One document matched: draft-templin-intarea-seal-32.xml
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<rfc category="std" docName="draft-templin-intarea-seal-32.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="18" month="October" 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
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 two primary
functions of IPv4 are to provide for 1) addressing, and 2) a
fragmentation and reassembly capability used to accommodate links with
diverse MTUs. While it is well known that the 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 (e.g., 1Gbps
Ethernet), however, this number may soon 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 are being
configured more and more to discard ICMP error messages coming from
the outside world. This is due in large part to the fact that
malicious spoofing of error messages in the Internet is 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 may be inconvenienced by
excessive ICMP error messages coming from the subnetwork that may be
either suspect or contain insufficient information 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>,
this approach requires tunnels to present a consistent MTU the same as
for ordinary links on the end-to-end path. 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. Due
to these many consideration, 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., IP, 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. SEAL accommodates links with diverse MTUs,
protects against off-path denial-of-service attacks, and can be
configured to enable efficient duplicate packet detection through the
use of a minimal mid-layer encapsulation.</t>
<t>SEAL specifically treats tunnels that traverse the subnetwork as
ordinary links that must support network layer services. As for any
link, tunnels that use SEAL must provide suitable networking services
including best-effort datagram delivery, integrity and consistent
handling of packets of various sizes. As for any link whose media
cannot provide suitable services natively, tunnels that use SEAL
employ link-level adaptation functions to meet the legitimate
expectations of the network layer service. As this is essentially a
link level adaptation, SEAL is therefore permitted to alter packets
within the subnetwork as long as it restores them to their original
form when they exit the subnetwork. The mechanisms described within
this document are designed precisely for this purpose.</t>
<t>SEAL encapsulation provides extended identification fields as well
as a mid-layer segmentation and reassembly capability that allows
simplified cutting and pasting of packets. Moreover, SEAL provides
dynamic mechanisms to ensure a functional path MTU on the path from
the ITE to the ETE. 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"><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"><vspace />a virtual interface
over which an encapsulating border node (host or router) receives
encapsulated packets from the subnetwork.</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 mid-layer or outer
encapsulations are added. Internet protocol numbers that identify
inner packets are found in the IANA Internet Protocol registry <xref
target="RFC3232"></xref>.</t>
<t hangText="mid-layer packet"><vspace />a packet resulting from
adding mid-layer encapsulating headers to an inner packet.</t>
<t hangText="outer IP packet"><vspace />a packet resulting from
adding an outer IP header (and possibly other outer headers) to a
mid-layer 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, IPvX, IPvY"><vspace />used to generically refer to
either IP protocol 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 sum of MHLEN and OHLEN<vspace /></t>
<t>ITE - Ingress Tunnel Endpoint<vspace /></t>
<t>MHLEN - the length of any mid-layer headers and
trailers<vspace /></t>
<t>MRU - Maximum Reassembly Unit<vspace /></t>
<t>MTU - Maximum Transmission Unit<vspace /></t>
<t>OHLEN - the length of any outer encapsulating headers and
trailers<vspace /></t>
<t>S_MRU - SEAL Maximum Reassembly Unit<vspace /></t>
<t>S_MSS - SEAL Maximum Segment Size<vspace /></t>
<t>SCMP - the SEAL Control Message Protocol<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 IPv4 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 it soon became
apparent that 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 a new Internetworking
architecture based on Routing and Addressing in Networks with Global
Enterprise Recursion (RANGER) <xref target="RFC5720"></xref><xref
target="RFC6139"></xref> and the Internet Routing Overlay Network (IRON)
<xref target="RFC6179"></xref>.</t>
<t>SEAL provides a network sublayer for encapsulation of an inner
network layer packet within outer encapsulating headers. For example,
for IPvX in IPvY encapsulation (e.g., as IPv4/SEAL/IPv6), the SEAL
header appears as a subnetwork encapsulation as seen by the inner IP
layer. SEAL can also be used as a sublayer within a transport layer
protocol data payload (e.g., as IPv4/UDP/SEAL/IPv6 similar to Teredo
<xref target="RFC4380"></xref>), 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>SEAL supports a segmentation and reassembly capability for adapting
the network layer to the underlying subnetwork characteristics, where
the Egress Tunnel Endpoint (ETE) determines how much or how little
reassembly it is willing to support. In the limiting case, the ETE can
avoid reassembly altogether and act as a passive observer that simply
informs the Ingress Tunnel Endpoint (ITE) of any MTU limitations and
otherwise discards all packets that arrive as multiple fragments. This
mode is useful for determining an appropriate MTU for tunneling between
performance-critical routers connected to high data rate subnetworks
such as the Internet DFZ, for unidirectional tunneling in which the ETE
is stateless, and for other uses in which reassembly would present too
great of a burden for the routers or end systems.</t>
<t>When the ETE supports reassembly, the tunnel can be used to transport
packets that are too large to traverse the path without fragmentation.
In this mode, the ITE determines the tunnel MTU based on the largest
packet the ETE is capable of reassembling rather than on the MTU of the
smallest link in the path. Therefore, tunnel endpoints that use SEAL can
transport packets that are much larger than the underlying subnetwork
links themselves can carry in a single piece.</t>
<t>SEAL tunnels may be configured over paths that include not only
ordinary physical links, but also virtual links that may include other
tunnels. An example application would be linking two geographically
remote supercomputer centers with large MTU links by configuring a SEAL
tunnel across the Internet. A second example would be support for sub-IP
segmentation over low-end links, i.e., especially over wireless
transmission media such as IEEE 802.15.4, broadcast radio links in
Mobile Ad-hoc Networks (MANETs), Very High Frequency (VHF) civil
aviation data links, etc.</t>
<t>Many other use case examples are anticipated, and will be identified
as further experience is gained.</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
connects an ITE to one or more ETE "neighbors" via tunneling across an
underlying enterprise network, or "subnetwork". The tunnel neighbor
relationship between the ITE and each ETE may be either unidirectional
or bidirectional.</t>
<t>A unidirectional tunnel neighbor relationship allows the near end
TE to send data packets forward to the far end TE, while the far end
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 for
SEAL are discussed in <xref
target="I-D.templin-intarea-vet"></xref>.</t>
</section>
<section title="SEAL Model of Operation">
<t>SEAL supports a multi-level segmentation and reassembly capability
for the transmission of unicast and multicast packets across an
underlying IP subnetwork which may include heterogeneous links. First,
the ITE can use IPv4 fragmentation to fragment inner IPv4 packets
before SEAL encapsulation if necessary. Secondly, the SEAL layer
itself provides a simple cutting-and-pasting capability for mid-layer
packets that can be used to avoid IP fragmentation on the outer
packet. Finally, ordinary IP fragmentation is permitted on the outer
packet after SEAL encapsulation, and allows the TEs to detect and tune
out any in-the-network fragmentation.</t>
<t>SEAL-enabled ITEs encapsulate each inner packet in any mid-layer
headers and trailers, segment the resulting mid-layer packet into
multiple segments if necessary, then append a SEAL header and any
outer encapsulations to each segment. As an example, for IPv6 within
IPv4 encapsulation a single-segment inner IPv6 packet encapsulated in
any mid-layer headers and trailers, followed by the SEAL header,
followed by any outer headers and trailers, followed by an outer IPv4
header would appear as shown in <xref target="encaps1"></xref>:</t>
<t><figure anchor="encaps1"
title="SEAL Encapsulation - Single Segment">
<artwork><![CDATA[ +--------------------+
~ outer IPv4 header ~
+--------------------+
I ~ other outer hdrs ~
n +--------------------+
n ~ SEAL Header ~
e +--------------------+ +--------------------+
r ~ mid-layer headers ~ ~ mid-layer headers ~
+--------------------+ +--------------------+
I --> | | --> | |
P --> ~ inner IPv6 ~ --> ~ inner IPv6 ~
v --> ~ Packet ~ --> ~ Packet ~
6 --> | | --> | |
+--------------------+ +--------------------+
P ~ mid-layer trailers ~ ~ mid-layer trailers ~
a +--------------------+ +--------------------+
c ~ outer trailers ~
k Mid-layer packet +--------------------+
e after mid-layer encaps.
