ID that appears in packets received from
the ITE.</t>
</section>
<section title="IP-Layer Reassembly">
<t>The ETE reassembles fragmented IP packets that are explcitly
addressed to itself. For IP fragments that are received via a SEAL
tunnel, the ETE SHOULD maintain conservative reassembly cache high-
and low-water marks. When the size of the reassembly cache exceeds
this high-water mark, the ETE SHOULD actively discard stale
incomplete reassemblies (e.g., using an Active Queue Management
(AQM) strategy) until the size falls below the low-water mark. The
ETE SHOULD also actively discard any pending reassemblies that
clearly have no opportunity for completion, e.g., when a
considerable number of new fragments have arrived before a fragment
that completes a pending reassembly arrives.</t>
<t>The ETE processes non-SEAL IP packets as specified in the
normative references, i.e., it performs any necessary IP reassembly
then discards the packet if it is larger than the reassembly buffer
size or delivers the (fully-reassembled) packet to the appropriate
upper layer protocol module.</t>
<t>For SEAL packets, the ITE performs any necessary IP reassembly
then submits the packet for SEAL decapsulation as specified in
Section 5.5.4. (Note that if the packet is larger than the
reassembly buffer size, the ITE still returns the leading portion of
the (partially) reassembled packet.)</t>
</section>
<section title="Decapsulation and Re-Encapsulation">
<t>For each SEAL packet accepted for decapsulation, when I==1 the
ETE first examines the Identification field. If the Identification
is not within the window of acceptable values for this ITE, the ETE
silently discards the packet.</t>
<t>Next, if V==1 the ETE verifies the ICV value (with the ICV field
itself reset to 0) and silently discards the packet if the value is
incorrect.</t>
<t>Next, if the packet arrived as multiple IP fragments and L ==0,
the ETE sends an SPTB message back to the ITE with MTU set to the
size of the largest fragment received minus HLEN (see: Section
5.6.1.1).</t>
<t>Next, if the packet arrived as multiple IP fragments and the
inner packet is larger than 1500 bytes, the ETE silently discards
the packet; otherwise, it continues to process the packet.</t>
<t>Next, if there is an incorrect value in a SEAL header field
(e.g., an incorrect "VER" field value), the ETE discards the packet.
If the SEAL header has C==0, the ETE also returns an SCMP "Parameter
Problem" (SPP) message (see Section 5.6.1.2).</t>
<t>Next, if the SEAL header has C==1, the ETE processes the packet
as an SCMP packet as specified in Section 5.6.2. Otherwise, the ETE
continues to process the packet as a SEAL data packet.</t>
<t>Next, if the SEAL header has (M==1 || Offset!==0) the ETE checks
to see if the other segments of this already-segmented SEAL packet
have arrived, i.e., by looking for additional segments that have the
same outer IP source address, destination address, source transport
port number (if present) and SEAL Identification value. If the other
segments have already arrived, the ETE discards the SEAL header and
other outer headers from the non-initial segments and appends them
onto the end of the first segment. Otherwise, the ETE caches the
segment for at most 60 seconds while awaiting the arrival of its
partners. During this process, the ETE discards any segments that
are overlapping with respect to segments that have already been
received. The ETE further SHOULD manage the SEAL reassembly cache
the same as described for the IP-Layer Reassembly cache in Section
5.5.3, i.e., it SHOULD perform an early discard for any pending
reassemblies that have low probability of completion.</t>
<t>Next, if the SEAL header in the (reassembled) packet has A==1,
the ETE sends an SPTB message back to the ITE with MTU=0 (see:
Section 5.6.1.1).</t>
<t>Finally, the ETE discards the outer headers and processes the
inner packet according to the header type indicated in the SEAL
NEXTHDR field. If the inner (TTL / Hop Limit) field encodes the
value 0, the ETE silently discards the packet. Otherwise, if the
next hop toward the inner destination address is via a different
interface than the SEAL packet arrived on, the ETE discards the SEAL
header and delivers the inner packet either to the local host or to
the next hop interface if the packet is not destined to the local
host.</t>
<t>If the next hop is on the same interface the SEAL packet arrived
on, however, the ETE submits the packet for SEAL re-encapsulation
beginning with the specification in Section 5.4.3 above and without
decrementing the value in the inner (TTL / Hop Limit) field. In this
process, the packet remains within the tunnel (i.e., it does not
exit and then re-enter the tunnel); hence, the packet is not
