One document matched: draft-templin-intarea-seal-38.xml
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<rfc category="std" docName="draft-templin-intarea-seal-38.txt"
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<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="17" month="November" year="2011" />
<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, 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.</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 (including tunnels
of one form or another) over an actual network that supports the
Internet Protocol (IP) <xref target="RFC0791"></xref><xref
target="RFC2460"></xref>. 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 insertion of an outer IP header reduces the effective path MTU
visible to the inner network layer. When IPv4 is used, this reduced MTU
can be accommodated through the use of IPv4 fragmentation, but</t>
<t>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><xref target="FOLK"></xref><xref
target="RFC4963"></xref>. Additionally, classical path MTU discovery
<xref target="RFC1191"></xref> has known operational issues that are
exacerbated by in-the-network tunnels <xref
target="RFC2923"></xref><xref target="RFC4459"></xref>. 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,
IPv4 is ubiquitously deployed as the Layer 3 protocol. The primary
functions of IPv4 are to provide for routing, addressing, and a
fragmentation and reassembly capability used to accommodate links with
diverse MTUs. While it is well known that the IPv4 address space is
rapidly becoming depleted, there is a lesser-known but growing
consensus that other IPv4 protocol limitations have already or may
soon become problematic.</t>
<t>First, the IPv4 header Identification field is only 16 bits in
length, meaning that at most 2^16 unique packets with the same
(source, destination, protocol)-tuple may be active in the Internet at
a given time <xref target="I-D.ietf-intarea-ipv4-id-update"></xref>.
Due to the escalating deployment of high-speed links, however, this
number has become too small by several orders of magnitude for high
data rate packet sources such as tunnel endpoints <xref
target="RFC4963"></xref>. 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"></xref>) to
the potential for major integrity issues (e.g., mis-association of the
fragments of multiple IP packets during reassembly <xref
target="RFC4963"></xref>).</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 (see Appendix
D). In this method, the source node provides explicit instructions to
routers in the path to discard the packet and return an ICMP error
message if an MTU restriction is encountered. However, this approach
has several serious shortcomings that lead to an overall
“brittleness” <xref target="RFC2923"></xref>.</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"></xref>. Furthermore, when a source node that
requires ICMP error message feedback when a packet is dropped due to
an MTU restriction does not receive the messages, a path MTU-related
black hole occurs. This means that the source will continue to send
packets that are too large and never receive an indication from the
network that they are being discarded. This behavior has been
confirmed through documented studies showing clear evidence of path
MTU discovery failures in the Internet today <xref
target="TBIT"></xref><xref target="WAND"></xref><xref
target="SIGCOMM"></xref>.</t>
<t>The issues with both IPv4 fragmentation and this
“classical” method of path MTU discovery are exacerbated
further when IP tunneling is used <xref target="RFC4459"></xref>. For
example, an ingress tunnel endpoint (ITE) may be required to forward
encapsulated packets into the subnetwork on behalf of hundreds,
thousands, or even more original sources within the end site that it
serves. If the ITE allows IPv4 fragmentation on the encapsulated
packets, persistent fragmentation could lead to undetected data
corruption due to Identification field wrapping. If the ITE instead
uses classical IPv4 path MTU discovery, it must rely on ICMP error
messages coming from the subnetwork that may be suspect, subject to
loss due to filtering middleboxes, or insufficiently provisioned for
translation into error messages to be returned to the original
sources.</t>
<t>Although recent works have led to the development of a robust
end-to-end MTU determination scheme <xref target="RFC4821"></xref>,
they do not excuse tunnels from delivering path MTU discovery feedback
when packets are lost due to size restrictions. 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 network layer protocols (e.g., IPv4, IPv6,
OSI, etc.) over IP subnetworks that connect Ingress and Egress Tunnel
Endpoints (ITEs/ETEs) of border nodes. It provides a modular
specification designed to be tailored to specific associated tunneling
protocols. A transport-mode of operation is also possible, and
described in Appendix C.</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 detection. The encapsulation further
ensures data origin authentication, packet header integrity and
anti-replay.</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>
<section title="Terminology and Requirements">
<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="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="ETE Link Path"><vspace />a subnetwork path from an ITE
to an ETE beginning with an underlying link of the ITE as the first
hop.</t>
<t hangText="inner packet"><vspace />an unencapsulated network layer
protocol packet (e.g., IPv6 <xref target="RFC2460"></xref>, IPv4
<xref target="RFC0791"></xref>, OSI/CLNP <xref
target="RFC1070"></xref>, 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"></xref>. 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"></xref> error
message, an ICMPv6 <xref target="RFC4443"></xref> error message,
etc.).</t>
<t hangText="Packet Too Big (PTB)"><vspace />a control plane message
indicating an MTU restriction (e.g., an ICMPv6 "Packet Too Big"
message <xref target="RFC4443"></xref>, an ICMPv4 "Fragmentation
Needed" message <xref target="RFC0792"></xref>, etc.).</t>
<t hangText="IP"><vspace />used to generically refer to either
Internet Protocol (IP) version, i.e., IPv4 or IPv6.</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>DF - the IPv4 header "Don't Fragment" flag <xref
target="RFC0791"></xref><vspace /></t>
<t>ETE - Egress Tunnel Endpoint<vspace /></t>
<t>HLEN - the length of the SEAL header plus outer
headers<vspace /></t>
<t>ICV - Integrity Check Vector<vspace /></t>
<t>ITE - Ingress Tunnel Endpoint<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>SNA - SCMP Neighbor Advertisement message<vspace /></t>
<t>SNS - SCMP Neighbor Solicitation 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>SEAL_PORT - a transport-layer service port number used for
SEAL<vspace /></t>
<t>SEAL_PROTO - an IP protocol number used for SEAL<vspace /></t>
<t>TE - Tunnel Endpoint (i.e., either ingress or egress)
<vspace /></t>
<t>VET - Virtual Enterprise Traversal<vspace /></t>
</list></t>
<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"></xref>. When used in lower case (e.g., must, must not,
etc.), these words MUST NOT be interpreted as described in <xref
target="RFC2119"></xref>, 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. SEAL, along with
the Virtual Enterprise Traversal (VET) <xref
target="I-D.templin-intarea-vet"></xref> tunnel virtual interface
abstraction, are the functional building blocks for the Internet Routing
Overlay Network (IRON) <xref target="I-D.templin-ironbis"></xref> and
Routing and Addressing in Networks with Global Enterprise Recursion
(RANGER) <xref target="RFC5720"></xref><xref target="RFC6139"></xref>