t Outer IPv4 packet
after SEAL and outer encaps.]]></artwork>
</figure></t>
<t>As a second example, for IPv4 within IPv6 encapsulation an inner
IPv4 packet requiring three SEAL segments would appear as three
separate outer IPv6 packets, where the mid-layer headers are carried
only in segment 0 and the mid-layer trailers are carried in segment 2
as shown in <xref target="encaps2"></xref>:</t>
<figure anchor="encaps2"
title="SEAL Encapsulation - Multiple Segments">
<artwork><![CDATA[+------------------+ +------------------+ +------------------+
~ outer IPv6 hdr ~ ~ outer IPv6 hdr ~ ~ outer IPv6 hdr ~
+------------------+ +------------------+ +------------------+
~ other outer hdrs ~ ~ other outer hdrs ~ ~ other outer hdrs ~
+------------------+ +------------------+ +------------------+
~ SEAL hdr (SEG=0) ~ ~ SEAL hdr (SEG=1) ~ ~ SEAL hdr (SEG=2) ~
+------------------+ +------------------+ +------------------+
| mid-layer hdrs | | inner IPv4 | | inner IPv4 Packet|
~ plus inner IPv4 ~ ~ Packet Segment ~ ~ Segment plus ~
~ Packet Segment ~ ~ (Length = L) ~ ~ mid-layer trails ~
| (Length = L) | | | | (Len may be !=L) |
+------------------+ +------------------+ +------------------+
~ outer trailers ~ ~ outer trailers ~ ~ outer trailers ~
+------------------+ +------------------+ +------------------+
Segment 0 (includes Segment 1 (no mid- Segment 2 (includes
mid-layer hdrs) layer encaps) mid-layer trails)]]></artwork>
</figure>
<t></t>
<t>The ITE inserts the SEAL header according to the specific tunneling
protocol. Examples include the following:<list style="symbols">
<t>For simple encapsulation of an inner network layer packet
within an outer IPvX 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
IPvX headers as: IPvX/SEAL/{inner packet}.</t>
<t>For encapsulations over transports such as UDP (e.g., in the
same manner as for <xref target="RFC4380"></xref>), the ITE
inserts the SEAL header between the outer transport layer header
and the mid-layer packet, e.g., as IPvX/UDP/SEAL/{mid-layer
packet}. (Here, the UDP header is seen as an "other outer header"
as depicted in <xref target="encaps2"></xref>.)</t>
</list>The SEAL header includes per-neighbor, per-link and
per-packet identification values which routers within the subnetwork
can use for duplicate packet detection and both TEs can use for SEAL
segmentation/reassembly.</t>
<t>The following sections specify the SEAL header format and
SEAL-related operations of the ITE and ETE.</t>
</section>
<section title="SEAL Header 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|I|F|M|R| NEXTHDR/SEG | LINK_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NBR_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID (when necessary) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>where the header fields are defined as:</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="I (1)"><vspace />the "Identification Field Included"
bit. Set to 1 if the SEAL header includes a 32-bit packet
Identification field (see below); set to 0 otherwise.</t>
<t hangText="F (1)"><vspace />the "First Segment" bit. Set to 1 if
this SEAL packet contains the first segment (i.e., Segment #0) of
a mid-layer packet.</t>
<t hangText="M (1)"><vspace />the "More Segments" bit. Set to 1 if
this SEAL packet contains a non-final segment of a multi-segment
mid-layer packet.</t>
<t hangText="R (1)"><vspace />the "Redirect Requested" bit. Set to
1 by the ITE to inform the ETE that a Redirect should be sent
(see:<xref target="I-D.templin-intarea-vet"> </xref>).</t>
<t hangText="NEXTHDR/SEG (8)">an 8-bit field. When 'F'=1, encodes
the next header Internet Protocol number the same as for the IPv4
protocol and IPv6 next header fields. When 'F'=0, encodes a
segment number of a multi-segment mid-layer packet. (The segment
number 0 is reserved.)</t>
<t hangText="LINK_ID (16)"><vspace />a 16-bit link identification
value, set to a unique value by the ITE for each underlying link
over which it will send encapsulated packets to the ETE. Used as a
neighbor selector adjunct for the NBR_ID.</t>
<t hangText="NBR_ID (32)"><vspace />a 32-bit neighbor
identification value. Set to a random value by the ETE in an
initial exchange with the ITE, and used as a tunnel neighbor
selector in conjunction with the LINK_ID.</t>
<t hangText="PKT_ID (32)"><vspace />a 32-bit per-packet
identification field. Present only when the I bit is set to 1 (see
above).</t>
</list>Setting of the various bits and fields of the SEAL header is
specified in the following sections.</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 ETEs 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 ITE can alternatively set an indefinite MTU on the tunnel
interface such that all inner packets are admitted into the
interface without regard to 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.</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 must
contain as much of the invoking packet as possible without the
entire message exceeding the network layer minimum MTU (e.g., 576
bytes for IPv4, 1280 bytes for IPv6, etc.). For SEAL packets that
would undergo recursive encapsulation, however, the inner layer must
send a SEAL PTB message instead of a PTB of the inner network layer
(see: Section 4.4.3).</t>
<t>In light of the above considerations, the ITE SHOULD configure an
indefinite MTU on tunnel *router* interfaces, since these may be
required to carry recursively-nested SEAL encapsulations. The ITE
MAY instead set a finite MTU on tunnel *host* interfaces. Any
necessary tunnel adaptations are then performed by the SEAL layer
within the tunnel interface as described in the following