discarded if the LEVEL field in the SEAL header contains the value
0.</t>
</section>
</section>
<section title="The SEAL Control Message Protocol (SCMP)">
<t>SEAL provides a companion SEAL Control Message Protocol (SCMP) that
uses the same message types and formats as for the Internet Control
Message Protocol for IPv6 (ICMPv6) <xref target="RFC4443"/>. As for
ICMPv6, each SCMP message includes a 32-bit header and a
variable-length body. The ITE encapsulates the SCMP message in a SEAL
header and outer headers as shown in <xref target="scmpencaps"/>:</t>
<t><figure anchor="scmpencaps" title="SCMP Message Encapsulation">
<artwork><![CDATA[ +--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| SCMP message header| --> | SCMP message header|
+--------------------+ +--------------------+
| | --> | |
~ SCMP message body ~ --> ~ SCMP message body ~
| | --> | |
+--------------------+ +--------------------+
SCMP Message SCMP Packet
before encapsulation after encapsulation]]></artwork>
</figure></t>
<t>The following sections specify the generation, processing and
relaying of SCMP messages.</t>
<section title="Generating SCMP Error Messages">
<t>ETEs generate SCMP error messages in response to receiving
certain SEAL data packets using the format shown in <xref
target="control2"/>:</t>
<t><figure anchor="control2" title="SCMP Error Message Format">
<artwork><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-Specific Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of the invoking SEAL data packet as possible |
~ (beginning with the SEAL header) without the SCMP ~
| packet exceeding MINMTU bytes (*) |
(*) also known as the "packet-in-error"]]></artwork>
</figure>The error message includes the 32-bit SCMP message
header, followed by a 32-bit Type-Specific Data field, followed by
the leading portion of the invoking SEAL data packet beginning with
the SEAL header as the "packet-in-error". The packet-in-error
includes as much of the invoking packet as possible extending to a
length that would not cause the entire SCMP packet following outer
encapsulation to exceed MINMTU bytes.</t>
<t>When the ETE processes a SEAL data packet for which the
Identification and ICV values are correct but an error must be
returned, it prepares an SCMP error message as shown in <xref
target="control2"/>. The ETE sets the Type and Code fields to the
same values that would appear in the corresponding ICMPv6 message
<xref target="RFC4443"/>, but calculates the Checksum beginning with
the SCMP message header using the algorithm specified for ICMPv4 in
<xref target="RFC0792"/>.</t>
<t>The ETE next encapsulates the SCMP message in the requisite SEAL
and outer headers as shown in <xref target="scmpencaps"/>. During
encapsulation, the ETE sets the outer destination address/port
numbers of the SCMP packet to the values associated with the ITE and
sets the outer source address/port numbers to its own outer
address/port numbers.</t>
<t>The ETE then sets (C=1; A=0; L=0; X=0; M=0; Offset=0) in the SEAL
header, then sets I, V, NEXTHDR and LEVEL to the same values that
appeared in the SEAL header of the data packet. If the neighbor
relationship between the ITE and ETE is unidirectional, the ETE next
sets the LINK_ID field to the same value that appeared in the SEAL
header of the data packet. Otherwise, the ETE sets the LINK_ID field
to the value it would use in sending a SEAL packet to this ITE.</t>
<t>When I==1, the ETE next sets the Identification field to an
appropriate value for the ITE. If the neighbor relationship between
the ITE and ETE is unidirectional, the ETE sets the Identification
field to the same value that appeared in the SEAL header of the data
packet. Otherwise, the ETE sets the Identification field to the
value it would use in sending the next SEAL packet to this ITE.</t>
<t>When V==1, the ETE then calculates and sets the ICV field the
same as specified for SEAL data packet encapsulation in Section
5.4.4.</t>
<t>Finally, the ETE sends the resulting SCMP packet to the ITE the
same as specified for SEAL data packets in Section 5.4.5.</t>
<t>The following sections describe additional considerations for
various SCMP error messages:</t>
<section title="Generating SCMP Packet Too Big (SPTB) Messages">
<t>An ITE generates an SCMP "Packet Too Big" (SPTB) message when
it receives a SEAL data packet that is larger than (MINMTU-HLEN)
but no larger than 1500 bytes. The ITE sends the SPTB message
toward the previous hop SEAL ITE with the MTU field set to
(MINMTU-HLEN) subject to rate limiting. In the case of nested
tunneling, the previous hop SEAL ITE is determined by examining
the source addressing information in the outer encapsulations of
the SEAL data packet submitted for nested encapsulation. In the
case of re-encapsulating tunneling, the SEAL node acts as an ETE
