One document matched: draft-templin-intarea-seal-36.xml
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<rfc category="std" docName="draft-templin-intarea-seal-36.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="31" 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, 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>,
they do not excuse tunnels from delivering path MTU discovery feedback
when packets are lost due to size restrictions. Moreover, in current
practice existing tunneling protocols mask the MTU issues by selecting
a "lowest common denominator" MTU that may be much smaller than
necessary for most paths and difficult to change at a later date.
Therefore, a new approach to accommodate tunnels over links with
diverse MTUs is necessary.</t>
</section>
<section title="Approach">
<t>For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected network routing region
and bounded by encapsulating border nodes. Example connected network
routing regions include Mobile Ad hoc Networks (MANETs), enterprise
networks and the global public Internet itself. Subnetwork border
nodes forward unicast and multicast packets over the virtual topology
across multiple IP and/or sub-IP layer forwarding hops that may
introduce packet duplication and/or traverse links with diverse
Maximum Transmission Units (MTUs).</t>
<t>This document introduces a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for tunneling network layer protocols (e.g., 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.</t>
<t>SEAL provides a minimal mid-layer encapsulation that accommodates
links with diverse MTUs and allows routers in the subnetwork to
perform efficient duplicate packet detection. The encapsulation
further ensures packet header integrity, data origin authentication
and anti-replay <xref target="I-D.ietf-savi-framework"></xref><xref
target="RFC4302"></xref>.</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 per-destination path MTU over
the tunnel. This is in contrast to static approaches which avoid MTU
issues by selecting a lowest common denominator MTU value that may be
overly conservative for the vast majority of tunnel paths and
difficult to change even when larger MTUs become available.</t>
<t>The following sections provide the SEAL normative specifications,
while the appendices present non-normative additional
considerations.</t>
</section>
</section>
<section title="Terminology and Requirements">
<t>The following terms are defined within the scope of this
document:</t>
<t><list style="hanging">
<t hangText="subnetwork"><vspace />a virtual topology configured
over a connected network routing region and bounded by encapsulating
border nodes.</t>
<t hangText="Ingress Tunnel Endpoint"><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 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="outer IP packet"><vspace />a packet resulting from
adding an outer IP header (and possibly other outer headers) to a
SEAL-encapsulated inner packet.</t>
<t hangText="packet-in-error"><vspace />the leading portion of an
invoking data packet encapsulated in the body of an error control
message (e.g., an ICMPv4 <xref target="RFC0792"></xref> error
message, an ICMPv6 <xref target="RFC4443"></xref> error message,
etc.).</t>
<t hangText="Packet Too Big (PTB)"><vspace />a control plane message
indicating an MTU restriction (e.g., an ICMPv6 "Packet Too Big"
message <xref target="RFC4443"></xref>, an ICMPv4 "Fragmentation
Needed" message <xref target="RFC0792"></xref>, etc.).</t>
<t hangText="IP"><vspace />used to generically refer to either
Internet Protocol (IP) version, i.e., IPv4 or IPv6.</t>
</list></t>
<t>The following abbreviations correspond to terms used within this
document and/or elsewhere in common Internetworking nomenclature:</t>
<t><list>
<t>DF - the IPv4 header "Don't Fragment" flag <xref
target="RFC0791"></xref><vspace /></t>
<t>ETE - Egress Tunnel Endpoint<vspace /></t>
<t>HLEN - the length of the SEAL header plus outer
headers<vspace /></t>
<t>ICV - Integrity Check Vector<vspace /></t>
<t>ITE - Ingress Tunnel Endpoint<vspace /></t>
<t>MTU - Maximum Transmission Unit<vspace /></t>
<t>SCMP - the SEAL Control Message Protocol<vspace /></t>
<t>SDU - SCMP Destination Unreachable message<vspace /></t>
<t>SNA - SCMP Neighbor Advertisement message<vspace /></t>
<t>SNS - SCMP Neighbor Solicitation message<vspace /></t>
<t>SPP - SCMP Parameter Problem message<vspace /></t>
<t>SPTB - SCMP Packet Too Big message<vspace /></t>
<t>SEAL - Subnetwork Encapsulation and Adaptation
Layer<vspace /></t>
<t>SEAL_PORT - a transport-layer service port number used for
SEAL<vspace /></t>
<t>SEAL_PROTO - an 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 the domain of
applicability also extends to subnetwork abstractions over enterprise
networks, ISP networks, SOHO networks, the global public Internet
itself, and any other connected network routing region. SEAL along with
the Virtual Enterprise Traversal (VET) <xref
target="I-D.templin-intarea-vet"></xref> tunnel virtual interface
abstraction are the functional building blocks for the Internet Routing
Overlay Network (IRON) <xref target="I-D.templin-ironbis"></xref> and
Routing and Addressing in Networks with Global Enterprise Recursion
(RANGER) <xref target="RFC5720"></xref><xref target="RFC6139"></xref>
architectures.</t>
<t>SEAL provides a network sublayer for encapsulation of an inner
network layer packet within outer encapsulating headers. SEAL can also
be used as a sublayer within a transport layer protocol data payload,
where transport layer encapsulation is typically used for Network
Address Translator (NAT) traversal as well as operation over subnetworks
that give preferential treatment to certain "core" Internet protocols
(e.g., TCP, UDP, etc.). The SEAL header is processed the same as for
IPv6 extension headers, i.e., it is not part of the outer IP header but
rather allows for the creation of an arbitrarily extensible chain of
headers in the same way that IPv6 does.</t>