sections.</t>
</section>
<section title="Tunnel Interface Soft State">
<t>The ITE maintains per-ETE soft state within the tunnel interface,
e.g., in a neighbor cache. The soft state includes the
following:</t>
<t><list style="symbols">
<t>a Mid-layer Header Length (MHLEN); set to the length of any
mid-layer encapsulation headers and trailers that must be added
before SEAL segmentation.</t>
<t>an Outer Header Length (OHLEN); set to the length of the
outer IP, SEAL and other outer encapsulation headers and
trailers.</t>
<t>a total Header Length (HLEN); set to MHLEN plus OHLEN.</t>
<t>a SEAL Maximum Segment Size (S_MSS). The ITE initializes
S_MSS to the minimum MTU of the underlying interfaces if the
underlying interface MTUs can be determined (otherwise, the ITE
initializes S_MSS to "infinity"). The ITE decreases or increases
S_MSS based on any SCMP "Packet Too Big (PTB)" messages received
(see Section 4.6).</t>
<t>a SEAL Maximum Reassembly Unit (S_MRU). If the ITE is not
configured to use SEAL segmentation, it initializes S_MRU to the
constant value 0 and ignores any S_MRU values reported by the
ETE. Otherwise, the ITE initializes S_MRU to "infinity" (i.e.,
the largest possible inner packet size) and decreases or
increases S_MRU based on any SCMP PTB messages received from the
ETE (see Section 4.6). When (S_MRU>(S_MSS*256)), the ITE uses
(S_MSS*256) as the effective S_MRU value.</t>
<t>a NBR_ID value that is coordinated with the ETE and used to
fill the SEAL header field of the same name for packets sent to
this ETE.</t>
<t>one or more LINK_ID values that are coordinated with the ETE
and used to fill the SEAL header field of the same name for
packets sent to this ETE.</t>
<t>for each LINK_ID, an outer IP address (and transport layer
port number when upper layer encapsulation is used) for use as
the destination addresses for each packet sent to this ETE.</t>
<t>a PKT_ID value that is randomly-initialized and
monotonically-incremented for each packet sent to this ETE.</t>
</list>Note that S_MSS and S_MRU include the length of the outer
and mid-layer encapsulating headers and trailers (i.e., HLEN), since
the ETE must retain the headers and trailers during reassembly. Note
also that the ITE maintains S_MSS and S_MRU as 32-bit values such
that inner packets larger than 64KB (e.g., IPv6 jumbograms <xref
target="RFC2675"></xref>) can be accommodated when appropriate for a
given subnetwork.</t>
</section>
<section title="Submitting Packets for Encapsulation">
<t>The ITE prepares each inner packet/fragment admitted into the
tunnel interface for encapsulation according to its length. For IPv4
inner packets with DF=0 in the IPv4 header, if the packet is larger
than S_MSS bytes the ITE fragments it into IPv4 fragments no larger
than S_MSS then submits each fragment for encapsulation separately.
For packets no larger than S_MSS bytes, the ITE instead submits the
unfragmented packet for encapsulation.</t>
<t>For all other inner packets, if the packet is larger than
(MAX(S_MRU, S_MSS) - HLEN), the ITE discards it and sends a PTB
message to the source with an MTU value of (MAX(S_MRU, S_MSS) -
HLEN); otherwise, the ITE submits the packet for encapsulation. The
ITE must include the length of the uncompressed headers and trailers
when calculating HLEN even if the tunnel is using header
compression. The ITE is also permitted to submit inner packets for
encapsulation if they can be accommodated in a single SEAL segment
(i.e., no larger than S_MSS) even if they are larger than the ETE
would be willing to reassemble if fragmented (i.e., larger than
S_MRU) - see: Section 4.5.1.</t>
<t>NB: A stateless MTU discovery capability is possible for paths in
which inner packet headers are transmitted in-the-clear, when the
inner network layer protocol does not permit in-the-network
fragmentation (e.g., IPv6), and when either the ITE or ETE sets
S_MRU to 0. In that case, the ITE submits each packet that is no
larger than the outgoing underlying interface for encapsulation and
statelessly processes any resulting SCMP PTB messages as described
in Section 4.6.2.1.</t>
</section>
<section title="Mid-Layer Encapsulation">
<t>After inner IP fragmentation (if necessary), the ITE next
encapsulates each inner packet/fragment in the MHLEN bytes of any
mid-layer headers and trailers. The ITE then submits the mid-layer
packet for SEAL segmentation and encapsulation.</t>
</section>
<section title="SEAL Segmentation">
<t>If the ITE is configured to use SEAL segmentation, it checks the
length of the resulting packet after mid-layer encapsulation to
determine whether segmentation is needed. If the length of the
resulting mid-layer packet plus OHLEN is larger than S_MSS but no
larger than S_MRU the ITE performs SEAL segmentation by breaking the
mid-layer packet into N segments (N <= 256) that are no larger
than (S_MSS - OHLEN) bytes each.</t>
<t>When the ITE performs SEAL segmentation, it MUST segment the
mid-layer packet such that the first segment includes at least the
mid-layer headers. (When the inner packet header is available
in-the-clear, the first segment MUST also include the inner header.)
Each segment except the final one MUST be of equal length, and the
first byte of each segment MUST begin immediately after the final
byte of the previous segment, i.e., the segments MUST NOT overlap.
The ITE SHOULD generate the smallest number of segments possible,
e.g., it SHOULD NOT generate 6 smaller segments when the packet
could be accommodated with 4 larger segments.</t>
<t>This SEAL segmentation process ignores the fact that the
mid-layer packet may be unfragmentable outside of the subnetwork.