with respect to the previous hop ITE and as an ITE with respect to
the next hop ETE. The node must therefore retain the outer
encapsulations that were used in the previous segment of this
(segmented) SEAL tunnel (i.e., as it transitions from ETE on the
previous hop segment to ITE on the next hop segment) so that it
will have the source addressing information for the previous hop
ITE.</t>
<t>An ETE generates an SPTB message when it receives a SEAL data
packet that arrived as multiple outer IP fragments and for which
L==0. The ETE prepares the SPTB message the same as for the
corresponding ICMPv6 PTB message, and writes the length of the
largest outer IP fragment received minus HLEN in the MTU field of
the message.</t>
<t>The ETE also generates an SPTB message when it accepts a SEAL
protocol data packet with A==1 in the SEAL header. The ETE
prepares the SPTB message the same as above, except that it writes
the value 0 in the MTU field.</t>
</section>
<section title="Generating Other SCMP Error Messages">
<t>An ETE generates an SCMP "Destination Unreachable" (SDU)
message under the same circumstances that an IPv6 system would
generate an ICMPv6 Destination Unreachable message.</t>
<t>An ETE generates an SCMP "Parameter Problem" (SPP) message when
it receives a SEAL packet with an incorrect value in the SEAL
header.</t>
<t>TEs generate other SCMP message types using methods and
procedures specified in other documents. For example, SCMP message
types used for tunnel neighbor coordinations are specified in VET
<xref target="I-D.templin-intarea-vet"/>.</t>
</section>
</section>
<section title="Processing SCMP Error Messages">
<t>An ITE may receive SCMP messages with C==1 in the SEAL header
after sending packets to an ETE. The ITE first verifies that the
outer addresses of the SCMP packet are correct, and (when I==1) that
the Identification field contains an acceptable value. The ITE next
verifies that the SEAL header fields are set correctly as specified
in Section 5.6.1. When V==1, the ITE then verifies the ICV value.
The ITE next verifies the Checksum value in the SCMP message header.
If any of these values are incorrect, the ITE silently discards the
message; otherwise, it processes the message as follows:</t>
<section title="Processing SCMP PTB Messages">
<t>After an ITE sends a SEAL data packet to an ETE, it may receive
an SPTB message with a packet-in-error containing the leading
portion of the packet (see: Section 5.6.1.1). For IP SPTB messages
with MTU==0, the ITE processes the message as confirmation that
the ETE received a SEAL data packet with A==1 in the SEAL header.
The ITE then discards the message.</t>
<t>For SPTB messages with MTU != 0, the ITE processes the message
as an indication of a packet size limitation as follows. If the
inner packet is itself a SEAL packet, and the inner packet length
is less than 1500, the ITE reduces its MINMTU value for this ITE.
If the inner packet is a non-SEAL IPv4 packet and the inner packet
length is less than 1500, the ITE instead sets RATE_LIMIT=1. For
all other cases, if the inner packet length is larger than 1500
and the MTU value is not substantially less than 1500 bytes, the
value is likely to reflect the true MTU of the restricting link on
the path to the ETE; otherwise, a router on the path may be
generating runt fragments.</t>
<t>In that case, the ITE can consult a plateau table (e.g., as
described in <xref target="RFC1191"/>) to rewrite the MTU value to
a reduced size. For example, if the ITE receives an IPv4 SPTB
message with MTU==256 and inner packet length 4KB, it can rewrite
the MTU to 2KB. If the ITE subsequently receives an IPv4 SPTB
message with MTU==256 and inner packet length 2KB, it can rewrite
the MTU to 1792, etc., to a minimum of 1500 bytes. If the ITE is
performing stateful MTU determination for this SEAL path, it then
writes the new MTU value minus HLEN in MAXMTU.</t>
<t>The ITE then checks its forwarding tables to discover the
previous hop toward the source address of the inner packet. If the
previous hop is reached via the same tunnel interface the SPTB
message arrived on, the ITE relays the message to the previous
hop. In order to relay the message, the first writes zero in the
Identification and ICV fields of the SEAL header within the
packet-in-error. The ITE next rewrites the outer SEAL header
fields with values corresponding to the previous hop and
recalculates the ICV using the ICV calculation parameters
associated with the previous hop. Next, the ITE replaces the
SPTB's outer headers with headers of the appropriate protocol
version and fills in the header fields as specified in Section
5.4.5, where the destination address/port correspond to the
previous hop and the source address/port correspond to the ITE.