<t>To accommodate MTU diversity, the Egress Tunnel Endpoint (ETE) acts
as a passive observer that simply informs the Ingress Tunnel Endpoint
(ITE) of any packet size limitations. This allows the ITE to return
appropriate path MTU discovery feedback even if the network path between
the ITE and ETE filters ICMP messages.</t>
<t>SEAL further ensures data origin authentication <xref
target="I-D.ietf-savi-framework"></xref>, packet header integrity, and
anti-replay. The SEAL framework resembles a lightweight version of the
IP Security (IPsec) <xref target="RFC4301"></xref> Authentication Header
(AH) <xref target="RFC4302"></xref>, however its purpose is to provide
minimal hop-by-hop authenticating services along a path while leaving
full data integrity, authentication and confidentiality services as an
end-to-end consideration.</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 a
third tunnel, etc. 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 emerges from 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>
</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 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
ITE to send data packets forward to the far end ETE, while the ETE
only returns control messages when necessary. A bidirectional tunnel
neighbor relationship is one over which both TEs can exchange both
data and control messages.</t>
<t>Implications of the VET unidirectional and bidirectional models are
discussed in <xref target="I-D.templin-intarea-vet"></xref>.</t>
</section>
<section title="SEAL Model of Operation">
<t>SEAL-enabled ITEs encapsulate each inner packet in a SEAL header,
any outer header encapsulations, and in some instances a SEAL trailer
as shown in <xref target="encaps1"></xref>:</t>
<t><figure anchor="encaps1" title="SEAL Encapsulation">
<artwork><![CDATA[ +--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
| SEAL Trailer |
+--------------------+
]]></artwork>
</figure></t>
<t>The ITE inserts the SEAL header according to the specific tunneling
protocol. For simple encapsulation of an inner network layer packet
within an outer IP header (e.g., <xref target="RFC1070"></xref><xref
target="RFC2003"></xref><xref target="RFC2473"></xref><xref
target="RFC4213"></xref>, etc.), the ITE inserts the SEAL header
between the inner packet and outer IP headers as: IP/SEAL/{inner
packet}.</t>
<t>For encapsulations over transports such as UDP (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 inner
packet, e.g., as IP/UDP/SEAL/{inner packet}. (Here, the UDP header is
seen as an "other outer header" as depicted in <xref
target="encaps1"></xref>.)</t>
<t>Finally, in some instances the ITE appends a SEAL trailer at the
end of the SEAL packet. In that case, the trailer is added after the
final byte of the encapsulated packet.</t>
</section>
<section title="SEAL Header and Trailer Format">
<t>The SEAL header is formatted as follows:</t>
<t><figure anchor="minimal" title="SEAL Header Format ">
<artwork><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|P|R|T|U|Z| NEXTHDR | PREFLEN | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ PREFIX (when present) ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
]]></artwork>
</figure></t>
<t>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="P (1)"><vspace />the "Prefix Included" bit. Set to 1
if the header includes a Prefix Field. Used for SCMP messages that
do not include a packet-in-error (see: <xref
target="I-D.templin-intarea-vet"></xref>), and for NULL SEAL data
packets used as probes (see: Section 4.4.6).</t>
<t hangText="R (1)"><vspace />the "Redirects Permitted" bit. For
data packets, set to 1 by the ITE to inform the ETE that the
source is accepting Redirects (see:<xref
target="I-D.templin-intarea-vet"> </xref>).</t>
<t hangText="T (1)"><vspace />the "Trailer Included" bit. Set to 1
if the ITE was obliged to include a trailer.</t>
<t hangText="U (1)"><vspace />the "Unfragmented Packet" bit. Set
to 1 by the ITE in SEAL data packets for which it wishes to
receive an explicit acknowledgement from the ETE if the packet
arrived unfragmented.</t>
<t hangText="Z (1)"><vspace />the "Reserved" bit. Must be set to 0
for this version of the SEAL specification.</t>
<t hangText="NEXTHDR (8)">an 8-bit field that encodes the next
header Internet Protocol number the same as for the IPv4 protocol
and IPv6 next header fields.</t>
<t hangText="PREFLEN (8)">an 8-bit field that encodes the length
of the prefix to be applied to the source address of inner
packets.</t>
<t hangText="LINK_ID (5)"><vspace />an 5-bit link identification
value, set to a unique value by the ITE for each underlying link
over which it will send encapsulated packets to ETEs. Up to 32
underlying links are therefore supported.</t>
<t hangText="LEVEL (3)"><vspace />an 3-bit nesting level; use to
limit the number of nestings of tunnels-within-tunnels. Set to an
integer value up to 7 in the initial SEAL encapsulation, and
decremented by 1 for each successive additional SEAL encapsulation
nesting level. Up to 8 levels of nesting are therefore
supported.</t>
<t hangText="PKT_ID (32)"><vspace />a 32-bit per-packet
identification field. Set to a monotonically-incrementing 32-bit
value for each SEAL packet transmitted to this ETE, beginning with
0.</t>
<t hangText="ICV1 (32)"><vspace />a 32-bit header integrity check
value that covers the leading 128 bytes of the packet beginning
with the SEAL header. The value 128 is chosen so that at least the
SEAL header as well as the inner packet network and transport
layer headers are covered by the integrity check.</t>
<t hangText="PREFIX (variable)"><vspace />a variable-length string
of bytes; present only when P=1. The length is found by
determining the equation Len=(Ceiling(PREFLEN / 32) * 4). For
example, if PREFLEN=63, the Prefix field is 8 bytes in length. The
Prefix field encodes an inner network layer prefix beginning with
the most significant bit, and with zero-padding in the least