The process is a mid-layer (not an IP layer) operation employed by
the ITE to adapt the mid-layer packet to the subnetwork path
characteristics, and the ETE will restore the packet to its original
form during reassembly. Therefore, the fact that the packet may have
been segmented within the subnetwork is not observable outside of
the subnetwork.</t>
</section>
<section title="SEAL Encapsulation">
<t>Following SEAL segmentation, the ITE next encapsulates each
segment in a SEAL header formatted as specified in Section 4.3.</t>
<t>For the first segment, the ITE then sets F=1, and sets M=1 if
there are more segments or sets M=0 otherwise. The ITE then sets
NEXTHDR to the Internet Protocol number corresponding to the
encapsulated inner packet. For example, the ITE sets NEXTHDR to the
value '4' for encapsulated IPv4 packets <xref
target="RFC2003"></xref>, the value '41' for encapsulated IPv6
packets <xref target="RFC2473"></xref><xref
target="RFC4213"></xref>, the value '50' for encapsulated IPsec/ESP
payloads <xref target="RFC4301"></xref><xref
target="RFC4303"></xref>, the value '80' for encapsulated OSI
packets <xref target="RFC1070"></xref>, etc.</t>
<t>For each non-initial segment of an N-segment mid-layer packet (N
<= 256), the ITE instead sets (F=0; M=1; SEG=1) in the SEAL
header of the first non-initial segment, sets (F=0; M=1; SEG=2) in
the next non-initial segment, etc., and sets (F=0; M=0; SEG=N-1) in
the final segment. (Note that the value SEG=0 is not used, since the
initial segment encodes a NEXTHDR value and not a SEG value.)</t>
<t>For each segment (i.e., both initial and non-initial), the ITE
then sets (C=0; R=0) and also sets A=1 if an explicit
acknowledgement is required (see Section 4.4.9). The ITE then sets
the NBR_ID and LINK_ID fields in order to identify itself to the
ETE.</t>
<t>Finally, for each SEAL segment of a multi-segment SEAL packet,
the ITE sets I=1 and includes the current PKT_ID value in a trailing
32-bit Identification field in the SEAL header of each segment. For
each SEAL packet that will be sent as a single segment, however, the
ITE MAY set I=0 and omit the trailing PKT_ID field. Whether or not
the PKT_ID field was included, the ITE then monotonically increments
the PKT_ID value (modulo 2^32) for the next SEAL packet to be sent
to the ETE. This allows the ETE to determine whether a large number
of SEAL packets have been received since an incomplete reassembly
was initiated.</t>
</section>
<section title="Outer Encapsulation">
<t>Following SEAL encapsulation, the ITE next encapsulates each SEAL
segment in the requisite outer headers and trailers 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). The ITE then writes the outer IP address for
this ETE in the destination address of the outer IP header.</t>
<t>When IPv4 is used as the outer encapsulation layer, the ITE
finally sets the DF flag in the IPv4 header of each segment. If the
path to the ETE correctly implements IP fragmentation (see: Section
4.4.9), the ITE sets DF=0; otherwise, it 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>
</section>
<section title="Sending SEAL Protocol Packets">
<t>Following outer encapsulation, the ITE sends each outer packet
that encapsulates a segment of the same mid-layer packet over the
same underlying link in canonical order, i.e., segment 0 first,
followed by segment 1, etc., and finally segment N-1.</t>
</section>
<section title="Probing Strategy">
<t>When IPv4 is used as the outer encapsulation layer, the ITE can
perform a qualification exchange over each underlying link to
determine whether each subnetwork path to the ETE correctly
implements IPv4 fragmentation. This procedure could be employed,
e.g., to determine whether there are any 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>To perform this qualification, the ITE prepares a probe packet
that is no larger than 576 bytes (e.g., a NULL packet with A=1 and
NEXTHDR="No Next Header" <xref target="RFC2460"></xref> in the SEAL
header), then splits the packet into two outer IPv4 fragments and
sends both fragments to the ETE over the same underlying link. If
the ETE returns an SCMP PTB message with Code=0 (see Section
4.6.1.1), then the subnetwork path correctly implements IPv4
fragmentation. If the ETE returns an SCMP PTB message with Code=2,
however, then a middlebox in the subnetwork is reassembling the IPv4
fragments before they are delivered to the ETE (i.e., in violation
of <xref target="RFC1812"></xref>).</t>
<t>In addition to any control plane probing, all SEAL encapsulated
data packets sent by the ITE are considered implicit probes. SEAL
encapsulated packets that use IPv4 as the outer layer of
encapsulation with DF=0 will elicit SCMP PTB messages from the ETE
if any IPv4 fragmentation occurs in the path. SEAL encapsulated
packets that use either IPv6 or IPv4 with DF=1 as the outer layer of
encapsulation may be dropped by a router on the path to the ETE
which will also return an ICMP PTB message to the ITE. If the
message includes enough information (see Section 4.4.10), the ITE
can then use the (NBR_ID, LINK_ID, PKT_ID)-tuple along with the
destination addresses within the packet-in-error to determine
whether the PTB message corresponds to one of its recent packet
transmissions.</t>
<t>The ITE should also send explicit probes, periodically, to verify
that the ETE is still reachable. The ITE sets A=1 in the SEAL header
of a packet to be used as an explicit probe, where the probe can be
either an ordinary data packet segment or a NULL packet (see above).
The probe will elicit an SCMP PTB message with Code=2 from the ETE
as an acknowledgement (see Section 4.6.1.1).</t>
</section>
<section title="Processing ICMP Messages">
<t>When the ITE sends outer IP packets, it may receive ICMP error
messages <xref target="RFC0792"></xref><xref
target="RFC4443"></xref> from either the ETE or routers within the
subnetwork. The ICMP messages include an outer IP header, followed
by an ICMP header, followed by a portion of the outer IP packet that
generated the error (also known as the "packet-in-error"). The ITE
can use the (NBR_ID, LINK_ID, PKT_ID)-tuple along with the source
and destination addresses within the packet-in-error to confirm that
the ICMP message came from either the ETE or an on-path router, and
can use any additional information to determine whether to accept or
discard the message.</t>
<t>The ITE should specifically process raw ICMPv4 Protocol
Unreachable messages and ICMPv6 Parameter Problem messages with Code
"Unrecognized Next Header type encountered" as a hint that the ETE
does not implement the SEAL protocol. The ITE can also process other
raw ICMPv4 messages as a hint that the path to the ETE may be
failing. Specific actions that the ITE may take in these cases are
out of scope.</t>
</section>
<section title="Black Hole Detection">
<t>In some subnetwork paths, ICMP error messages may be lost due to
filtering or may not contain enough information due to a router in
the path not observing the recommendations of <xref
target="RFC1812"></xref>. The ITE can use explicit probing as
described in Section 4.4.9 to determine whether the path to the ETE
is silently dropping packets (also known as a "black hole"). For
example, when the ITE is obliged to set DF=1 in the outer headers of
data packets it should send explicit probe packets, periodically, in
order to detect path MTU increases or decreases.</t>
</section>
</section>
<section title="ETE Specification">
<section title="Reassembly Buffer Requirements">
<t>The ETE SHOULD support the minimum IP-layer reassembly
requirements specified for IPv4 (i.e., 576 bytes <xref
target="RFC1812"></xref>) and IPv6 (i.e., 1500 bytes <xref
target="RFC2460"></xref>). The ETE SHOULD also support SEAL-layer
reassembly for inner packets of at least 1280 bytes in length and
MAY support reassembly for larger inner packets. The ETE records the
SEAL-layer reassembly buffer size in a soft-state variable "S_MRU"
(see: Section 4.5.2).</t>
<t>The ETE may instead omit the reassembly function altogether and
set S_MRU=0, but this may cause ITEs to experience tunnel MTU
underruns over some paths resulting in an unusable link. When
reassembly is supported, the ETE must retain the outer IP, SEAL and
other outer headers and trailers during both IP-layer and SEAL-layer
reassembly for the purpose of associating the fragments/segments of
the same packet, and must also configure a SEAL-layer reassembly
buffer that is no smaller than the IP-layer reassembly buffer.
