One document matched: draft-templin-intarea-seal-42.xml
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<rfc category="info" docName="draft-templin-intarea-seal-42.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="19" month="December" 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, 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 (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 encapsulation headers often include insufficiently provisioned
per-packet identification values. This can present issues for duplicate
packet detection and detection of packet reordering within the
subnetwork. 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 will be informed of the
MTU limitation through IPv6 path MTU discovery <xref
target="RFC1981"></xref>. 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><xref target="FOLK"></xref><xref
target="RFC4963"></xref>. Additionally, classical IPv4 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>.</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"></xref>,
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.</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 Internet at the same time <xref
target="I-D.ietf-intarea-ipv4-id-update"></xref>. (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"></xref>.</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"></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 IPv4 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 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, 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 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>
<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="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="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. Note that, if the ITE's interface connection to the underlying
link assigns multiple IP addresses, each address represents a
separate ETE link path.</t>
<t hangText="inner packet"><vspace />an unencapsulated network layer
protocol packet (e.g., IPv4 <xref target="RFC0791"></xref>, OSI/CLNP
<xref target="RFC0994"></xref>, IPv6 <xref target="RFC2460"></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>
</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>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>
<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 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><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>
<t>In many aspects, SEAL also very closely resembles the Generic Routing
Encapsulation (GRE) framework <xref target="RFC1701"></xref>. SEAL can
therefore be applied in the same use cases that are traditionally
addressed by GRE, and can also provide additional capabilities as
described in this document.</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 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 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 certain cases) 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 ~
| | --> | |
+--------------------+ +----------+---------+
. 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, 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}. In that case, the UDP
header is seen as an "other outer header" as depicted in <xref
target="encaps1"></xref>.</t>
<t>In certain cases, the ITE also appends a 16-bit trailer at the end
of the SEAL packet. In that case, the trailer is added after the final
byte of the encapsulated packet and need not be aligned on an even
word boundary.</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|A|R|I|V|T| NEXTHDR | PREFLEN | 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. 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="I (1)"><vspace />the "Identification Included"
bit.</t>
<t hangText="V (1)"><vspace />the "ICV included" bit.</t>
<t hangText="T (1)"><vspace />the "Trailer included" bit for IPv4
ETE link paths. Reserved for future use for IPv6 ETE link
paths.</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
packet.</t>
<t hangText="LINK_ID (5)"><vspace />a 5-bit link identification
value, set to a unique value by the ITE for each link path over
which it will send encapsulated packets to the ETE (up to 32 link
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 ETE link 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
monotonically-incrementing 32-bit value for each SEAL packet
transmitted to this ETE, beginning with 0.</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>When (T==1), the SEAL encapsulation also includes a 16-bit
trailing integrity check vector ("ICV2") formatted as follows:</t>
<t><figure anchor="trailer" title="SEAL Trailer Format">
<artwork><![CDATA[ 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV2(A) | ICV2(B) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure><list style="hanging">
<t hangText="ICV2 (16)"><vspace />a 16-bit ICV2 value; present
only when T==1. The value is calculated by the 8-bit Fletcher's
algorithm given in <xref target="RFC1146"></xref>, where the "A"
result is placed in the most significant byte and the "B" result