The ITE then sends the message to the previous hop the same as if
it were issuing a new SPTB message. (Note that, in this process,
the values within the SEAL header of the packet-in-error are
meaningless to the previous hop and therefore cannot be used by
the previous hop for authentication purposes.)</t>
<t>If the previous hop is not reached via the same tunnel
interface, the ITE instead transcribes the message into a format
appropriate for the inner packet (i.e., the same as described for
transcribing ICMP messages in Section 5.4.7) and sends the
resulting transcribed message to the original source. (NB: if the
inner packet within the SPTB message is an IPv4 SEAL packet with
DF==0, the ITE should set DF=1 and re-calculate the IPv4 header
checksum while transcribing the message in order to avoid bogon
filters.) The ITE then discards the SPTB message.</t>
<t>Note that the ITE may receive an SPTB message from another ITE
that is at the head end of a nested level of encapsulation. The
ITE has no security associations with this nested ITE, hence it
should consider this SPTB message the same as if it had received
an ICMP PTB message from an ordinary router on the path to the
ETE. That is, the ITE should examine the packet-in-error field of
the SPTB message and only process the message if it is able to
recognize the packet as one it had previously sent.</t>
</section>
<section title="Processing Other SCMP Error Messages">
<t>An ITE may receive an SDU message with an appropriate code
under the same circumstances that an IPv6 node would receive an
ICMPv6 Destination Unreachable message. The ITE either transcribes
or relays the message toward the source address of the inner
packet within the packet-in-error the same as specified for SPTB
messages in Section 5.6.2.1.</t>
<t>An ITE may receive an SPP message when the ETE receives a SEAL
packet with an incorrect value in the SEAL header. The ITE should
examine the SEAL header within the packet-in-error to determine
whether a different setting should be used in subsequent packets,
but does not relay the message further.</t>
<t>TEs process other SCMP message types using methods and
procedures specified in other documents. For example, SCMP message
types used for tunnel neighbor coordinations are specified in VET
<xref target="I-D.templin-intarea-vet"/>.</t>
</section>
</section>
</section>
</section>
<section title="Link Requirements">
<t>Subnetwork designers are expected to follow the recommendations in
Section 2 of <xref target="RFC3819"/> when configuring link MTUs.</t>
</section>
<section title="End System Requirements">
<t>End systems are encouraged to implement end-to-end MTU assurance
(e.g., using Packetization Layer Path MTU Discovery (PLPMTUD) per <xref
target="RFC4821"/>) even if the subnetwork is using SEAL.</t>
<t>When end systems use PLPMTUD, SEAL will ensure that the tunnel
behaves as a link in the path that assures an MTU of at least 1500 bytes
while not precluding discovery of larger MTUs. The PMPMTUD mechanism
will therefore be able to function as designed in order to discover and
utilize larger MTUs.</t>
</section>
<section title="Router Requirements">
<t>Routers within the subnetwork are expected to observe the standard IP
router requirements, including the implementation of IP fragmentation
and reassembly as well as the generation of ICMP messages <xref
target="RFC0792"/><xref target="RFC1122"/><xref target="RFC1812"/><xref
target="RFC2460"/><xref target="RFC4443"/><xref target="RFC6434"/>.</t>
<t>Note that, even when routers support existing requirements for the
generation of ICMP messages, these messages are often filtered and
discarded by middleboxes on the path to the original source of the
message that triggered the ICMP. It is therefore not possible to assume
delivery of ICMP messages even when routers are correctly
implemented.</t>
</section>
<section title="Nested Encapsulation Considerations">
<t>SEAL supports nested tunneling for up to 8 layers of encapsulation.