Hence, the ETE:</t>
<t><list style="symbols">
<t>SHOULD configure an outer IP-layer reassembly buffer of at
least the minimum specified for the outer IP protocol
version.</t>
<t>SHOULD configure a SEAL-layer reassembly buffer S_MRU size of
at least (1280 + HELN) bytes, and</t>
<t>MUST be capable of discarding inner packets that require
IP-layer and/or SEAL-layer reassembly and that are larger than
(S_MRU - HLEN).</t>
</list></t>
<t>The ETE is permitted to accept inner packets that did not undergo
IP-layer and/or SEAL-layer reassembly even if they are larger than
(S_MRU - HELN) bytes. Hence, S_MRU is a maximum *reassembly* size,
and may be less than the largest packet size the ETE is able to
receive when no reassembly is required.</t>
</section>
<section title="Tunnel Interface Soft State">
<t>The ETE maintains a per-interface default S_MRU value to be
applied for all ITEs, and can optionally maintain individual per-ITE
S_MRU values that override the default.</t>
<t>The ETE may also maintain per-ITE soft state to associate
(NBR_ID, LINK_ID, PKT_ID)-tuples with the inner and/or mid-layer
source addresses used by ITEs, e.g., for ingress filtering purposes
(see Section 4.5.5).</t>
</section>
<section title="IP-Layer Reassembly">
<t>The ETE submits unfragmented SEAL protocol IP packets for
SEAL-layer reassembly as specified in Section 4.5.4. The ETE instead
performs standard IP-layer reassembly for multi-fragment SEAL
protocol IP packets as follows.</t>
<t>The ETE should maintain conservative IP-layer reassembly cache
high- and low-water marks. When the size of the reassembly cache
exceeds this high-water mark, the ETE should actively discard
incomplete reassemblies (e.g., using an Active Queue Management
(AQM) strategy) until the size falls below the low-water mark. The
ETE should also actively discard any pending reassemblies that
clearly have no opportunity for completion, e.g., when a
considerable number of new fragments have arrived before a fragment
that completes a pending reassembly arrives. Following successful
IP-layer reassembly, the ETE submits the reassembled packet for
SEAL-layer reassembly as specified in Section 4.5.4.</t>
<t>When the ETE processes the IP first fragment (i.e., one with MF=1
and Offset=0 in the IP header) of a fragmented SEAL packet for which
the (NBR_ID, LINK_ID, PKT_ID)-tuple belongs to a neighboring ITE, it
sends an SCMP PTB message with Code=0 back to the ITE (see Section
4.6.1.1). When the ETE processes an IP fragment that would cause the
reassembled outer packet to be larger than the IP-layer reassembly
buffer following reassembly, it discontinues the reassembly and
discards any further fragments of the same packet.</t>
</section>
<section title="SEAL-Layer Reassembly">
<t>Following IP reassembly (if necessary), the ETE examines each
SEAL data packet (i.e., those with C=0 in the SEAL header) to
determine whether an SCMP error message is required. If the packet
has an incorrect value in the SEAL header the ETE discards the
packet and returns an SCMP "Parameter Problem" message (see Section
4.6.1.2). Next, if the SEAL header has A=1 and the packet did not
arrive as multiple outer IP fragments, the ETE sends an SCMP PTB
message with Code=2 back to the ITE (see Section 4.6.1.1). The ETE
next submits single-segment mid-layer packets for decapsulation and
delivery to upper layers (see Section 4.5.5). The ETE instead
performs SEAL-layer reassembly for multi-segment mid-layer packets
with I=1 in the SEAL header as follows.</t>
<t>The ETE adds each segment of a multi-segment mid-layer packet
with I=1 in the SEAL header to a SEAL-layer pending-reassembly queue
according to the (NBR_ID, LINK_ID, PKT_ID)-tuple found in the SEAL
header. The ETE performs SEAL-layer reassembly through simple
in-order concatenation of the encapsulated segments of the same
mid-layer packet from N consecutive SEAL segments. SEAL-layer
reassembly requires the ETE to maintain a cache of recently received
segments for a hold time that would allow for nominal inter-segment
delays. When a SEAL reassembly times out, the ETE discards the
incomplete reassembly and returns an SCMP "Time Exceeded" message to
the ITE using the cached SEAL first segment as the packet-in-error
(see Section 4.6.1.2). As for IP-layer reassembly, the ETE should
also maintain a conservative reassembly cache high- and low-water
mark and should actively discard any pending reassemblies that
clearly have no opportunity for completion, e.g., when a
considerable number of new SEAL packets have been received before a
packet that completes a pending reassembly has arrived.</t>
<t>If the ETE receives a SEAL packet for which a segment with the
same (NBR_ID, LINK_ID, PKT_ID)-tuple is already in the queue, it
must determine whether to accept the new segment and release the
old, or drop the new segment. If accepting the new segment would
cause an inconsistency with other segments already in the queue
(e.g., differing segment lengths), the ETE drops the segment that is
least likely to complete the reassembly. When the ETE receives a
SEAL segment would cause the size of the reassembled packet to
exceed S_MRU, the ETE schedules the reassembly resources for garbage
collection and sends an SCMP PTB message with Code=1 back to the ITE
(see Section 4.6.1.1).</t>
<t>After all segments are gathered, the ETE reassembles the packet
by concatenating the segments encapsulated in the N consecutive SEAL
packets beginning with the initial segment (i.e., SEG=0) and
followed by any non-initial segments 1 through N-1. That is, for an
N-segment mid-layer packet, reassembly entails the concatenation of
the SEAL-encapsulated packet segments with the same value in the
Identification field and with (F=1, M=1) in the first SEAL header,
followed by (F=0, M=1, SEG=1) in the next SEAL header, followed by
(F=0, M=1, SEG=2), etc., up to (F=0, M=0, SEG=(N-1)) in the final
SEAL header. Following successful SEAL-layer reassembly, the ETE
submits the reassembled mid-layer packet for decapsulation and
delivery to upper layers as specified in Section 4.5.5.</t>
<t>The ETE must not perform SEAL-layer reassembly for multi-segment
mid-layer packets with I=0 in the SEAL header. The ETE instead drops
all segments with (I=0 && (F=0 || M=1)) in the SEAL header
and sends an SCMP Parameter Problem message back to the ITE.</t>
</section>
<section title="Decapsulation and Delivery to Upper Layers">
<t>Following any necessary IP- and SEAL-layer reassembly, the ETE
performs ingress filtering on the mid-layer and/or inner source
addresses if necessary (e.g., via a Reverse-Path Forwarding (RPF)
lookup) to determine whether they are correct for the (NBR_ID,
LINK_ID, PKT_ID)-tuple encoded in the SEAL header. (When the outer