significant bits when PREFLEN is not properly divisible by 32.</t>
</list>When present, The SEAL trailer is formatted as follows:</t>
<t><figure anchor="min2" title="SEAL Trailer Format ">
<artwork><![CDATA[ 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>where the trailer includes a single 32-bit field formatted
as follows:</t>
<t><list style="hanging">
<t hangText="ICV2 (32)"><vspace />a 32-bit packet body integrity
check value. Present only when T=1, and covers the remaining
length of the encapsulated packet beyond the leading 128 bytes
(i.e., the remaining portion that was not covered by ICV1). Added
as a trailing 32 bit field following the final byte of the
encapsulated SEAL packet. Need not be aligned on an even byte
boundary.</t>
</list></t>
</section>
<section title="ITE Specification">
<section title="Tunnel Interface Soft State">
<t>The ITE maintains a per-ETE integrity check vector (ICV)
calculation algorithm and a symmetric secret key to calculate the
ICV(s) in packets it will send to this ETE. The ITE also maintains a
window of PKT_ID values for the packets it has recently sent to this
ETE.</t>
<t>For each underlying link of each ETE, the ITE maintains the
boolean variable "USE_ICV2" and "USE_MIN_MTU" initialized to
"FALSE".</t>
<t>When the ITE performs stateful MTU determination (see Section
4.4.9), it further caches a static MTU value for each ETE link
path.</t>
</section>
<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 (sometimes known as "MSS
clamping").</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., 576 bytes for
IPv4, 1280 bytes for IPv6, etc.).</t>
<t>In light of the above considerations, the ITE SHOULD configure an
indefinite MTU on tunnel *router* interfaces. The ITE MAY instead
set a finite MTU on tunnel *host* interfaces.</t>
</section>
<section title="Submitting Packets for Encapsulation">
<t>For each inner packet, if the packet is itself a SEAL packet
(i.e., one with either SEAL_PROTO in the IP protocol/next-header
field, or with SEAL_PORT in the transport layer destination port
field) and the LEVEL field of the SEAL header contains the value 0,
the ITE silently discards the packet.</t>
<t>Otherwise, the ITE calculates HLEN as the sum of the lengths of
the SEAL header plus outer transport and network layer headers that
will be used for encapsulation of the inner packet. The ITE must
include the length of the uncompressed outer headers when
calculating HLEN even if the tunnel is using header compression. The
ITE next sets the variable "LMTU" to the MTU of the underlying link
minus HLEN. If LMTU is less than 1280, the ITE also sets the boolean
variables USE_ICV2 and USE_MIN_MTU for this ETE link path to TRUE.
The ITE then prepares the inner packet for encapsulation according
to its length.</t>
<t>For IPv4 inner packets with DF=0 in the IPv4 header, if the
packet is not the first fragment of a SEAL packet (i.e., not a
packet that includes a SEAL header) the ITE fragments the packet
into inner fragments no larger than the minimum of LMTU and 512
bytes. The ITE then submits each inner fragment for SEAL
encapsulation as specified in Section 4.4.4.</t>
<t>For all other inner packets, if USE_MIN_MTU is TRUE the ITE
resets LMTU=1280. Next, if the packet is no larger than the maximum
of LMTU and 1280 bytes the ITE submits it for SEAL encapsulation.
Otherwise, the ITE sends a packet-too-big error message toward the
source address of the inner packet.</t>
<t>To send the 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 with the MTU field set to the maximum of LMTU and 1280 bytes
(see: Section 4.6.1). Otherwise, the ITE prepares an ordinary PTB
message appropriate to the inner protocol version.</t>
<t>When preparing the PTB message, the ITE sets the MTU field to the
maximum of LMTU and 1280 bytes. (When the inner packet is an IPv4
packet that includes the leading portion of a SEAL data packet, the
ITE first sets DF=1 in the inner header then re-calculates the inner
header checksum before generating the PTB. This is a precautionary
step to appease network middleboxes that would discard an ICMPv4 PTB
message in which the IPv4 header of the inner packet within the
packet-in-error has DF=0.)</t>
<t>After sending the (S)PTB message, the ITE discards the inner
packet.</t>
</section>
<section title="SEAL Encapsulation">
<t>The ITE next encapsulates the inner packet in a SEAL header
formatted as specified in Section 4.3. The ITE sets NEXTHDR to the
protocol number corresponding to the address family of the
encapsulated inner packet. For example, the ITE sets NEXTHDR to the
value '4' for encapsulated IPv4 packets <xref
target="RFC2003"></xref>, '41' for encapsulated IPv6 packets <xref
target="RFC2473"></xref><xref target="RFC4213"></xref>, '80' for
encapsulated OSI/CLNP packets <xref target="RFC1070"></xref>,
etc.</t>
<t>The ITE then sets R=1 if redirects are permitted (see: <xref
target="I-D.templin-intarea-vet"></xref>) and sets PREFLEN to the
length of the prefix to be applied to the inner source address. The
ITE's claimed PREFLEN is subject to verification by the ETE; hence,
the ITE MUST set PREFLEN to the exact prefix length that it is
authorized to use. (Note that if this process is entered via
re-encapsulation (see: Section 4.5.4), PREFLEN and R are instead
copied from the SEAL header of the re-encapsulated packet. This
implies that the PREFLEN and R values are propagated across a
re-encapsulating chain of ITE/ETEs that must all be authorized to
represent the prefix.)</t>
<t>The ITE next sets (C=0; P=0; Z=0), then sets LINK_ID to the value
assigned to the underlying link and sets PKT_ID to a
monotonically-increasing integer value, beginning with the value 0
in the first packet transmitted to this ETE. The ITE also sets U=1
if it needs to determine whether the ETE will receive it without
fragmentation, e.g., to test whether a middlebox is reassembling
fragmented packets before they arrive at the ETE (see: Section
4.4.8), for stateful MTU determination (see Section 4.4.9), etc.