is placed in the least significant byte.</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 can instead
set a finite MTU on tunnel *host* interfaces.</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 ETE link 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
neighbor; 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>For each ETE link path, the ITE must also account for
encapsulation header lengths. The ITE therefore maintains the per
ETE link path constant values "SHLEN" set to 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 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 ETE link paths that use IPv4 as the outer encapsulation
protocol, the ITE also maintains the variables "USE_DF" set to
FALSE, and "USE_TRAIL" set to TRUE if PATH_MTU is less than MIN_MTU
(otherwise set to FALSE).</t>
<t>The ITE may instead maintain the packet sizing variables and
constants 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 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 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 should fragment the packet into inner fragments no larger
than 512 bytes unless it has operational assurance that the path can
support a larger inner fragment size. 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). (NB: 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.)</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 SEAL header includes an
Identification field when USE_ID is TRUE, followed by an ICV field
when USE_ICV is TRUE. When USE_TRAIL is TRUE, the ITE also leaves
room for a trailing ICV2 field at the end of the packet.</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 4.4.6)
or for stateful MTU determination (see Section 4.4.9). Otherwise,
the ITE sets A=0.</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>The ITE then sets LINK_ID to the value assigned to the underlying
ETE link 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"></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>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>The ITE then sets the (I, V, T) flags and initializes any header
extension fields as follows:</t>
<t><list style="symbols">
<t>When USE_ID is TRUE, the ITE sets I=1 and writes a
monotonically-increasing integer value for this ETE in the
Identification field (beginning with 0 in the first packet
transmitted). Otherwise, the ITE sets I=0.</t>
<t>When USE_ICV is TRUE, the ITE sets V=1 and initializes the
ICV field to 0; otherwise, it sets V=0.</t>
<t>When USE_TRAIL is TRUE, the ITE sets T=1; otherwise, it sets
T=0.</t>
</list>When USE_TRAIL is TRUE, the next calculates the trailing
ICV2 value using the 8-bit Fletcher checksum algorithm given in
Appendix I of <xref target="RFC1146"></xref>. Beginning with the
SEAL header, the ITE calculates the checksum over the entire packet
then places the "A" result in the first byte of the trailing ICV2
field and places the "B" result in the second byte.</t>
<t>When USE_ICV is TRUE, the ITE then 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. (If the packet contains fewer than 128 bytes, the ITE
does not include the trailing ICV2 field (if present) in the ICV
calculation.)</t>
<t>The ITE then adds the outer encapsulating headers 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 transport (when necessary) and IP
layer headers. When a transport layer header is included, the ITE
writes the port number for SEAL in the transport destination service
port field and writes the protocol number of the transport protocol
in the outer IP header protocol field. Otherwise, the ITE writes the
protocol number for SEAL in the outer IP header protocol field.</t>
<t>The ITE then sets the other fields of the outer transport and IP
layer headers as specified in Sections 5.5.4 and 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 re-encapsulation procedures specified in Section 5.5.6
of <xref target="I-D.templin-intarea-vet"></xref>.</t>
<t>For IPv4 ETE link paths, if USE_DF is 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. (For IPv6 ETE link paths, 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>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 an ordinary router within the
subnetwork or from another ITE on the path to the ETE (i.e., in case
of nested encapsulations). 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 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 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.)</t>
<t>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 whether or not
stateful MTU determination is used (and for IPv4 also sets
(USE_TRAIL=TRUE; USE_DF=FALSE)), 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 ETE link paths, 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"></xref>, 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 1280 bytes for testing purposes so that the
reassembled packet will not be discarded by the ETE. The ITE can
also construct a NULL probe packet instead of using ordinary SEAL
data packets.</t>
<t>To generate a NULL probe packet, the ITE creates a packet buffer
beginning with the same outer headers, SEAL header and an inner
network layer header that would appear in an ordinary data packet.