architectures.</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 the same as for
IPv6 extension headers, i.e., it is not part of the outer IP header but
rather allows for the creation of an arbitrarily extensible chain of
headers in the same way that IPv6 does.</t>
<t>To accommodate MTU diversity, the Egress Tunnel Endpoint (ETE) acts
as a passive observer that simply informs the Ingress Tunnel Endpoint
(ITE) of any packet size limitations. This allows the ITE to return
appropriate path MTU discovery feedback even if the network path between
the ITE and ETE filters ICMP messages.</t>
<t>SEAL further ensures data origin authentication, packet header
integrity, and anti-replay. The SEAL framework is therefore similar to
the IP Security (IPsec) Authentication Header (AH) <xref
target="RFC4301"></xref><xref target="RFC4302"></xref>, however it
provides only minimal hop-by-hop authenticating services along a path
while leaving full data integrity, authentication and confidentiality
services as an end-to-end consideration. While SEAL performs data origin
authentication, the origin site must also perform the necessary ingress
filtering in order to provide full source address verification <xref
target="I-D.ietf-savi-framework"></xref>.</t>
</section>
<section title="SEAL Specification">
<t>The following sections specify the operation of SEAL:</t>
<section title="VET Interface Model">
<t>SEAL is an encapsulation sublayer used within VET non-broadcast,
multiple access (NBMA) tunnel virtual interfaces. Each VET interface
is configured over one or more underlying interfaces attached to
subnetwork links. The VET interface connects an ITE to one or more ETE
"neighbors" via tunneling across an underlying subnetwork, where
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 VET unidirectional and bidirectional models are
discussed in <xref target="I-D.templin-intarea-vet"></xref>.</t>
</section>
<section title="SEAL Model of Operation">
<t>SEAL-enabled ITEs encapsulate each inner packet in a SEAL header,
any outer header encapsulations, and in some instances a SEAL trailer
as shown in <xref target="encaps1"></xref>:</t>
<t><figure anchor="encaps1" title="SEAL Encapsulation">
<artwork><![CDATA[ +--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
| SEAL Trailer |
+--------------------+
]]></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 (e.g., <xref target="RFC1070"></xref><xref
target="RFC2003"></xref><xref target="RFC2473"></xref><xref
target="RFC4213"></xref>, etc.), the ITE inserts the SEAL header
between the inner packet and outer IP headers as: IP/SEAL/{inner
packet}.</t>
<t>For encapsulations over transports such as UDP, the ITE inserts the
SEAL header between the outer transport layer header and the inner
packet, e.g., as IP/UDP/SEAL/{inner packet} (similar to <xref
target="RFC4380"></xref>). In that case, the UDP header is seen as an
"other outer header" as depicted in <xref
target="encaps1"></xref>.</t>
<t>When necessary, the ITE also appends a SEAL trailer at the end of
the SEAL packet. In that case, the trailer is added after the final
byte of the encapsulated packet.</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 are discussed in Section 4 of <xref
target="RFC2473"></xref>.</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, IP address} pair
as the ingress attachment point to a subnetwork link path to the ETE.
The ITE therefore maintains path MTU state on a per ETE link path
basis, although it may instead maintain only the
lowest-common-denominator values for all of the ETE's link paths in
order to reduce state.</t>
<t>Finally, the SEAL ITE ensures that the inner network layer protocol
will see a minimum MTU of 1280 bytes over each ETE link path
regardless of the outer network layer protocol version, i.e., even if
a small amount of fragmentation and reassembly are necessary.</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|P|R|T|U|Z| NEXTHDR | PREFLEN | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ PREFIX (when present) ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
]]></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="P (1)"><vspace />the "Prefix Included" bit. Set to 1
if the header includes a Prefix Field. Used for SCMP messages that
do not include a packet-in-error (see: <xref
target="I-D.templin-intarea-vet"></xref>), and for NULL SEAL data
packets used as probes (see: Section 4.4.6).</t>
<t hangText="R (1)"><vspace />the "Redirects Permitted" bit. For
data packets, set to 1 by the ITE to inform the ETE that the
source is accepting Redirects (see:<xref
target="I-D.templin-intarea-vet"> </xref>).</t>
<t hangText="T (1)"><vspace />the "Trailer Included" bit. Set to 1
if the ITE was obliged to include a trailer.</t>
<t hangText="U (1)"><vspace />the "Unfragmented Packet" bit. Set
to 1 by the ITE in SEAL data packets for which it wishes to
receive an explicit acknowledgement from the ETE if the packet
arrives unfragmented.</t>
<t hangText="Z (1)"><vspace />the "Reserved" bit. Must be set to 0
for this version of the SEAL specification.</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="PREFLEN (8)">an 8-bit field that encodes the length
of the prefix to be applied to the source address of the inner
packets (when P==0) or the prefix included in the PREFIX field
(when P==1).</t>
<t hangText="LINK_ID (5)"><vspace />a 5-bit link identification
value, set to a unique value by the ITE for each underlying link
as the first hop of a path over which it will send encapsulated
packets to ETEs. Up to 32 ETE link paths are therefore supported
for each ETE.</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="PKT_ID (32)"><vspace />a 32-bit per-packet
identification field. Set to a monotonically-incrementing 32-bit
value for each SEAL packet transmitted to this ETE, beginning with
0.</t>
<t hangText="ICV1 (32)"><vspace />a 32-bit header integrity check
value that covers the leading 128 bytes of the packet beginning
with the SEAL header. The value 128 is chosen so that at least the
SEAL header as well as the inner packet network and transport
layer headers are covered by the integrity check.</t>
<t hangText="PREFIX (variable)"><vspace />a variable-length string
of bytes; present only when P==1. The field length is determined
by calculating Len=(Ceiling(PREFLEN / 32) * 4). For example, if
PREFLEN==63, the field is 8 bytes in length and encodes the
leading 63 bits of the inner network layer prefix beginning with
the most significant bit.</t>
</list>When T==1, SEAL encapsulation also includes a trailer
formatted as follows:</t>
<t><figure anchor="min2" title="SEAL Trailer 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list style="hanging">
<t hangText="ICV2 (32)"><vspace />a 32-bit packet integrity check
value. Present only when T==1, and covers the remaining length of
the encapsulated packet beyond the leading 128 bytes (i.e., the
remaining portion that was not covered by ICV1). Added as a
trailing 32 bit field following the final byte of the encapsulated
SEAL packet and used to detect reassembly misassociations. Need
not be aligned on an even byte boundary.</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 VET NBMA tunnel virtual interfaces may support
a large set of ETE link paths that accept widely varying maximum
packet sizes, however, a number of factors should be taken into
consideration when selecting a tunnel interface MTU.</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"></xref>.</t>
<t>The ITE should therefore set a tunnel interface MTU of at least
1500 bytes plus extra room to accommodate any additional
encapsulations that may occur on the path from the original source.