source address and/or port number for the ITE is known, they are
also included in the ingress filtering check.) If ingress filtering
determines that the source addresses are incorrect, the ETE silently
drops the packet.</t>
<t>Following ingress filtering, the ETE performs any mid-layer
transformations on the mid-layer packet and delivers the inner
packet to the upper-layer protocol identified either in the SEAL
NEXTHDR field or the next header field of the mid-layer packet
(i.e., if the packet included mid-layer encapsulations). The ETE
instead silently discards the inner packet if it was a NULL packet
(see Section 4.4.9).</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 basic message types and formats as for the Internet
Control Message Protocol for IPv6 (ICMPv6) <xref
target="RFC4443"></xref>. The SCMP message begins with a 4 byte header
that includes 8-bit Type and Code fields followed by a 16-bit Checksum
field. The header is then followed by a variable-length Message Body,
as shown in <xref target="control1"></xref>:</t>
<t><figure anchor="control1" title="SCMP 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Message Body ~
| |]]></artwork>
</figure>When the TE prepares an SCMP message, it sets the Type and
Code fields to the same values that would appear in the corresponding
ICMPv6 message, then formats the Message Body the same as for the
corresponding ICMPv6 message. The TE then calculates the SCMP message
Checksum the same as specified for ICMPv6, except that it does not
prepend a pseudo-header of the outer IP header. The TE then
encapsulates the SCMP message in the outer headers and trailers 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 ~
+--------------------+ --> +--------------------+
~ outer trailers ~
SCMP Message +--------------------+
before encapsulation
SCMP Packet
after encapsulation]]></artwork>
</figure></t>
<t>The following sections specify the generation and processing of
SCMP messages.</t>
<section title="Generating SCMP Error Messages">
<t>TEs 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 (4/8 bytes) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking SEAL data packet as possible |
~ possible (beginning immediately after the SEAL header) ~
| 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 or 8 byte Type-Specific Data field, followed
by the leading portion of the invoking SEAL data packet (beginning
immediately after the SEAL header) as the "packet-in-error". The
packet-in-error includes as much of the leading portion of the
invoking SEAL data packet as possible beginning immediately after
the SEAL header and extending to a length that would not cause the
entire SCMP packet following outer encapsulation to exceed 576
bytes.</t>
<t>When an ETE processes a SEAL data packet that passes ingress
filtering (see: Section 4.5.5) but for which 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 in
the SCMP header according to the appropriate error message type,
fills out the Type-Specific Data field, includes the
packet-in-error, then calculates and sets the Checksum.</t>
<t>The ETE next encapsulates the SCMP message in the requisite 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 outer source address/port numbers
of the original data packet and sets the outer source address/port
numbers of the SCMP message to the outer destination address/port
numbers of the data packet. The ETE then sets the SCMP packet's SEAL
header fields to the same values that appeared in the SEAL header of
the data packet except that it sets (A=0; C=1). The ETE then sends
the resulting SCMP packet to the source of the original SEAL data
packet.</t>
<t>NB: A simplified implementation of this method entails creating a
copy of the original data packet, inserting the SCMP message header
and Type-Specific Data fields between the SEAL header and mid-layer
headers, truncating the resulting message to 576 bytes, then
rewriting the header fields as described above.</t>
<t>The following sections describe additional considerations for
various SCMP error messages:</t>
<section title="Generating SCMP Packet Too Big (PTB) Messages">
<t>An ETE generates an SCMP "Packet Too Big" (PTB) message under
one of the following cases:</t>
<t><list style="symbols">
<t>Case 0: when it receives the outer first fragment (e.g., an
IP packet with MF=1 and Offset=0 in the outer header) of a
SEAL protocol data packet that arrived as multiple outer
fragments, or:</t>
<t>Case 1: when it receives a SEAL protocol data packet that
encodes a segment that would cause the size of the reassembled
packet to exceed S_MRU, or:</t>
<t>Case 2: when it receives a SEAL protocol data packet with
A=1 in the SEAL header that did not arrive as multiple outer
fragments (i.e., one that does not also match Case 0).</t>
</list></t>
<t>The ETE prepares an SCMP PTB message the same as for the
corresponding ICMPv6 PTB message, except that the "Type-Specific
Data" field is 8 bytes in length and encodes a 4-byte MSS field
followed by a 4-byte MRU field as shown in <xref
target="control2.5"></xref>:</t>
<t><figure anchor="control2.5"
title="SCMP PTB Message Type-Specific Data Field 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSS (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MRU (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>For case 0 above, the ETE writes the value in the outer header
length field into the MSS field of the Type-Specific Data; for
cases 1 and 2, the ETE instead writes the value 0 into the MSS
field. For all cases, the ETE then writes the S_MRU value for this
ITE in the MRU field of the Type-Specific Data (i.e., even if the
value is 0).</t>
<t>The ETE then writes the value '2' in the Type field (i.e., the
same value that would appear in an ICMPv6 PTB message) then writes
the value 0, 1 or 2 in the Code field of the PTB message according
to whether the reason for generating the message was due to the
corresponding case number from the list of cases above. The ETE
finally copies the leading portion of the SEAL data packet
(beginning immediately after the SEAL header) into the
packet-in-error field and calculates the Checksum the same as for
any SCMP message.</t>
<t>NB: Unlike cases 0 and 1 above, case 2 does not necessarily
signify packet loss. Instead, it is a control plane
acknowledgement of a data plane probe. Also, if the ETE generates
a Case 0 SCMP PTB message it MUST NOT also generate a Case 2 PTB
message on behalf of the same SEAL segment.</t>
</section>
<section title="Generating Other SCMP Error Messages">
<t>An ETE generates an SCMP "Destination Unreachable" message
under the same circumstances that an IPv6 system would generate an
ICMPv6 Destination Unreachable message. The ETE formats the SCMP
Destination Unreachable message the same as for ICMPv6 Destination