Otherwise, the ITE sets U=0.</t>
<t>Next, if the inner packet is not itself a SEAL packet the ITE
sets LEVEL to an integer value between 0 and 7 as a specification of
the number of additional layers of nested SEAL encapsulations
permitted. Otherwise, the ITE sets LEVEL to the value that appears
in the inner packet's SEAL header minus 1.</t>
<t>Next, if the outer protocol version is IPv4, the variable
USE_ICV2 for this ETE link path is TRUE, and the inner packet is
larger than 128 bytes minus the SEAL header length but no larger
than 1280 bytes, the ITE sets T=1. Otherwise, the ITE sets T=0.</t>
<t>The ITE finally sets ICV1 to 0 and calculates the packet ICVs.
The ICVs are calculated using an algorithm agreed on by the ITE and
ETE. The algorithm uses a symmetric secret key so that the ETE can
verify that the ICVs were generated by the ITE.</t>
<t>The ITE first calculates the ICV value over the leading 128 bytes
of the packet beginning with the SEAL header (or up to the end of
the packet if there are fewer than 128 bytes) then places result in
the ICV1 field in the header. If T=1, the ITE next calculates the
ICV over the remainder of the packet and places the result in the
ICV2 field in the SEAL trailer. The ITE then submits the packet for
outer encapsulation.</t>
</section>
<section title="Outer Encapsulation">
<t>Following SEAL encapsulation, the ITE next encapsulates the
packet in the requisite outer headers according to the specific
encapsulation format (e.g., <xref target="RFC1070"></xref>, <xref
target="RFC2003"></xref>, <xref target="RFC2473"></xref>, <xref
target="RFC4213"></xref>, etc.), except that it writes 'SEAL_PROTO'
in the protocol field of the outer IP header (when simple IP
encapsulation is used) or writes 'SEAL_PORT' in the outer
destination transport service port field (e.g., when IP/UDP
encapsulation is used).</t>
<t>When UDP encapsulation is used, the ITE sets the UDP header
fields as specified in Section 5.5.4 of <xref
target="I-D.templin-intarea-vet"></xref>. The ITE then performs
outer IP header encapsulation as specified in Section 5.5.5 of <xref
target="I-D.templin-intarea-vet"></xref>. If this process is entered
via re-encapsulation (see: Section 4.5.4), the ITE instead follows
the outer IP/UDP re-encapsulation procedures specified in Section
5.5.6 of <xref target="I-D.templin-intarea-vet"></xref>.</t>
<t>When IPv4 is used as the outer encapsulation layer, 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. The ITE MAY
instead set DF=1 if it is using stateful MTU determination (see
Section 4.4.9).</t>
<t>When IPv6 is used as the outer encapsulation layer, the "DF" flag
is absent but implicitly set to 1. The packet therefore will not be
fragmented within the subnetwork, since IPv6 deprecates
in-the-network fragmentation.</t>
<t>Next, the ITE uses IP fragmentation if necessary to fragment the
packet into outer IP fragments that are no larger than the MTU of
the underlying link.</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 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.</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 raw ICMP error
messages<xref target="RFC0792"></xref><xref target="RFC4443"></xref>
from either the ETE or from a router within the subnetwork. Each
ICMP message includes an outer IP header, followed by an ICMP
header, followed by a portion of the SEAL data packet that generated
the error (also known as the "packet-in-error") beginning with the
outer IP header.</t>
<t>The ITE should process 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 ICMP messages
that do not include sufficient information in the packet-in-error 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>
<t>For other ICMP messages, the ITE examines the packet-in-error
beginning with the SEAL header. If the value in the PKT_ID field is
not within the window of packets the ITE has recently sent to this
ETE, or if the value in the SEAL header ICV1 field is incorrect, the
ITE discards the message.</t>
<t>Next, the ITE processes the ICMP message if there is operational
assurance that it has not been crafted by a malicious middlebox,
e.g., if the source of the ICMP message is within the same
administrative domain as the ITE. If the received ICMP message is a
PTB, and the MTU field encodes a non-zero value, the ITE sets the
variable "PMTU" to the MTU value minus HLEN for this ETE link path.