In this model, the SEAL ITE has a tunnel neighbor relationship only with
ETEs at its own nesting level, i.e., it does not have a tunnel neighbor
relationship with other ITEs, nor with ETEs at other nesting levels.</t>
<t>Therefore, when an ITE 'A' within an inner nesting level needs to
return an error message to an ITE 'B' within an outer nesting level, it
generates an ordinary ICMP error message the same as if it were an
ordinary router within the subnetwork. 'B' can then perform message
validation as specified in Section 5.4.7, but full message origin
authentication is not possible.</t>
<t>Since ordinary ICMP messages are used for coordinations between ITEs
at different nesting levels, nested SEAL encapsulations should only be
used when the ITEs are within a common administrative domain and/or when
there is no ICMP filtering middlebox such as a firewall or NAT between
them. An example would be a recursive nesting of mobile networks, where
the first network receives service from an ISP, the second network
receives service from the first network, the third network receives
service from the second network, etc.</t>
<t>NB: As an alternative, the SCMP protocol could be extended to allow
ITE 'A' to return an SCMP message to ITE 'B' rather than return an ICMP
message. This would conceptually allow the control messages to pass
through firewalls and NATs, however it would give no more message origin
authentication assurance than for ordinary ICMP messages. It was
therefore determined that the complexity of extending the SCMP protocol
was of little value within the context of the anticipated use cases for
nested encapsulations.</t>
</section>
<section title="Reliability Considerations">
<t>Although a SEAL tunnel may span an arbitrarily-large subnetwork
expanse, the IP layer sees the tunnel as a simple link that supports the
IP service model. Links with high bit error rates (BERs) (e.g., IEEE
802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms <xref
target="RFC3366"/> to increase packet delivery ratios, while links with
much lower BERs typically omit such mechanisms. Since SEAL tunnels may
traverse arbitrarily-long paths over links of various types that are
already either performing or omitting ARQ as appropriate, it would
therefore be inefficient to require the tunnel endpoints to also perform
ARQ.</t>
</section>
<section title="Integrity Considerations">
<t>The SEAL header includes an integrity check field that covers the
SEAL header and at least the inner packet headers. This provides for
header integrity verification on a segment-by-segment basis for a
segmented re-encapsulating tunnel path.</t>
<t>Fragmentation and reassembly schemes must also consider
packet-splicing errors, e.g., when two fragments from the same packet
are concatenated incorrectly, when a fragment from packet X is
reassembled with fragments from packet Y, etc. The primary sources of
such errors include implementation bugs and wrapping IPv4 ID fields.</t>
<t>In particular, the IPv4 16-bit ID field can wrap with only 64K
packets with the same (src, dst, protocol)-tuple alive in the system at
a given time <xref target="RFC4963"/>. When the IPv4 ID field is
re-written by a middlebox such as a NAT or Firewall, ID field wrapping
can occur with even fewer packets alive in the system.</t>
<t>When outer IPv4 fragmentation is unavoidable, SEAL institutes rate
limiting so that the number of packets admitted into the tunnel by the
ITE does not exceed the number of unique packets that may be alive
within the Internet.</t>
</section>
<section title="IANA Considerations">
<t>The IANA is requested to allocate a User Port number for "SEAL" in
the 'port-numbers' registry for the TCP and UDP protocols.</t>
<t>The IANA is further requested to allocate an IP protocol number for
"SEAL" in the "protocol-numbers" registry.</t>
<t>.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>SEAL provides a segment-by-segment data origin authentication and
anti-replay service across the (potentially) multiple segments of a
re-encapsulating tunnel. It further provides a segment-by-segment
integrity check of the headers of encapsulated packets, but does not
verify the integrity of the rest of the packet beyond the headers unless