Unreachable messages, i.e., it writes the value '1' in the Type
field, writes the same Code and Type-Specific Data values, and
includes the leading portion of the SEAL data packet (beginning
immediately after the SEAL header) in the packet-in-error
field.</t>
<t>An ETE generates an SCMP "Parameter Problem" message when it
receives a SEAL packet with an incorrect value in the SEAL header,
and generates an SCMP "Time Exceeded" message when it garbage
collects an incomplete SEAL data packet reassembly. The ETE
formats the messages the same as for the corresponding ICMPv6
messages, i.e., it includes the same Type, Code and Type-Specific
Data values, and includes the leading portion of the SEAL data
packet (beginning immediately after the SEAL header) in the
packet-in-error field.</t>
</section>
</section>
<section title="Processing SCMP Error Messages">
<t>For each SCMP error message it receives, the TE first verifies
that the (NBR_ID, LINK_ID, PKT_ID)-tuple in the SEAL header as well
as the outer addresses of the SCMP packet are correct, then verifies
that the Checksum in the SCMP message header is correct. If the
identifying information and/or checksum are incorrect, the TE
discards the message; otherwise, it processes the message as
follows:</t>
<section title="Processing SCMP PTB Messages">
<t>An ITE may receive an SCMP PTB message with a packet-in-error
containing the leading portion of the mid-layer packet after it
sends a SEAL data packet to an ETE (see: Section 4.6.1.1) . The
ITE first records the value in the PTB message MRU field as the
new S_MRU value for this ETE. If the PTB message has Code=2 in the
SCMP message header, the ITE processes the message as both a
response to an explicit probe request and an indication that the
tunnel neighbor is responsive, i.e., in the same manner implied
for IPv6 Neighbor Unreachability Detection "hints of forward
progress" (see: <xref target="RFC4861"></xref>). If the PTB has
Code=0 /1 in the SCMP header, however, the ITE processes the
message as an indication of a packet size limitation as
follows.</t>
<t>If the PTB has Code =0, the ITE also examines the value in the
PTB message MSS field to determine a new S_MSS value. If the MSS
value is no less than 1280, the ITE records the value as the new
S_MSS. If the MSS value is less than the current S_MSS value and
also less than 1280, however, the ITE can discern that IP
fragmentation is occurring but it cannot determine the true MTU of
the restricting link due to the possibility that a router on the
path is generating runt first fragments. In this latter case, the
ITE searches for a reduced S_MSS value through an iterative
searching strategy that parallels the IPv4 Path MTU Discovery
"plateau table" procedure described in Sections 5 and 7 of <xref
target="RFC1191"></xref>. This searching strategy may entail
multiple iterations in which the ITE sends additional SEAL data
packets using a reduced S_MSS and receives additional SCMP PTB
messages, but the process should quickly converge. During this
process, it is essential that the ITE reduce S_MSS based on the
first SCMP PTB message received under the current S_MSS size, and
refrain from further reducing S_MSS until SCMP PTB messages
pertaining to packets sent under the new S_MSS are received.</t>
<t>For each PTB with Code=0/1, If the inner packet headers in the
packet-in-error are available in-the-clear, the inner packet is
not an IPv4 packet with DF=0, and the inner packet is larger than
(MAX(S_MRU, S_MSS) - HLEN), the ITE can send a transcribed PTB
message appropriate for the inner packet to the original source
with MTU set to (MAX(S_MRU, S_MSS) - HLEN).</t>
<t>NB: A stateless MTU discovery capability is possible for paths
in which inner packet headers are transmitted in-the-clear, when
the inner network layer protocol does not permit in-the-network
fragmentation (e.g., IPv6), and when either the ITE or ETE sets
S_MRU to 0. In that case, when the ITE receives an SCMP PTB with
Code=0 it can send a transcribed PTB message back to the source.
If the SCMP PTB reports an MSS size no less than 1280, the ETE
writes the MSS value into the MTU field of the transcribed PTB
message; otherwise, it consults a plateau table to report a
reduced size as described above. For example, if the ITE receives
an SCMP PTB message with Code=0, MSS=256, and inner header length
1500, it can send a transcribed PTB message listing an MTU of 1400
back to the source. If the ITE subsequently receives an SCMP PTB
message with Code=0, MSS=256, and inner header length 1400, it can
send a transcribed PTB message listing an MTU of 1300 back to the
source, etc.</t>
</section>
<section title="Processing Other SCMP Error Messages">
<t>An ITE may receive other SCMP "Destination Unreachable"
messages with an appropriate code under the same circumstances
that an IPv6 node would receive an ICMPv6 Destination Unreachable
message. The ITE processes the message the same as for the
corresponding ICMPv6 Destination Unreachable messages.</t>
<t>An ITE may receive an SCMP "Parameter Problem" message when the
ETE receives a SEAL packet with an incorrect value in the SEAL
header. The ITE should examine the incorrect SEAL header field
setting to determine whether a different setting should be used in
subsequent packets.</t>
<t>.An ITE may receive an SCMP "Time Exceeded" message when the
ETE garbage collects an incomplete SEAL data packet reassembly.
The ITE should consider the message as an indication of
congestion.</t>
</section>
</section>
<section title="Generating and Processing Other SCMP Message Types">
<t>TEs generate and process other SCMP message types using methods
and procedures outside the scope of this document. For example, SCMP
message types used for tunnel neighbor coordinations are specified
in <xref target="I-D.templin-intarea-vet"></xref>.</t>
</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>SEAL provides robust mechanisms for returning PTB messages; however,
end systems that send unfragmentable IP packets larger than 1500 bytes
are strongly encouraged to implement their own end-to-end MTU assurance,
e.g., using Packetization Layer Path MTU Discovery per <xref
target="RFC4821"></xref>.</t>
</section>
<section title="Router Requirements">
<t>IPv4 routers within the subnetwork are strongly encouraged to
implement IPv4 fragmentation such that the first fragment is the largest
and approximately the size of the underlying link MTU, i.e., they should
avoid generating runt first fragments.</t>
<t>IPv6 routers within the subnetwork are required to generate the
necessary PTB messages when they drop outer IPv6 packets due to an MTU
restriction.</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>Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-overlapping.