If PMTU is less than 1280, the ITE sets USE_ICV2 for the ETE link
path to TRUE.</t>
<t>Next, the ITE transcribes the ICMP message into a message to
return to the previous hop. If the previous hop toward the inner
source address is reached via the same tunnel interface the SEAL
data packet was sent on, the ITE transcribes the ICMP message into
an SCMP message (see: Section 4.6.1) and forwards it to the previous
hop. Otherwise, the ITE transcribes the ICMP message into a message
appropriate for the inner protocol version and forwards it to the
inner source address.</t>
<t>To transcribe the message, the ITE extracts the inner packet from
within the ICMP message packet-in-error field and generates a new
SCMP/ICMP message corresponding to the type of the received ICMP
message.</t>
<t>For (S)PTB messages, the ITE writes the maximum of PMTU and 1280
bytes in the MTU field. This calculation allows the largest MTU size
possible for packets that do not require fragmentation on the path
to the ETE.</t>
</section>
<section title="Middlebox Reassembly Testing">
<t>The ITE can perform a qualification exchange over each underlying
ETE link path to ensure that the subnetwork correctly delivers IP
fragments to the ETE. This procedure can be used, e.g., to determine
whether there are middleboxes on the path that violate the <xref
target="RFC1812"></xref>, Section 5.2.6 requirement that: "A router
MUST NOT reassemble any datagram before forwarding it".</t>
<t>When possible, the ITE should use knowledge of its topological
arrangement as an aide in determining when middlebox reassembly
testing is necessary. For example, if the ITE knows that the ITE is
located somewhere in the public Internet, middlebox reassembly
testing is unnecessary. If the ITE is aware that the ETE is located
behind a NAT or a firewall, however, then middlebox reassembly
testing is recommended.</t>
<t>The ITE performs the middlebox reassembly test by setting U=1 in
the header of a SEAL data packet to be used as a probe. Next, the
ITE encapsulates the packet in the appropriate outer headers, splits
it into two outer IP fragments, then sends both fragments to the ETE
over the same underlying link.</t>
<t>If the ETE returns an SCMP PTB message with MTU=0 (see Section
4.6.1.1), then a middlebox in the subnetwork is reassembling the
fragments before they are delivered to the ETE. In that case, the
ETE sets both USE_ICV2 and USE_MIN_MTU to TRUE so that future
packets will be protected from reassembly misassociations.</t>
<t>NB: In order to account for reassembly buffer restrictions and/or
packet loss, the ITE should select only relatively small SEAL data
packets for testing purposes and should perform a series of tests
(e.g., 3-5 tests with 1sec between tests) instead of a single
isolated test. The ITE can also construct a NULL test packet instead
of using ordinary SEAL data packets for testing. To create the test
packet, the ITE prepares a data packet with (C=0; P=1; R=0; T=0;
U=1; Z=0) in the SEAL header, writes the length of the ITE's claimed
prefix in the PREFLEN field, and writes the ITE's claimed prefix in
the PREFIX field. The ITE then sets NEXTHDR according to the address
family of the PREFIX, i.e., it sets NEXTHDR to the value '4' for an
IPv4 prefix, '41' for an IPv6 prefix , '80' for an OSI/CLNP prefix,
etc. The ITE can further add padding following the SEAL header to a
length that would not cause the size of the packet to exceed 512
bytes before outer encapsulation. The ITE then sets PKT_ID, LINK_ID
and LEVEL to appropriate values for this ETE, calculates the ICV
over the first 128 bytes of the packet beginning with the SEAL
header, and writes the value in the ICV1 field.</t>
</section>
<section title="Stateful MTU Determination">
<t>The above sections specify a generic stateless MTU determination
capability that works over all ITE->ETE paths. However, the
stateless method can be overly-conservative when the ETE is situated
behind a middlebox that performs IPv4 reassembly.</t>
<t>If the ITE discovers that the ETE is behind a middlebox that
performs IPv4 reassembly (e.g., via static configuration, via
testing as described in Section 4.4.8, etc.), it can instead
implement a stateful MTU determination capability. To do so, it
sends probe messages of various size to the ETE with U=1 in the SEAL
header and DF=1 in the outer IPv4 header. The ITE can then cache the
size of the largest packet for which it receives a probe reply from
the ETE as the MTU to use for this ETE, and thereafter ignores
USE_MIN_MTU and USE_ICV2.</t>
<t>For example, the ITE could send probes of 1300 bytes, followed by
1400 bytes, followed by 1460 bytes, followed by 1500 bytes, then set
the MTU for this ETE link path to the size of the largest probe
packet for which it receives a reply.</t>
<t>After determining the largest possible MTU, the ITE sends further
SEAL packets to this ETE with DF=1 and without including ICV2.
However, the ITE should periodically re-probe the path to determine
whether routing changes have resulted in a reduced path MTU.</t>
</section>
</section>
<section title="ETE Specification">
<section title="Tunnel Interface Soft State">
<t>The ETE maintains a per-ITE ICV calculation algorithm and a
symmetric secret key to verify the ICV(s) in the SEAL header and
trailer. The ETE also maintains a window of PKT_ID values for the
packets it has recently received from this ITE.</t>
</section>
<section title="Reassembly Buffer Requirements">
<t>The ETE must maintain a minimum IP reassembly buffer size of 1500
bytes for both IPv4 <xref target="RFC0791"></xref> and IPv6 <xref
target="RFC2460"></xref>. The ETE must also be capable of partially
reassembling and delivering at least the leading 1280 byte portion
of the inner packet even if the completely reassembled packet would
exceed that size.</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 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>
</section>
<section title="IP-Layer Reassembly">
<t>The ETE processes non-SEAL IP packets as specified in the
normative references, i.e., it performs any necessary IP reassembly
then delivers the (reassembled) packet to the appropriate upper
layer protocol module.</t>
<t>The ETE gathers the outer IP fragments of a SEAL packet 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 delivers this (partially)
reassembled packet for decapsulation as specified in Section
4.5.4.</t>
</section>
<section title="Decapsulation and Re-Encapsulation">
<t>For each SEAL packet submitted for decapsulation, the ETE first
examines the PKT_ID and ICV1 fields. If the PKT_ID is not within the
window of acceptable values for this ITE, or if the ICV1 field
includes an incorrect value, the ETE silently discards the
packet.</t>
<t>Next, if the packet arrived as multiple IP fragments and the SEAL
header has T=0, the ETE sends an SPTB message back to the ITE (see
Section 4.6.1.1) with MTU set to the size of the largest IP fragment
received. If the packet arrived as multiple IP fragments and the
(partial) inner packet is larger than 1280 bytes, the ETE then
silently discards the packet.</t>
<t>Next, if the SEAL header has T=1 the ETE silently discards the
packet if the inner packet is larger than 1280 bytes. Otherwise, the
ETE verifies the ICV2 value over the remainder of the packet if T=1
and silently discards the packet if the value is incorrect.</t>
<t>If the SEAL header has C=1, the ETE then 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>If there is an incorrect value in a SEAL header field, the
returns an SCMP "Parameter Problem" (SPP) message (see Section
4.6.1.2) then discards the packet. Otherwise, if the SEAL header has
U=1 and the packet did not require IP-layer reassembly, the ETE next
sends an SPTB message with MTU=0 back to the ITE (see Section
4.6.1.1).</t>
<t>Next, if the SEAL header has P=1 the ETE then discards the
packet. Otherwise, the ETE discards the outer headers and processes
the inner packet according to the header type indicated in the SEAL
NEXTHDR field.</t>
<t>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.</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>.