The ITE writes source address taken from the ITE's claimed prefix
and a NULL destination address in the inner network layer header,
then pads the packet with random data to a length that is at least
128 bytes but no more than 1280 bytes.</t>
<t>The ITE then sets (C=0; R=0; T=0) in the SEAL header of the probe
packet, writes the length of the ITE's claimed prefix in the PREFLEN
field 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 ETE link 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 ETE link 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 ETE link path correctly supports
fragmentation; otherwise, the ITE sets PATH_MTU=MIN_MTU and sets
(USE_TRAIL=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. 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 A=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 ETE link paths, if the largest successful probe is no
larger than MIN_MTU the ITE then sets (USE_TRAIL=TRUE;
USE_DF=FALSE); otherwise, the ITE sets (USE_TRAIL=FALSE;
USE_DF=TRUE).</t>
</section>
<section title="Detecting Path MTU Changes">
<t>When stateful determination is used, the ITE can periodically
reset PATH_MTU to the MTU of the underlying link and/or re-probe the
path to determine whether PATH_MTU 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 ETE link paths, when the path correctly implements
fragmentation and USE_TRAIL is TRUE, the ITE can periodically reset
USE_TRAIL=FALSE to determine whether the path still requires a
trailing ICV2 field. 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_TRAIL=TRUE.</t>
</section>
</section>
<section title="ETE Specification">
<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 ETE link 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 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. The ETE then submits the
(fully- or partially-reassembled) packet for SEAL decapsulation as
specified in Section 4.5.3.</t>
</section>
<section title="Decapsulation and Re-Encapsulation">
<t>For each SEAL packet submitted 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. For IPv4, if T==1 and the packet is no larger than 1280
bytes the ITE next verifies the ICV2 value and silently discards the
packet if the value is incorrect. (Note that the ITE must verify the
ICV2 value even if the packet arrives unfragmented in case a
middlebox is performing reassembly.)</t>
<t>Next, if the packet arrived as multiple IPv4 fragments and T==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
4.6.1.1).</t>
<t>Next, if the packet arrived as multiple IP fragments and the
inner packet is larger than 1280 bytes, the ETE then 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 4.6.1.2).</t>
<t>Next, if the SEAL header has C==1, the ETE processes the packet
as an SCMP packet as specified in Section 4.6.2. Otherwise, the ETE
continues to process the packet as a SEAL data packet.</t>
<t>Next, if the SEAL header has A==1, the ETE sends an SPTB message
back to the ITE with MTU=0 (see: Section 4.6.1.1).</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 destination address of the packet is
NULL 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 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 32-bit 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 invoking SEAL data packet as possible |
~ (beginning with the SEAL header) without the SCMP ~
| packet exceeding 576 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 576 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"></xref>. The ETE sets the Type and Code fields to
the same values that would appear in the corresponding ICMPv6
message <xref target="RFC4443"></xref>, but calculates the Checksum
beginning with the SCMP message header using the algorithm specified
for ICMPv4 in <xref target="RFC0792"></xref>.</t>
<t>The ETE next encapsulates the SCMP message in the requisite SEAL
and outer headers as shown in <xref target="scmpencaps"></xref>.
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; R=0; T=0) in the SEAL header, then
sets I, V, NEXTHDR, PREFLEN, 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
4.4.4.</t>
<t>Finally, the ETE sends the resulting SCMP packet to the ITE the
same as specified for SEAL data packets in Section 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 T==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"></xref>.</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 4.6.1. 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 4.6.1.1). 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 IPv4 SPTB messages with MTU != 0, the ITE instead processes
the message as an indication of a packet size limitation as
follows. If the inner packet is no larger than 1280 bytes, the ITE
sets (USE_TRAIL=TRUE; USE_DF=FALSE). If the inner packet is larger
than 1280 bytes, the ITE instead examines the SPTB message MTU
field. If the MTU value is not less than (MIN_MTU-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 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 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. (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 4.4.7) 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>
</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 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 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>
<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="IANA Considerations">
<t>The IANA is instructed to allocate a System Port number for "SEAL" in
the 'port-numbers' registry for the TCP, UDP, DCCP and SCTP
protocols.</t>
<t>The IANA is further instructed to allocate an IP protocol number for
"SEAL" in the "protocol-numbers" registry.</t>
<t>Considerations for port and protocol number assignments appear in
<xref target="RFC2780"></xref><xref target="RFC5226"></xref><xref
target="RFC6335"></xref>.</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 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"></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
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.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.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.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 be inefficient to require the tunnel endpoints to also perform
ARQ.</t>
</section>
<section title="Integrity">
<t>The SEAL header includes an integrity check field that covers the
SEAL header and at least the inner packet headers (or up to the end of
the packet if IPv4 fragmentation is needed). 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"></xref>. 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 therefore provides
a trailing checksum as a first-pass filter to detect reassembly
mis-associations. Any reassembly mis-associations not detected by the
checksum will very likely be detected later by upper layer
checksums.</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 an unspecified '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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