The ITE can also set smaller MTU values; however, care must be taken
not to set so small a value that original sources would experience
an MTU underflow. In particular, IPv6 sources must see a minimum
path MTU of 1280 bytes, and IPv4 sources should see a minimum path
MTU of 576 bytes.</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) bit set to 0, if the
packet is larger than the tunnel interface MTU the inner IPv4 layer
uses IPv4 fragmentation to break the packet into fragments no larger
than the tunnel interface MTU. The ITE then admits each fragment
into the interface as an independent packet.</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 minimum MTU (e.g., 1280 bytes
for IPv6, 576 bytes for IPv4, etc.).</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 subnetwork
adaptation is handled from within the interface. The ITE MAY instead
set a finite MTU on tunnel *host* interfaces.</t>
</section>
<section title="Tunnel Neighbor Soft State">
<t>Within the tunnel virtual interface, the ITE maintains a per
tunnel neighbor (i.e., a per-ETE) integrity check vector (ICV)
calculation algorithm and (when data origin authentication is
required) a symmetric secret key to calculate the ICV(s) in packets
it will send to this ETE. The ITE also maintains a window of PKT_ID
values for the packets it has recently sent to this ETE.</t>
<t>For each ETE link path, the ITE must account for the lengths of
the headers to be used for encapsulation. The ITE therefore
maintains the per ETE link path constant values "SHLEN" set to
length of the SEAL header, "UHLEN" set to the length of the UDP
encapsulating header (or 0 if UDP encapsulation is not used),
"IHLEN" set to the length of the outer IP layer header, and "HLEN"
set to (SHLEN+UHLEN+IHLEN). (The ITE must include the length of the
uncompressed headers even if header compression is enabled when
calculating these lengths.) In addition, the ETE maintains a
constant value "MIN_MTU" set to 1280+HLEN as well as a variable
"PATH_MTU" initialized to the MTU of the underlying link.</t>
<t>For IPv4, the ITE also maintains the per ETE link path boolean
variables "USE_DF" (initialized to "FALSE") and "USE_TRAILER"
(initialized to "TRUE" if PATH_MTU is less than MIN_MTU; otherwise
initialized to "FALSE") .</t>
<t>The ITE may instead maintain *HLEN, MIN_MTU, PATH_MTU, USE_DF,
and USE_TRAILER as per ETE (rather than per ETE link path) values.
In that case, the values reflect the lowest-common-denominator MTU
across all of the ETE's link paths.</t>
</section>
<section title="Pre-Encapsulation">
<t>For each inner packet admitted into the tunnel interface, if the
packet is itself a SEAL packet (i.e., one with either SEAL_PROTO in
the IP protocol/next-header field, or with SEAL_PORT in the
transport layer destination port field) and the LEVEL field of the
SEAL header contains the value 0, the ITE silently discards the
packet.</t>
<t>Otherwise, for IPv4 inner packets with DF==0 in the IPv4 header,
if the packet is larger than 512 bytes and is not the first fragment
of a SEAL packet (i.e., not a packet that includes a SEAL header)
the ITE fragments the packet into inner fragments no larger than 512
bytes. The ITE then submits each inner fragment for SEAL
encapsulation as specified in Section 4.4.4.</t>
<t>For all other packets, if the packet is no larger than
(MAX(PATH_MTU, MIN_MTU)-HLEN) for the corresponding ETE link path,
the ITE submits it for SEAL encapsulation as specified in Section
4.4.4. Otherwise, the ITE sends a PTB error message toward the
source address of the inner packet.</t>
<t>To send the PTB message, the ITE first checks its forwarding
tables to discover the previous hop toward the source address of the
inner packet. If the previous hop is reached via the same tunnel
interface, the ITE sends an SCMP PTB (SPTB) message to the previous
hop (see: Section 4.6.1.1) with the MTU field set to (MAX(PATH_MTU,
MIN_MTU)-HLEN). Otherwise, the ITE sends an ordinary PTB message
appropriate to the inner protocol version with the MTU field set to
(MAX(PATH_MTU, MIN_MTU)-HLEN).</t>
<t>After sending the (S)PTB message, the ITE discards the inner
packet.</t>
</section>
<section title="SEAL Encapsulation">
<t>The ITE next encapsulates the inner packet in a SEAL header
formatted as specified in Section 4.3. The ITE sets NEXTHDR to the
protocol number corresponding to the address family of the
encapsulated inner packet. For example, the ITE sets NEXTHDR to the
value '4' for encapsulated IPv4 packets <xref
target="RFC2003"></xref>, '41' for encapsulated IPv6 packets <xref
target="RFC2473"></xref><xref target="RFC4213"></xref>, '80' for
encapsulated OSI/CLNP packets <xref target="RFC1070"></xref>,
etc.</t>
<t>The ITE then sets R=1 if redirects are permitted (see: <xref
target="I-D.templin-intarea-vet"></xref>) and sets PREFLEN to the
length of the prefix to be applied to the inner source address. The
ITE's claimed PREFLEN is subject to verification by the ETE; hence,
the ITE MUST set PREFLEN to the exact prefix length that it is
authorized to use. (Note that if this process is entered via