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, but it does not calculate the SCMP message checksum. The TE
then formats the Message Body the same as for the corresponding ICMPv6
message. The TE then encapsulates the SCMP message in the SEAL header
and trailer as well as the 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 ~
+--------------------+ --> +--------------------+
| SEAL Trailer |
+--------------------+
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 | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-Specific Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking SEAL data packet as |
~ 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 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 extending to a length that
would not cause the entire SCMP packet following outer encapsulation
to exceed 576 bytes.</t>
<t>When the ETE processes a SEAL data packet for which the ICVs are
correct but an error must be returned, it prepares an SCMP error
message as shown in <xref target="control2"></xref>. The ETE sets
the Type and Code fields in the SCMP header according to the
appropriate error message type, sets the Reserved field to 0, fills
out the Type-Specific Data field and includes the
packet-in-error.</t>
<t>The ETE next encapsulates the SCMP message in the requisite SEAL
header, outer headers and SEAL trailer as shown in <xref
target="scmpencaps"></xref>. During encapsulation, the ETE sets the
outer destination address/port numbers of the SCMP packet to the
outer source address/port numbers of the original SEAL data packet
and sets the outer source address/port numbers to its own outer
address/port numbers.</t>
<t>The ETE then sets (C=1; P=0; R=0; U=0) in the SEAL header, then
sets NEXTHDR, PREFLEN, LEVEL, LINK_ID and PKT_ID to the same values
that appeared in the SEAL data packet header. If the SCMP message
includes more than 128 bytes beginning with the SEAL header, the ETE
next sets T=1; otherwise it sets T=0.</t>
<t>The ETE then calculates and sets the ICV1 field (and also the
ICV2 field if T=1) the same as specified for SEAL data packet
encapsulation in Section 4.4.4. Next, the ETE encapsulates the SCMP
message in the requisite outer encapsulations and sends the
resulting SCMP packet to the ITE the same as specified for SEAL data
packets in Section 4.4.5.</t>
<t>The following sections describe additional considerations for
various SCMP error messages:</t>
<section title="Generating SCMP Packet Too Big (SPTB) Messages">
<t>An ETE generates an SCMP "Packet Too Big" (SPTB) message when
it receives a SEAL data packet that arrived as multiple outer IP
fragments but for which T=0 in the SEAL header. 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 in the MTU field of the message.</t>
<t>The ETE also generates an SPTB message when it accepts a SEAL
protocol data packet which did not undergo IP reassembly and with
U=1 in the SEAL header. The ETE prepares the SPTB message the same
as above, except that it writes the value 0 in the MTU field.</t>
</section>
<section title="Generating Other SCMP Error Messages">
<t>An ETE generates an SCMP "Destination Unreachable" (SDU)
message under the same circumstances that an IPv6 system would
generate an ICMPv6 Destination Unreachable message.</t>
<t>An ETE generates an SCMP "Parameter Problem" (SPP) message when
it receives a SEAL packet with an incorrect value in the SEAL
header. IN THIS CASE ALONE, the ETE prepares the packet-in-error
beginning with the SEAL header instead of beginning immediately
after the SEAL header.</t>
<t>TEs generate other SCMP message types using methods and
procedures specified in other documents. For example, SCMP message
types used for tunnel neighbor coordinations are specified in VET
<xref target="I-D.templin-intarea-vet"></xref>.</t>
</section>
</section>
<section title="Processing SCMP Error Messages">
<t>For each SCMP error message it receives, the ITE first verifies
that the outer addresses of the SCMP packet are correct and that the
PKT_ID is within its window of values for this ETE. The ITE then
verifies the ICV1 and ICV2 values. If the identifying information
and/or ICVs are incorrect, the ITE discards the message; otherwise,
it processes the message as follows:</t>
<section title="Processing SCMP PTB Messages">
<t>After an ITE sends a SEAL data packet to an ETE, it may receive
an SPTB message with a packet-in-error containing the leading
portion of the inner packet (see: Section 4.6.1.1). If the SPTB
message has MTU=0, the ITE processes the message as confirmation
that the ETE received an unfragmented SEAL data packet with U=1 in
the SEAL header. The ITE then processes the SPTB message as a path
qualification probe response and discards the message. If the SPTB
message has MTU!=0, the ITE instead processes the message as an
indication of a packet size limitation.</t>
<t>If the MTU value in the SPTB message is no larger than
1280+HLEN, and the length of the inner packet within the
packet-in-error is no larger than 1280, the ITE sets the boolean
variable USE_ICV2 for this ETE link path to TRUE. The ITE then
discards the SPTB message.</t>
<t>If the MTU value in the SPTB message is not substantially less
that 1500, the value is likely to represent the true MTU of the
restricting link on the path to the ETE; otherwise, a router on
the path may be generating runt fragments. 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 SPTB message with
MTU=256 and inner header length 1500, it can rewrite the MTU to
1400. If the ITE subsequently receives an SPTB message with
MTU=256 and inner header length 1400, it can rewrite the MTU to
1300, etc.</t>
<t>The ITE then checks it's forwarding tables to determine the
previous hop on the reverse path toward the source address of the
inner packet in the packet-in-error. If the previous hop is