fragmentation is unavoidable. SEAL therefore considers full message
integrity checking, authentication and confidentiality as end-to-end
considerations.</t>
<t>An amplification/reflection/buffer overflow attack is possible when
an attacker sends IP fragments with spoofed source addresses to an ETE
in an attempt to clog the ETE's reassembly buffer and/or cause the ETE
to generate a stream of SCMP messages returned to a victim ITE. The SCMP
message ICV, Identification, as well as the inner headers of the
packet-in-error, provide mitigation for the ETE to detect and discard
SEAL segments with spoofed source addresses.</t>
<t>The SEAL header is sent in-the-clear the same as for the outer IP and
other outer headers. In this respect, the threat model is no different
than for IPv6 extension headers. Unlike IPv6 extension headers, however,
the SEAL header can be protected by an integrity check that also covers
the inner packet headers.</t>
<t>Security issues that apply to tunneling in general are discussed in
<xref target="RFC6169"/>.</t>
</section>
<section title="Related Work">
<t>Section 3.1.7 of <xref target="RFC2764"/> provides a high-level
sketch for supporting large tunnel MTUs via a tunnel-level segmentation
and reassembly capability to avoid IP level fragmentation.</t>
<t>Section 3 of <xref target="RFC4459"> </xref> describes inner and
outer fragmentation at the tunnel endpoints as alternatives for
accommodating the tunnel MTU.</t>
<t>Section 4 of <xref target="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.</t>
<t>IPsec/AH is <xref target="RFC4301"/><xref target="RFC4301"/> is used
for full message integrity verification between tunnel endpoints,
whereas SEAL only ensures integrity for the inner packet headers. The
AYIYA proposal <xref target="I-D.massar-v6ops-ayiya"/> uses similar
means for providing message authentication and integrity.</t>
<t>SEAL, along with the Virtual Enterprise Traversal (VET) <xref
target="I-D.templin-intarea-vet"/> tunnel virtual interface abstraction,
are the functional building blocks for the Internet Routing Overlay
Network (IRON) <xref target="I-D.templin-ironbis"/> and Routing and
Addressing in Networks with Global Enterprise Recursion (RANGER) <xref
target="RFC5720"/><xref target="RFC6139"/> architectures.</t>
<t>The concepts of path MTU determination through the report of
fragmentation and extending the IPv4 Identification field were first
proposed in deliberations of the TCP-IP mailing list and the Path MTU
Discovery Working Group (MTUDWG) during the late 1980's and early
1990's. An historical analysis of the evolution of these concepts, as
well as the development of the eventual PMTUD mechanism, appears in
<xref target="RFC5320"/>.</t>
</section>
<section title="Implementation Status">
<t>An early implementation of the first revision of SEAL <xref
target="RFC5320"/> is available at: http://isatap.com/seal.</t>
</section>
<section anchor="acknowledge" title="Acknowledgments">
<t>The following individuals are acknowledged for helpful comments and
suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver
Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, Ian
Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph Droms,
Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci, Joel
Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden,
Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt Mathis,
Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark Townsley, Ole
Troan, Margaret Wasserman, Magnus Westerlund, Robin Whittle, James
Woodyatt, and members of the Boeing Research & Technology NST
DC&NT group.</t>
<t>Discussions with colleagues following the publication of <xref
target="RFC5320"/> have provided useful insights that have resulted in
significant improvements to this, the Second Edition of SEAL.</t>
<t>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.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.0791"?>
<?rfc include="reference.RFC.0792"?>
<?rfc include="reference.RFC.1122"?>
<?rfc include="reference.RFC.4443"?>
<?rfc include="reference.RFC.3971"?>
<?rfc include="reference.RFC.4861"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.2460"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.1063"?>
<?rfc include="reference.RFC.0994"?>