re-encapsulation (see: Section 4.5.4), PREFLEN and R are instead
copied from the SEAL header of the re-encapsulated packet. This
implies that the PREFLEN and R values are propagated across a
re-encapsulating chain of ITE/ETEs that must all be authorized to
represent the prefix.)</t>
<t>Next, the ITE sets (C=0; P=0; Z=0), then sets LINK_ID to the
value assigned to the underlying ETE link path and sets PKT_ID to a
monotonically-increasing integer value for this ETE, beginning with
0 in the first packet transmitted. The ITE also sets U=1 if it needs
to determine whether the ETE will receive the packet without
fragmentation, e.g., for ETE reachability determination (see:
Section 4.4.6), to test whether a middlebox on the path is
reassembling fragmented packets before they arrive at the ETE (see:
Section 4.4.8), for stateful MTU determination (see Section 4.4.9),
etc. Otherwise, the ITE sets U=0.</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 this is an IPv4 ETE link path with USE_TRAILER==TRUE,
and the inner packet is larger than (128-SHLEN-UHLEN) bytes but no
larger than 1280 bytes, the ITE sets T=1. Otherwise, the ITE sets
T=0. The ITE then adds the outer encapsulating headers, calculates
the ICV(s) and performs any necessary outer fragmentation as
specified in Section 4.4.5.</t>
</section>
<section title="Outer Encapsulation">
<t>Following SEAL encapsulation, the ITE next encapsulates the
packet in the requisite outer headers according to the specific
encapsulation format (e.g., <xref target="RFC1070"></xref>, <xref
target="RFC2003"></xref>, <xref target="RFC2473"></xref>, <xref
target="RFC4213"></xref>, etc.), except that it writes 'SEAL_PROTO'
in the protocol field of the outer IP header (when simple IP
encapsulation is used) or writes 'SEAL_PORT' in the outer
destination transport service port field (e.g., when IP/UDP
encapsulation is used).</t>
<t>When UDP encapsulation is used, the ITE sets the UDP header
fields as specified in Section 5.5.4 of <xref
target="I-D.templin-intarea-vet"></xref> (where the UDP header
length field includes the length of the SEAL trailer, if present).
The ITE then performs outer IP header encapsulation as specified in
Section 5.5.5 of <xref target="I-D.templin-intarea-vet"></xref>. If
this process is entered via re-encapsulation (see: Section 4.5.4),
the ITE instead follows the outer IP/UDP re-encapsulation procedures
specified in Section 5.5.6 of <xref
target="I-D.templin-intarea-vet"></xref>.</t>
<t>When IPv4 is used as the outer encapsulation layer, if
USE_DF==FALSE the ITE sets DF=0 in the IPv4 header to allow the
packet to be fragmented within the subnetwork if it encounters a
restricting link. Otherwise, the ITE sets DF=1.</t>
<t>When IPv6 is used as the outer encapsulation layer, the "DF" flag
is absent but implicitly set to 1. The packet therefore will not be
fragmented within the subnetwork, since IPv6 deprecates
in-the-network fragmentation.</t>
<t>The ITE next sets ICV1=0 in the SEAL header and calculates the
packet ICVs. The ICVs are calculated using an algorithm agreed on by
the ITE and ETE. When data origin authentication is required, the
algorithm uses a symmetric secret key so that the ETE can verify
that the ICVs were generated by the ITE.</t>
<t>The ITE first calculates the ICV over the leading 128 bytes of
the packet (or up to the end of the packet if there are fewer than
128 bytes) beginning with the UDP header (if present) then places
result in the ICV1 field in the header. If T==1, the ITE next
calculates the ICV over the remainder of the packet and places the
result in the ICV2 field in the SEAL trailer. The ITE then submits
the packet for outer encapsulation.</t>
<t>Next, the ITE uses IP fragmentation if necessary to fragment the
encapsulated packet into outer IP fragments that are no larger than
PATH_MTU. By virtue of the pre-encapsulation packet size
calculations specified in Section 4.4.3, fragmentation will
therefore only occur for outer packets that are larger than PATH_MTU
but no larger than MIN_MTU. (Note that, for IPv6, fragmentation must
be performed by the ITE itself, while for IPv4 the fragmentation
could instead be performed by a router in the ETE link path.)</t>
<t>The ITE then sends each outer packet/fragment via the underlying
link corresponding to LINK_ID.</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 will return an ICMP message.</t>
<t>The ITE can also send an SCMP Router/Neighbor Solicitation
message to elicit an SCMP Router/Neighbor Advertisement response
(see: <xref target="I-D.templin-intarea-vet"></xref>) as
verification that the ETE is still reachable via a specific link
path.</t>
<t>The ITE processes ICMP messages as specified in Section
4.4.7.</t>
<t>The ITE processes SCMP messages as specified in Section
4.6.2.</t>
</section>
<section title="Processing ICMP Messages">
<t>When the ITE sends SEAL packets, it may receive ICMP error
messages<xref target="RFC0792"></xref><xref target="RFC4443"></xref>
from another ITE on the path to the ETE (i.e., in case of nested
encapsulations) or from an ordinary router within the subnetwork.