reached over a different interface than the SPTB message arrived
on, the ITE 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.</t>
<t>If the previous hop is reached over the same tunnel interface
that the SPTB message arrived on, the ITE instead relays the
message to the previous hop. In order to relay the message, the
ITE rewrites the SEAL header fields with values corresponding to
the previous hop. Next, the ITE replaces the SPTB's outer headers
with headers of the appropriate protocol version and fills in the
header fields as specified in Sections 5.5.4-5.5.6 of <xref
target="I-D.templin-intarea-vet"></xref>, where the destination
address/port correspond to the previous hop and the source
address/port correspond to the ITE. The ITE then sends the message
to the previous hop the same as if it were issuing a new SPTB
message.</t>
</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="IANA Considerations">
<t>The IANA is instructed to allocate an IP protocol number for
'SEAL_PROTO' in the 'protocol-numbers' registry.</t>
<t>The IANA is instructed to allocate a Well-Known Port number for
'SEAL_PORT' in the 'port-numbers' registry.</t>
<t>The IANA is instructed to establish a "SEAL Protocol" registry to
record SEAL Version values. This registry should be initialized to
include the initial SEAL Version number, i.e., Version 0.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>SEAL provides a segment-by-segment data origin authentication and
anti-replay service across the (potentially) multiple segments of a
re-encapsulating tunnel. It further provides a segment-by-segment
integrity check of the headers of encapsulated packets, but does not
verify the integrity of the rest of the packet beyond the headers unless
fragmentation is unavoidable. SEAL therefore considers full message
integrity checking, authentication and confidentiality as end-to-end
considerations in a manner that is compatible with securing mechanisms
such as TLS/SSL <xref target="RFC5246"></xref>.</t>
<t>An amplification/reflection 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 SCMP
message ICVs, PKT_ID, as well as the inner headers of the
packet-in-error, provide mitigation for the ETE to detect and discard
SEAL segments with spoofed source addresses.</t>
<t>The SEAL header is sent in-the-clear the same as for the outer IP and
other outer headers. In this respect, the threat model is no different
than for IPv6 extension headers. Unlike IPv6 extension headers, however,
the SEAL header is protected by an integrity check that also covers the
inner packet headers.</t>
<t>Security issues that apply to tunneling in general are discussed in
<xref target="RFC6169"></xref>.</t>
</section>
<section title="Related Work">
<t>Section 3.1.7 of <xref target="RFC2764"></xref> provides a high-level
sketch for supporting large tunnel MTUs via a tunnel-level segmentation
and reassembly capability to avoid IP level fragmentation. This
capability was implemented in the first edition of SEAL, but is now
deprecated.</t>
<t>Section 3 of <xref target="RFC4459"> </xref> describes inner and
outer fragmentation at the tunnel endpoints as alternatives for
accommodating the tunnel MTU.</t>
<t>Section 4 of <xref target="RFC2460"></xref> specifies a method for
inserting and processing extension headers between the base IPv6 header
and transport layer protocol data. The SEAL header is inserted and
processed in exactly the same manner.</t>
<t>IPsec/AH is <xref target="RFC4301"></xref><xref
target="RFC4301"></xref> is used for full message integrity verification
between tunnel endpoints, whereas SEAL only ensures integrity for the
inner packet headers. The AYIYA proposal <xref
target="I-D.massar-v6ops-ayiya"></xref> uses similar means for providing
full message authentication and integrity.</t>
<t>The concepts of path MTU determination through the report of
fragmentation and extending the 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. An 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 anchor="acknowledge" title="Acknowledgments">
<t>The following individuals are acknowledged for helpful comments and
suggestions: Jari Arkko, Fred Baker, Iljitsch van Beijnum, Oliver
Bonaventure, Teco Boot, Bob Braden, Brian Carpenter, Steve Casner, Ian
Chakeres, Noel Chiappa, Remi Denis-Courmont, Remi Despres, Ralph Droms,
Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci, Joel
Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob Hinden,
Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt Mathis,
Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark Townsley, Ole
Troan, Margaret Wasserman, Magnus Westerlund, Robin Whittle, James
Woodyatt, and members of the Boeing Research & Technology NST
DC&NT group.</t>
<t>Discussions with colleagues following the publication of RFC5320 have
provided useful insights that have resulted in significant improvements
to this, the Second Edition of SEAL.</t>
<t>Path MTU determination through the report of fragmentation was first
proposed by Charles Lynn on the TCP-IP mailing list in 1987. Extending
the IP identification field was first proposed by Steve Deering on the
MTUDWG mailing list in 1989.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.0791"?>
<?rfc include="reference.RFC.0792"?>
<?rfc include="reference.RFC.4443"?>
<?rfc include="reference.RFC.3971"?>
<?rfc include="reference.RFC.4861"?>
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.2460"?>
</references>
<references title="Informative References">
<?rfc include="reference.RFC.1063"?>
<?rfc include="reference.RFC.1191"?>
<?rfc include="reference.RFC.1981"?>
<?rfc include="reference.RFC.2003"?>
<?rfc include="reference.RFC.2473"?>
<?rfc include="reference.RFC.2923"?>
<?rfc include="reference.RFC.3366"?>
<?rfc include="reference.RFC.3819"?>