<?rfc include="reference.RFC.1146"?>
<?rfc include="reference.RFC.1191"?>
<?rfc include="reference.RFC.1701"?>
<?rfc include="reference.RFC.1981"?>
<?rfc include="reference.RFC.2003"?>
<?rfc include="reference.RFC.2473"?>
<?rfc include="reference.RFC.2827"?>
<?rfc include="reference.RFC.2923"?>
<?rfc include="reference.RFC.3366"?>
<?rfc include="reference.RFC.3819"?>
<?rfc include="reference.RFC.4213"?>
<?rfc include="reference.RFC.1812"?>
<?rfc include="reference.RFC.2780"?>
<?rfc include="reference.RFC.4301"?>
<?rfc include="reference.RFC.4302"?>
<?rfc include="reference.RFC.5246"?>
<?rfc include="reference.RFC.4459"?>
<?rfc include="reference.RFC.4821"?>
<?rfc include="reference.RFC.4963"?>
<?rfc include="reference.RFC.2764"?>
<?rfc include="reference.RFC.2675"?>
<?rfc include="reference.RFC.5445"?>
<?rfc include="reference.RFC.1070"?>
<?rfc include="reference.RFC.3232"?>
<?rfc include="reference.RFC.4191"?>
<?rfc include="reference.RFC.4987"?>
<?rfc include="reference.RFC.5720"?>
<?rfc include="reference.RFC.5226"?>
<?rfc include="reference.RFC.5320"?>
<?rfc include="reference.RFC.6335"?>
<?rfc include="reference.I-D.templin-intarea-vet"?>
<?rfc include="reference.I-D.ietf-savi-framework"?>
<?rfc include="reference.I-D.templin-ironbis"?>
<?rfc include="reference.RFC.6139"?>
<?rfc include="reference.RFC.5927"?>
<?rfc include="reference.RFC.6169"?>
<?rfc include="reference.RFC.6434"?>
<?rfc include="reference.RFC.6438"?>
<?rfc include="reference.I-D.ietf-6man-udpzero"?>
<?rfc include="reference.I-D.ietf-intarea-ipv4-id-update"?>
<?rfc include="reference.I-D.massar-v6ops-ayiya"?>
<?rfc include="reference.I-D.generic-6man-tunfrag"?>
<?rfc include="reference.I-D.taylor-v6ops-fragdrop"?>
<reference anchor="FRAG">
<front>
<title>Fragmentation Considered Harmful</title>
<author fullname="Christopher Kent" initials="C" surname="Kent">
<organization/>
</author>
<author fullname="Jeffrey Mogul" initials="J" surname="Mogul">
<organization/>
</author>
<date month="October" year="1987"/>
</front>
</reference>
<reference anchor="FOLK">
<front>
<title>Beyond Folklore: Observations on Fragmented Traffic</title>
<author fullname="Colleen Shannon" initials="C" surname="Shannon">
<organization/>
</author>
<author fullname="David Moore" initials="D" surname="Moore">
<organization/>
</author>
<author fullname="k claffy" initials="k" surname="claffy">
<organization/>
</author>
<date month="December" year="2002"/>
</front>
</reference>
<reference anchor="MTUDWG">
<front>
<title>IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November 1989 -
February 1995.</title>
<author fullname="" initials="" surname="">
<organization/>
</author>
<date month="" year=""/>
</front>
</reference>
<reference anchor="TCP-IP">
<front>
<title>Archive/Hypermail of Early TCP-IP Mail List,
http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May 1987 - May
1990.</title>
<author fullname="" initials="" surname="">
<organization/>
</author>
<date month="" year=""/>
</front>
</reference>
<reference anchor="TBIT">
<front>
<title>Measuring Interactions Between Transport Protocols and
Middleboxes</title>
<author fullname="Alberto Medina" initials="A" surname="Medina">
<organization/>
</author>
<author fullname="Mark Allman" initials="M" surname="Allman">
<organization/>
</author>
<author fullname="Sally Floyd" initials="S" surname="Floyd">
<organization/>
</author>
<date month="October" year="2004"/>
</front>
</reference>
<reference anchor="WAND">
<front>
<title>Inferring and Debugging Path MTU Discovery Failures</title>
<author fullname="Matthew Luckie" initials="M" surname="Luckie">
<organization/>
</author>
<author fullname="Kenjiro Cho" initials="K" surname="Cho">
<organization/>
</author>
<author fullname="Bill Owens" initials="B" surname="Owens">
<organization/>
</author>
<date month="October" year="2005"/>
</front>
</reference>
<reference anchor="SIGCOMM">
<front>
<title>Measuring Path MTU Discovery Behavior</title>
<author fullname="Matthew Luckie" initials="M" surname="Luckie">
<organization/>
</author>
<author fullname="Ben Stasiewicz" initials="B" surname="Stasiewicz">
<organization/>
</author>
<date month="November" year="2010"/>
</front>
</reference>
</references>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 11:53:23 |