Each ICMP message includes an outer IP header, followed by an ICMP
header, followed by a portion of the SEAL data packet that generated
the error (also known as the "packet-in-error") beginning with the
outer IP header.</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 ETE does not implement
the SEAL protocol. The ITE can also process other ICMP messages that
do not include sufficient information in the packet-in-error as a
hint that the ETE link path may be failing. Specific actions that
the ITE may take in these cases are out of scope.</t>
<t>For other ICMP messages, the should use any outer header
information available as a first-pass authentication filter (e.g.,
to determine if the source of the message is within the same
administrative domain as the ITE) and discards the message if first
pass filtering fails.</t>
<t>Next, the ITE examines the packet-in-error beginning with the
SEAL header. If the value in the PKT_ID field is not within the
window of packets the ITE has recently sent to this ETE, or if the
value in the SEAL header ICV1 field is incorrect, the ITE discards
the message.</t>
<t>Next, if the received ICMP message is a PTB the ITE sets the
temporary variable "PMTU" for this ETE link path to the MTU value in
the PTB message. If PMTU==0, the ITE consults a plateau table (e.g.,
as described in <xref target="RFC1191"></xref>) to determine PMTU
based on the length field in the outer IP header of the
packet-in-error. (For example, if the ITE receives a PTB message
with MTU==0 and length 1500, it can set PMTU=1450. If the ITE
subsequently receives a PTB message with MTU==0 and length 1450, it
can set PMTU=1400, etc.) If the ITE is performing stateful MTU
determination for this ETE link path (see Section 4.4.9), the ITE
next sets PATH_MTU=PMTU. If PMTU is less than MIN_MTU, the ITE sets
PATH_MTU=PMTU (and for IPv4 also sets USE_TRAILER=TRUE), then
discards the message.</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 previous hop
toward the inner source address within the packet-in-error is
reached via the same tunnel interface the SEAL data packet was sent
on, the ITE transcribes the ICMP message into an SCMP message.
Otherwise, the ITE transcribes the ICMP message into a message
appropriate for the inner protocol version.</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
4.6.1. 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>For IPv4, the ITE can perform a qualification exchange over an
ETE link path to ensure that the subnetwork correctly delivers
fragments to the ETE. This procedure can be used, e.g., to determine
whether there are middleboxes on the path that violate the <xref
target="RFC1812"></xref>, Section 5.2.6 requirement that: "A router
MUST NOT reassemble any datagram before forwarding it".</t>
<t>When possible, the ITE should use knowledge of its topological
arrangement as an aid in determining when middlebox reassembly
testing is necessary. For example, if the ITE is aware that the ETE
is located somewhere in the public Internet, middlebox reassembly
testing is unnecessary. If the ITE is aware that the ETE is located
behind a NAT or a firewall, however, then middlebox reassembly
testing is recommended.</t>
<t>The ITE can perform a middlebox reassembly test by setting U=1 in
the header of a SEAL data packet to be used as a probe. Next, the
ITE encapsulates the packet in the appropriate outer headers, splits
it into two outer IPv4 fragments, then sends both fragments over the
same ETE link path.</t>
<t>While performing the test, the ITE should select only inner
packets that are no larger than 1280 bytes for testing purposes in
order to avoid reassembly buffer overruns. The ITE can also
construct a NULL test packet instead of using ordinary SEAL data
packets for testing.</t>
<t>To create the NULL packet, the ITE prepares a data packet with
(C=0; P=1; R=0; T=0; U=1; Z=0) in the SEAL header, writes the length
of the ITE's claimed prefix in the PREFLEN field, and writes the
ITE's claimed prefix in the PREFIX field. The ITE then sets NEXTHDR
according to the address family of the PREFIX, i.e., it sets NEXTHDR
to the value '4' for an IPv4 prefix, '41' for an IPv6 prefix , '80'
for an OSI/CLNP prefix, etc.</t>
<t>The ITE can further add padding following the PREFIX field to a
length that would not cause the size of the NULL packet to exceed
1280 bytes before encapsulation. The ITE then sets LINK_ID, LEVEL
and PKT_ID to the appropriate values for this ETE link path and
calculates ICV1 the same as for an ordinary SEAL data packet.</t>
<t>The ITE should send a series of test packets (e.g., 3-5 tests
with 1sec intervals between tests) instead of a single isolated test
in case of packet loss, and will eventually receive an SPTB message
from the ITE (see: Section 4.6.2.1). If the ETE returns an SCMP PTB
message with MTU != 0, then the ETE link path correctly supports
fragmentation.</t>
<t>If the ETE returns an SCMP PTB message with MTU==0, however, then
a middlebox in the subnetwork is reassembling the fragments before
forwarding them to the ETE. In that case, the ITE sets
PATH_MTU=MIN_MTU and sets (USE_TRAILER=TRUE; USE_DF=FALSE). The ITE
may instead enable stateful MTU determination for this ETE link path
as specified in Section 4.4.9 to attempt to discover larger
MTUs.</t>
<t>NB: Examples of middleboxes that may perform reassembly include
stateful NATs and firewalls. Such devices could still allow for
stateless MTU determination if they gather the fragments of a
fragmented IPv4 SEAL data packet for packet analysis purposes but
then forward the fragments on to the final destination rather than
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 ETE link path. For example, when the ETE is situated
behind a middlebox that performs IPv4 reassembly (see: Section
4.4.8) it is imperative that fragmentation of large packets be
avoided on the path to the middlebox. In other instances (e.g., when
the ETE link path includes performance-constrained links), the ITE
may deem it necessary to cache a conservative static MTU in order to
avoid sending large packets that would only be dropped due to an MTU
restriction somewhere on the path.</t>
<t>To determine a static MTU value, the ITE can send a series of
probe packets of various sizes to the ETE with U=1 in the SEAL
header and DF=1 in the outer IP header. The ITE can then cache the
size of the largest packet for which it receives a probe reply from
the ETE as the PATH_MTU value this ETE link path.</t>
<t>For example, the ITE could send NULL probe packets of 1500 bytes,
followed by 1450 bytes, followed by 1400 bytes, etc. then set
PATH_MTU for this ETE link path to the size of the largest probe