This condition is naturally enforced due to the fact that each
consecutive SEAL segment begins at offset 0 with respect to the previous
SEAL segment.</t>
<t>An amplification/reflection attack is possible when an attacker sends
IP first fragments with spoofed source addresses to an ETE in an attempt
to generate a stream of SCMP messages returned to a victim ITE. The
(NBR_ID, LINK_ID, PKT_ID)-tuple as well as the mid-layer and inner
headers of the packet 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 (outside of any IPsec/ESP
encapsulations) 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. As for IPv6 extension headers, the SEAL header is protected
only by L2 integrity checks and is not covered under any L3 integrity
checks.</t>
<t>SCMP messages carry the (NBR_ID, LINK_ID, PKT_ID)-tuple as well as
the mid-layer and inner headers of the packet-in-error. Therefore, when
an ITE receives an SCMP message it can unambiguously associate it with
the SEAL data packet that triggered the error.</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, which is in
part the same approach used by SEAL. SEAL could therefore be considered
as a fully functioned manifestation of the method postulated by that
informational reference.</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; however, the SEAL protocol specifies a
mid-layer segmentation and reassembly capability that is distinct from
both inner and outer fragmentation.</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>The concepts of path MTU determination through the report of
fragmentation and extending the IP Identification field were first
proposed in deliberations of the TCP-IP mailing list and the Path MTU
Discovery Working Group (MTUDWG) during the late 1980's and early
1990's. SEAL supports a report fragmentation capability using bits in an
extension header (the original proposal used a spare bit in the IP
header) and supports ID extension through a 16-bit field in an extension
header (the original proposal used a new IP option). A historical
analysis of the evolution of these concepts, as well as the development
of the eventual path MTU discovery mechanism for IP, appears in Appendix
D of this document.</t>
</section>
<section title="SEAL Advantages over Classical Methods">
<t>The SEAL approach offers a number of distinct advantages over the
classical path MTU discovery methods <xref target="RFC1191"></xref>
<xref target="RFC1981"></xref>:</t>
<t><list style="numbers">
<t>Classical path MTU discovery always results in packet loss when
an MTU restriction is encountered. Using SEAL, IP fragmentation
provides a short-term interim mechanism for ensuring that packets
are delivered while SEAL adjusts its packet sizing parameters.</t>
<t>Classical path MTU may require several iterations of dropping
packets and returning PTB messages until an acceptable path MTU
value is determined. Under normal circumstances, SEAL determines the
correct packet sizing parameters in a single iteration.</t>
<t>Using SEAL, ordinary packets serve as implicit probes without
exposing data to unnecessary loss. SEAL also provides an explicit
probing mode not available in the classic methods.</t>
<t>Using SEAL, ETEs encapsulate SCMP error messages in outer and
mid-layer headers such that packet-filtering network middleboxes
will not filter them the same as for "raw" ICMP messages that may be
generated by an attacker.</t>
<t>The SEAL approach ensures that the tunnel either delivers or
deterministically drops packets according to their size, which is a
required characteristic of any IP link.</t>
<t>Most importantly, all SEAL packets have Identification values
that are sufficiently long to be used for duplicate packet detection
purposes and to associate ICMP error messages with actual packets
sent without requiring per-packet state; hence, SEAL avoids certain
denial-of-service attack vectors open to the classical methods.</t>
</list></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.4303"?>
<?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.RFC.6179"?>
<?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"?>
<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. Since SEAL supports segmentation at a layer below IP,
SEAL therefore presents a case in which the link unit of loss (i.e., a
SEAL segment) is smaller than the end-to-end retransmission unit (e.g.,
a TCP segment).</t>
<t>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 to perform ARQ.</t>
<t>When the SEAL ITE has knowledge that the tunnel will traverse a
subnetwork with non-negligible loss due to, e.g., interference, link
errors, congestion, etc., it can solicit Segment Reports from the ETE
periodically to discover missing segments for retransmission within a
single round-trip time. However, retransmission of missing segments may
require the ITE to maintain considerable state and may also result in
considerable delay variance and packet reordering.</t>
<t>SEAL may also use alternate reliability mechanisms such as Forward
Error Correction (FEC). A simple FEC mechanism may merely entail
gratuitous retransmissions of duplicate data, however more efficient
alternatives are also possible. Basic FEC schemes are discussed in <xref
target="RFC5445"></xref>.</t>
<t>The use of ARQ and FEC mechanisms for improved reliability are for
further study.</t>
</section>
<section title="Integrity">
<t>Each link in the path over which a SEAL tunnel is configured is
responsible for link layer integrity verification for packets that
traverse the link. As such, when a multi-segment SEAL packet with N
segments is reassembled, its segments will have been inspected by N
independent link layer integrity check streams instead of a single
stream that a single segment SEAL packet of the same size would have
received. Intuitively, a reassembled packet subjected to N independent
integrity check streams of shorter-length segments would seem to have
integrity assurance that is no worse than a single-segment packet
subjected to only a single integrity check steam, since the integrity
check strength diminishes in inverse proportion with segment length. In
any case, the link-layer integrity assurance for a multi-segment SEAL
packet is no different than for a multi-fragment IPv6 packet.</t>
<t>Fragmentation and reassembly schemes must also consider
packet-splicing errors, e.g., when two segments from the same packet are
concatenated incorrectly, when a segment from packet X is reassembled
with segments from packet Y, etc. The primary sources of such errors
include implementation bugs and wrapping IP ID fields. In terms of
implementation bugs, the SEAL segmentation and reassembly algorithm is
much simpler than IP fragmentation resulting in simplified
implementations. In terms of wrapping ID fields, when IPv4 is used as
the outer IP protocol, the 16-bit IP 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. However, SEAL ensures that any outer IPv4
fragmentation and reassembly will be short-lived and tuned out as soon
as the ITE receives an SCMP PTB message, and SEAL segmentation and
reassembly uses a much longer Identification field. Therefore,
reassembly mis-associations of IP fragments nor of SEAL segments should
be prohibitively rare.</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 IPv4/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 IPv4/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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