<?rfc include="reference.RFC.4213"?>
<?rfc include="reference.RFC.1812"?>
<?rfc include="reference.RFC.4380"?>
<?rfc include="reference.RFC.4301"?>
<?rfc include="reference.RFC.4302"?>
<?rfc include="reference.RFC.5246"?>
<?rfc include="reference.RFC.4459"?>
<?rfc include="reference.RFC.4821"?>
<?rfc include="reference.RFC.4963"?>
<?rfc include="reference.RFC.2764"?>
<?rfc include="reference.RFC.2675"?>
<?rfc include="reference.RFC.5445"?>
<?rfc include="reference.RFC.1070"?>
<?rfc include="reference.RFC.3232"?>
<?rfc include="reference.RFC.4191"?>
<?rfc include="reference.RFC.4987"?>
<?rfc include="reference.RFC.5720"?>
<?rfc include="reference.I-D.templin-intarea-vet"?>
<?rfc include="reference.I-D.ietf-savi-framework"?>
<?rfc include="reference.I-D.templin-ironbis"?>
<?rfc include="reference.RFC.6139"?>
<?rfc include="reference.RFC.5927"?>
<?rfc include="reference.RFC.6169"?>
<?rfc include="reference.I-D.ietf-intarea-ipv4-id-update"?>
<?rfc include="reference.I-D.templin-aero"?>
<?rfc include="reference.I-D.massar-v6ops-ayiya"?>
<reference anchor="FRAG">
<front>
<title>Fragmentation Considered Harmful</title>
<author fullname="Christopher Kent" initials="C" surname="Kent">
<organization></organization>
</author>
<author fullname="Jeffrey Mogul" initials="J" surname="Mogul">
<organization></organization>
</author>
<date month="October" year="1987" />
</front>
</reference>
<reference anchor="FOLK">
<front>
<title>Beyond Folklore: Observations on Fragmented Traffic</title>
<author fullname="Colleen Shannon" initials="C" surname="Shannon">
<organization></organization>
</author>
<author fullname="David Moore" initials="D" surname="Moore">
<organization></organization>
</author>
<author fullname="k claffy" initials="k" surname="claffy">
<organization></organization>
</author>
<date month="December" year="2002" />
</front>
</reference>
<reference anchor="MTUDWG">
<front>
<title>IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November 1989 -
February 1995.</title>
<author fullname="" initials="" surname="">
<organization></organization>
</author>
<date month="" year="" />
</front>
</reference>
<reference anchor="TCP-IP">
<front>
<title>Archive/Hypermail of Early TCP-IP Mail List,
http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May 1987 - May
1990.</title>
<author fullname="" initials="" surname="">
<organization></organization>
</author>
<date month="" year="" />
</front>
</reference>
<reference anchor="TBIT">
<front>
<title>Measuring Interactions Between Transport Protocols and
Middleboxes</title>
<author fullname="Alberto Medina" initials="A" surname="Medina">
<organization></organization>
</author>
<author fullname="Mark Allman" initials="M" surname="Allman">
<organization></organization>
</author>
<author fullname="Sally Floyd" initials="S" surname="Floyd">
<organization></organization>
</author>
<date month="October" year="2004" />
</front>
</reference>
<reference anchor="WAND">
<front>
<title>Inferring and Debugging Path MTU Discovery Failures</title>
<author fullname="Matthew Luckie" initials="M" surname="Luckie">
<organization></organization>
</author>
<author fullname="Kenjiro Cho" initials="K" surname="Cho">
<organization></organization>
</author>
<author fullname="Bill Owens" initials="B" surname="Owens">
<organization></organization>
</author>
<date month="October" year="2005" />
</front>
</reference>
<reference anchor="SIGCOMM">
<front>
<title>Measuring Path MTU Discovery Behavior</title>
<author fullname="Matthew Luckie" initials="M" surname="Luckie">
<organization></organization>
</author>
<author fullname="Ben Stasiewicz" initials="B" surname="Stasiewicz">
<organization></organization>
</author>
<date month="November" year="2010" />
</front>
</reference>
</references>
<section title="Reliability">
<t>Although a SEAL tunnel may span an arbitrarily-large subnetwork
expanse, the IP layer sees the tunnel as a simple link that supports the
IP service model. Links with high bit error rates (BERs) (e.g., IEEE
802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms <xref
target="RFC3366"></xref> to increase packet delivery ratios, while links
with much lower BERs typically omit such mechanisms. Since SEAL tunnels
may traverse arbitrarily-long paths over links of various types that are
already either performing or omitting ARQ as appropriate, it would
therefore often be inefficient to also require the tunnel endpoints to
also perform ARQ.</t>
</section>
<section title="Integrity">
<t>The SEAL header includes an ICV field that covers the SEAL header and
at least the inner packet headers. This provides for header integrity
verification on a segment-by-segment basis for a segmented
re-encapsulating tunnel path. When fragmentation is needed, the SEAL
packet also contains a trailer with a secondary ICV that covers the
remainder of the packet.</t>
<t>Fragmentation and reassembly schemes must consider packet-splicing
errors, e.g., when two fragments from the same packet are concatenated
incorrectly, when a fragment from packet X is reassembled with fragments
from packet Y, etc. The primary sources of such errors include
implementation bugs and wrapping IP ID fields.</t>
<t>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</t>
<t>SEAL avoids reassembly mis-associations through the use of extended
ICVs, and also discards any reassembled packets larger than 1280
bytes.</t>
</section>
<section title="Transport Mode">
<t>SEAL can also be used in "transport-mode", e.g., when the inner layer
comprises upper-layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL (e.g., by
inserting a 'SEAL_OPTION' TCP option during connection establishment)
for the carriage of protocol data encapsulated as 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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