packet for which it receives an SPTB reply message. While probing
with NULL probe packets, the ITE processes any ICMP PTB message it
receives as a potential indication of probe failure then discards
the message.</t>
<t>For IPv4, if the largest successful probe is larger than MIN_MTU
the ITE then sets (USE_TRAILER=FALSE; USE_DF=TRUE) for this ETE link
path; otherwise, the ITE sets (USE_TRAILER=TRUE; USE_DF=FALSE).</t>
</section>
<section title="Detecting Path MTU Changes">
<t>For IPv6, the ITE can periodically reset PATH_MTU to the MTU of
the underlying link to determine whether the ETE link path now
supports larger packet sizes. If the path still has a too-small MTU,
the ITE will receive a PTB message that reports a smaller size.</t>
<t>For IPv4, when USE_TRAILER==TRUE and PATH_MTU is larger than
MIN_MTU the ITE can periodically reset USE_TRAILER=FALSE to
determine whether the ETE link path still requires trailers. If the
ITE receives an SPTB message for an inner packet that is no larger
than 1280 bytes (see: Section 4.6.1.1), the ITE should again set
USE_TRAILER=TRUE.</t>
<t>When stateful MTU determination is used, the ITE should
periodically re-probe the path as described in Section 4.4.9 to
determine whether routing changes have resulted in a reduced or
increased PATH_MTU.</t>
</section>
</section>
<section title="ETE Specification">
<section title="Tunnel Neighbor Soft State">
<t>The ETE maintains a per-ITE ICV calculation algorithm and (when
data origin authentication is required) a symmetric secret key to
verify the ICV(s) in the SEAL header and trailer. The ETE also
maintains a window of PKT_ID values for the packets it has recently
received from this ITE.</t>
</section>
<section title="IP-Layer Reassembly">
<t>The ETE must maintain a minimum IP-layer reassembly buffer size
of 1500 bytes for both IPv4 <xref target="RFC0791"></xref> and IPv6
<xref target="RFC2460"></xref>.</t>
<t>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
until it has received at least the first 1280 bytes beyond the SEAL
header or up to the end of the packet. For IPv4, the ETE then
submits the (fully- or partially-reassembled) packet for
decapsulation as specified in Section 4.5.3. For IPv6, the ETE only
submits the packet if it was fully-reassembled and no larger than
the reassembly buffer size.</t>
</section>
<section title="Decapsulation and Re-Encapsulation">
<t>For each SEAL packet submitted for decapsulation, the ETE first
examines the PKT_ID and ICV1 fields. If the PKT_ID is not within the
window of acceptable values for this ITE, or if the ICV1 field
includes an incorrect value, the ETE silently discards the
packet.</t>
<t>Next, if the SEAL header has T==1 and the inner packet is larger
than 1280 bytes the ETE silently discards the packet. If the SEAL
header has T==1 and the inner packet is no larger than 1280 bytes,
the ETE instead verifies the ICV2 value and silently discards the
packet if the value is incorrect.</t>
<t>Next, if the SEAL header has C==0 and there is an incorrect value
in a SEAL header field (e.g., an incorrect "VER" field value), the
ETE returns an SCMP "Parameter Problem" (SPP) message (see Section
4.6.1.2) and discards the packet.</t>
<t>Next, if the packet arrived as multiple IPv4 fragments and the
inner packet is larger than 1280 bytes, the ETE sends an SPTB
message back to the ITE with MTU set to the size of the largest
fragment received minus HLEN (see: Section 4.6.1.1) then discards
the packet. If the packet arrived as multiple IPv6 fragments and the
inner packet is larger than 1280 bytes, the ETE instead silently
discards the packet.</t>
<t>Next, if the packet arrived as multiple IPv4 fragments, the SEAL
header has (C==0; T==0), and the inner packet is larger than
(128-SHLEN-UHLEN) bytes, the ETE sends an SPTB message back to the
ITE with MTU set to the size of the largest fragment received minus
HLEN (see: Section 4.6.1.1) then continues to process the
packet.</t>
<t>Next, if the SEAL header has C==1, the ETE processes the packet
as an SCMP packet as specified in Section 4.6.2. Otherwise, the ETE
continues to process the packet as a SEAL data packet.</t>
<t>Next, if the packet arrived unfragmented and the SEAL header has
U==1, the ETE sends an SPTB message back to the ITE with MTU=0 (see:
Section 4.6.1.1).</t>
<t>Next, if the SEAL header has P==1 the ETE discards the (NULL)
packet.</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 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 4.4.3 above. In this
process, the packet remains within the tunnel interface (i.e., it
does not exit and then re-enter the interface); hence, the packet is
not discarded if the LEVEL field in the SEAL header contains the
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"></xref>. As
for ICMPv6, each SCMP message includes a 4-byte header and a
variable-length body. The TE encapsulates the SCMP message in a SEAL
header and outer headers as shown in <xref
target="scmpencaps"></xref>:</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"></xref>:</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 inner packet within the invoking |
~ SEAL data packet as possible without the SCMP ~
| packet exceeding 576 bytes (*) |
(*) also known as the "packet-in-error"]]></artwork>
</figure>The error message includes the 4 byte SCMP message
header, followed by a 4 byte Type-Specific Data field, followed by
the leading portion of the inner packet within the invoking SEAL
data packet (i.e., beginning immediately after the SEAL header) as
the "packet-in-error". The packet-in-error includes as much of the
inner packet as possible extending to a length that would not cause
the entire SCMP packet following outer encapsulation to exceed 576
bytes.</t>
<t>When the ETE processes a SEAL data packet for which the ICVs are
correct but an error must be returned, it prepares an SCMP error
message as shown in <xref target="control2"></xref>. The ETE sets
the Type and Code fields to the same values that would appear in the
corresponding ICMPv6 message and calculates the Checksum beginning
with the SCMP message header and continuing to the end of the
message. (When calculating the Checksum, the TE sets the Checksum
field itself to 0.)</t>
<t>The ETE next encapsulates the SCMP message in the requisite SEAL
header, outer headers and SEAL trailer as shown in <xref
target="scmpencaps"></xref>. During encapsulation, the ETE sets the
outer destination address/port numbers of the SCMP packet to the
outer source address/port numbers of the original SEAL data packet
and sets the outer source address/port numbers to its own outer
address/port numbers.</t>
<t>The ETE then sets (C=1; R=0; T=0; U=0; Z=0) in the SEAL header,
then sets NEXTHDR, PREFLEN, LINK_ID, LEVEL, and PKT_ID to the same
values that appeared in the SEAL header of the data packet. If the
SEAL data packet header had P==1, the ETE also copies the PREFIX
field from the data packet into the SEAL header and sets P=1;
otherwise, it sets P=0.</t>
<t>The ETE then calculates and sets the ICV1 field the same as
specified for SEAL data packet encapsulation in Section 4.4.4. Next,
the ETE encapsulates the SCMP message in the requisite outer
encapsulations and sends the resulting SCMP packet to the ITE the
same as specified for SEAL data packets in Section 4.4.5.</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 ETE generates an SCMP "Packet Too Big" (SPTB) message when
it receives a SEAL data packet that arrived as multiple outer IPv4
fragments and for which the reassembled inner packet would be
larger than 1280 bytes. The ETE also generates an SPTB when it
receives the fragments of a fragmented IPv4-encapsulated SEAL data
packet with T==0 in the SEAL header but that following reassembly
would be larger than (128-SHLEN-UHLEN) bytes but no larger than
1280 bytes. The ETE prepares the SPTB message the same as for the
corresponding ICMPv6 PTB message, and writes the length of the
largest outer IP fragment received minus HLEN in the MTU field of
the message.</t>
<t>The ETE also generates an SPTB message when it accepts a SEAL
protocol data packet which did not undergo IP fragmentation and
with U==1 in the SEAL header. The ETE prepares the SPTB message
the same as above, except that it writes the value 0 in the MTU
field.</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. IN THIS CASE ALONE, the ETE prepares the packet-in-error
beginning with the SEAL header instead of beginning immediately
after 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"></xref>.</t>
</section>
</section>
<section title="Processing SCMP Error Messages">
<t>An ITE may receive SCMP messages after sending packets to an ETE.
The ITE first verifies that the outer addresses of the SCMP packet
are correct, and that the PKT_ID is within its window of values for
this ETE. The ITE next verifies that the SEAL header fields are set
correctly as specified in Section 4.6.1. The ITE then verifies the
ICV1 value. If the outer addresses, SEAL header information and/or
ICV1 value are incorrect, the ITE silently discards the message;
otherwise, it processes the message as follows:</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 inner packet (see: Section 4.6.1.1). For IP SPTB
messages with MTU==0, the ITE processes the message as
confirmation that the ETE received an unfragmented SEAL data
packet with U==1 in the SEAL header. The ITE then discards the
message.</t>
<t>For IPv4 SPTB messages with MTU != 0, the ITE instead processes
the message as an indication of a packet size limitation as
follows. The ITE first determines the inner packet length by
subtracting SHLEN from the length field in the UDP header within
the packet-in-error (and also subtracting the length of the SEAL
trailer when T=1). If the inner packet is no larger than 1280
bytes, the ITE sets USE_TRAILER=TRUE. If the inner packet is
larger than 1280 bytes, the ITE instead examines the SPTB message
MTU field. If the MTU value is not substantially less than
(1500-HLEN), the value is likely to reflect the true MTU of the
restricting link on the path to the ETE; otherwise, a router on
the path may be generating runt fragments.</t>
<t>In that case, the ITE can consult a plateau table (e.g., as
described in <xref target="RFC1191"></xref>) to rewrite the MTU
value to a reduced size. For example, if the ITE receives an IPv4
SPTB message with MTU==256 and inner packet length 1500, it can
rewrite the MTU to 1450. If the ITE subsequently receives an IPv4
SPTB message with MTU==256 and inner packet length 1450, it can
rewrite the MTU to 1400, etc. If the ITE is performing stateful
MTU determination for this ETE link path, it then writes the new
MTU value in PATH_MTU.</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 ITE rewrites the SEAL
header fields with values corresponding to the previous hop and
recalculates the ICV1 values using the ICV calculation parameters
associated with the previous hop. Next, the ITE replaces the
SPTB's outer headers with headers of the appropriate protocol
version and fills in the header fields as specified in Sections
5.5.4-5.5.6 of <xref target="I-D.templin-intarea-vet"></xref>,
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.</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 4.4.7) and sends the
resulting transcribed message to the original source. The ITE then
discards the SPTB message.</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 4.6.2.1.</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"></xref>.</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"></xref> 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 per <xref
target="RFC4821"></xref>) even if the subnetwork is using SEAL.</t>
</section>
<section title="Router Requirements">
<t>Routers within the subnetwork are expected to observe the router
requirements found in the normative references, including the
implementation of IP fragmentation and reassembly <xref
target="RFC1812"></xref><xref target="RFC2460"></xref> as well as the
generation of ICMP messages <xref target="RFC0792"></xref><xref
target="RFC4443"></xref>.</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 any ITEs/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 4.4.7, 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>
</section>
<section title="IANA Considerations">
<t>The IANA is instructed to allocate an IP protocol number for
'SEAL_PROTO' in the 'protocol-numbers' registry.</t>
<t>The IANA is instructed to allocate a Well-Known Port number for
'SEAL_PORT' in the 'port-numbers' registry.</t>
<t>The IANA is instructed to establish a "SEAL Protocol" registry to
record SEAL Version values. This registry should be initialized to
include the initial SEAL Version number, i.e., Version 0.</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 in a manner that is compatible with securing mechanisms
such as TLS/SSL <xref target="RFC5246"></xref>.</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 ICVs, PKT_ID, as well as the inner headers of the
packet-in-error, provide mitigation for the ETE to detect and discard
SEAL segments with spoofed source addresses.</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 is 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"></xref>.</t>
</section>
<section title="Related Work">
<t>Section 3.1.7 of <xref target="RFC2764"></xref> provides a high-level
sketch for supporting large tunnel MTUs via a tunnel-level segmentation
and reassembly capability to avoid IP level fragmentation. This
capability was implemented in the first edition of SEAL, but is now
deprecated.</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"></xref> 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><xref
target="RFC4301"></xref> 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"></xref> uses similar means for providing
full message authentication and integrity.</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 path MTU discovery mechanism,
appears in Appendix D of this document.</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 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.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.1191"?>
<?rfc include="reference.RFC.1981"?>
<?rfc include="reference.RFC.2003"?>
<?rfc include="reference.RFC.2473"?>
<?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.4380"?>
<?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.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.I-D.ietf-intarea-ipv4-id-update"?>
<?rfc include="reference.I-D.templin-aero"?>
<?rfc include="reference.I-D.massar-v6ops-ayiya"?>
<reference anchor="FRAG">
<front>
<title>Fragmentation Considered Harmful</title>
<author fullname="Christopher Kent" initials="C" surname="Kent">
<organization></organization>
</author>
<author fullname="Jeffrey Mogul" initials="J" surname="Mogul">
<organization></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></organization>
</author>
<author fullname="David Moore" initials="D" surname="Moore">
<organization></organization>
</author>
<author fullname="k claffy" initials="k" surname="claffy">
<organization></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></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></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></organization>
</author>
<author fullname="Mark Allman" initials="M" surname="Allman">
<organization></organization>
</author>
<author fullname="Sally Floyd" initials="S" surname="Floyd">
<organization></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></organization>
</author>
<author fullname="Kenjiro Cho" initials="K" surname="Cho">
<organization></organization>
</author>
<author fullname="Bill Owens" initials="B" surname="Owens">
<organization></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></organization>
</author>
<author fullname="Ben Stasiewicz" initials="B" surname="Stasiewicz">
<organization></organization>
</author>
<date month="November" year="2010" />
</front>
</reference>
</references>
<section title="Reliability">
<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"></xref> to increase packet delivery ratios, while links
with much lower BERs typically omit such mechanisms. Since SEAL tunnels
may traverse arbitrarily-long paths over links of various types that are
already either performing or omitting ARQ as appropriate, it would
therefore often be inefficient to also require the tunnel endpoints to
also perform ARQ.</t>
</section>
<section title="Integrity">
<t>The SEAL header includes an ICV field that covers the SEAL header and
at least the inner packet headers. This provides for header integrity
verification on a segment-by-segment basis for a segmented
re-encapsulating tunnel path. When IPv4 fragmentation is needed, the
SEAL packet also contains a trailer with a secondary ICV that covers the
remainder of the packet.</t>
<t>Fragmentation and reassembly schemes must consider packet-splicing
errors, e.g., when two fragments from the same packet are concatenated
incorrectly, when a fragment from packet X is reassembled with fragments
from packet Y, etc. The primary sources of such errors include
implementation bugs and wrapping IPv4 ID fields.</t>
<t>In terms of wrapping ID fields, the IPv4 16-bit ID field can wrap
with only 64K packets with the same (src, dst, protocol)-tuple alive in
the system at a given time <xref target="RFC4963"></xref> increasing the
likelihood of reassembly mis-associations</t>
<t>When reassembly is unavoidable, SEAL provides an extended ICV to
detect reassembly mis-associations for packets no larger than 1280 bytes
and also discards any reassembled packets larger than 1280 bytes.</t>
</section>
<section title="Transport Mode">
<t>SEAL can also be used in "transport-mode", e.g., when the inner layer
comprises upper-layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL (e.g., by
inserting a 'SEAL_OPTION' TCP option during connection establishment)
for the carriage of protocol data encapsulated as IP/SEAL/TCP. In this
sense, the "subnetwork" becomes the entire end-to-end path between the
TCP peers and may potentially span the entire Internet.</t>
<t>If both TCPs agree on the use of SEAL, their protocol messages will
be carried as IP/SEAL/TCP and the connection will be serviced by the
SEAL protocol using TCP (instead of an encapsulating tunnel endpoint) as
the transport layer protocol. The SEAL protocol for transport mode
otherwise observes the same specifications as for Section 4.</t>
</section>
<section title="Historic Evolution of PMTUD">
<t>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 <xref target="TCP-IP"></xref>. The
discussion that followed provided significant reference material for
[FRAG]. An IETF Path MTU Discovery Working Group <xref
target="MTUDWG"></xref> was formed in late 1989 with charter to produce
an RFC. Several variations on a very few basic proposals were
entertained, including:</t>
<t><list style="numbers">
<t>Routers record the PMTUD estimate in ICMP-like path probe
messages (proposed in [FRAG] and later <xref
target="RFC1063"></xref>)</t>
<t>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)</t>
<t>A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw
RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)</t>
<t>Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
1990)</t>
<t>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 <xref
target="RFC1191"></xref> by Mogul and Deering).</t>
</list></t>
<t>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 <xref target="RFC1191"></xref> and later <xref
target="RFC1981"></xref>.</t>
<t>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:</t>
<t><list style="symbols">
<t>routers upgrade their software faster than hosts</t>
<t>PCs could not reassemble fragmented packets</t>
<t>Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets</t>
<t>Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)</t>
<t>the 16-bit IP_ID field could wrap around and disrupt reassembly
at high packet arrival rates</t>
</list>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.</t>
</section>
</back>
</rfc>
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