Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track October 28, 2011
Expires: April 30, 2012
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-35.txt
Abstract
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on April 30, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 6
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 8
4. SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 9
4.1. VET Interface Model . . . . . . . . . . . . . . . . . . . 10
4.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 10
4.3. SEAL Header Format . . . . . . . . . . . . . . . . . . . . 11
4.4. SEAL Trailer Format . . . . . . . . . . . . . . . . . . . 13
4.5. ITE Specification . . . . . . . . . . . . . . . . . . . . 13
4.5.1. Tunnel Interface Soft State . . . . . . . . . . . . . 13
4.5.2. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
4.5.3. Submitting Packets for Encapsulation . . . . . . . . . 15
4.5.4. SEAL Encapsulation . . . . . . . . . . . . . . . . . . 16
4.5.5. Outer Encapsulation . . . . . . . . . . . . . . . . . 17
4.5.6. Probing Strategy . . . . . . . . . . . . . . . . . . . 18
4.5.7. Processing ICMP Messages . . . . . . . . . . . . . . . 19
4.6. ETE Specification . . . . . . . . . . . . . . . . . . . . 20
4.6.1. Tunnel Interface Soft State . . . . . . . . . . . . . 20
4.6.2. Reassembly Buffer Requirements . . . . . . . . . . . . 20
4.6.3. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 20
4.6.4. Decapsulation and Re-Encapsulation . . . . . . . . . . 21
4.7. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 22
4.7.1. Generating SCMP Error Messages . . . . . . . . . . . . 22
4.7.2. Processing SCMP Error Messages . . . . . . . . . . . . 24
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 26
6. End System Requirements . . . . . . . . . . . . . . . . . . . 26
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 26
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 27
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 28
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
12.1. Normative References . . . . . . . . . . . . . . . . . . . 29
12.2. Informative References . . . . . . . . . . . . . . . . . . 29
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 32
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 32
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 33
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 33
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Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 35
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1. Introduction
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) [RFC0791][RFC2460]. Those virtual
topologies have elements that appear as one hop in the virtual
topology, but are actually multiple IP or sub-IP layer hops. These
multiple hops often have quite diverse properties that are often not
even visible to the endpoints of the virtual hop. This introduces
failure modes that are not dealt with well in current approaches.
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 [FRAG][FOLK][RFC4963].
Additionally, classical path MTU discovery [RFC1191] has known
operational issues that are exacerbated by in-the-network tunnels
[RFC2923][RFC4459]. The following subsections present further
details on the motivation and approach for addressing these issues.
1.1. Motivation
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.
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 [I-D.ietf-intarea-ipv4-id-update]. 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 [RFC4963]. 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
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annoyances (e.g., in-the-network router fragmentation [RFC1981]) to
the potential for major integrity issues (e.g., mis-association of
the fragments of multiple IP packets during reassembly [RFC4963]).
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" [RFC2923].
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 [RFC5927].
Furthermore, when a source node that requires ICMP error message
feedback when a packet is dropped due to an MTU restriction does not
receive the messages, a path MTU-related black hole occurs. This
means that the source will continue to send packets that are too
large and never receive an indication from the network that they are
being discarded. This behavior has been confirmed through documented
studies showing clear evidence of path MTU discovery failures in the
Internet today [TBIT][WAND][SIGCOMM].
The issues with both IPv4 fragmentation and this "classical" method
of path MTU discovery are exacerbated further when IP tunneling is
used [RFC4459]. For example, an ingress tunnel endpoint (ITE) may be
required to forward encapsulated packets into the subnetwork on
behalf of hundreds, thousands, or even more original sources 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.
Although recent works have led to the development of a robust end-to-
end MTU determination scheme [RFC4821], 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
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paths and difficult to change at a later date. Therefore, a new
approach to accommodate tunnels over links with diverse MTUs is
necessary.
1.2. Approach
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).
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 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 [I-D.ietf-savi-framework][RFC4302].
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.
The following sections provide the SEAL normative specifications,
while the appendices present non-normative additional considerations.
2. Terminology and Requirements
The following terms are defined within the scope of this document:
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subnetwork
a virtual topology configured over a connected network routing
region and bounded by encapsulating border nodes.
Ingress Tunnel Endpoint
a virtual interface over which an encapsulating border node (host
or router) sends encapsulated packets into the subnetwork.
Egress Tunnel Endpoint
a virtual interface over which an encapsulating border node (host
or router) receives encapsulated packets from the subnetwork.
inner packet
an unencapsulated network layer protocol packet (e.g., IPv6
[RFC2460], IPv4 [RFC0791], OSI/CLNP [RFC1070], etc.) before any
outer encapsulations are added. Internet protocol numbers that
identify inner packets are found in the IANA Internet Protocol
registry [RFC3232].
outer IP packet
a packet resulting from adding an outer IP header (and possibly
other outer headers) to a SEAL-encapsulated inner packet.
packet-in-error
the leading portion of an invoking data packet encapsulated in the
body of an error control message (e.g., an ICMPv4 [RFC0792] error
message, an ICMPv6 [RFC4443] error message, etc.).
Packet Too Big (PTB)
a control plane message indicating an MTU restriction (e.g., an
ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
"Fragmentation Needed" message [RFC0792], etc.).
IP
used to generically refer to either IP protocol version, i.e.,
IPv4 or IPv6.
The following abbreviations correspond to terms used within this
document and/or elsewhere in common Internetworking nomenclature:
DF - the IPv4 header "Don't Fragment" flag [RFC0791]
ETE - Egress Tunnel Endpoint
HLEN - the length of the SEAL header plus outer headers
ICV - Integrity Check Vector
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ITE - Ingress Tunnel Endpoint
MTU - Maximum Transmission Unit
SCMP - the SEAL Control Message Protocol
SDU - SCMP Destination Unreachable message
SNA - SCMP Neighbor Advertisement message
SNS - SCMP Neighbor Solicitation message
SPP - SCMP Parameter Problem message
SPTB - SCMP Packet Too Big message
SEAL - Subnetwork Encapsulation and Adaptation Layer
SEAL_PORT - a transport-layer service port number used for SEAL
SEAL_PROTO - an IPv4 protocol number used for SEAL
TE - Tunnel Endpoint (i.e., either ingress or egress)
VET - Virtual Enterprise Traversal
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 [RFC2119]. When used
in lower case (e.g., must, must not, etc.), these words MUST NOT be
interpreted as described in [RFC2119], but are rather interpreted as
they would be in common English.
3. Applicability Statement
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)
[I-D.templin-intarea-vet] tunnel virtual interface abstraction are
the functional building blocks for the Internet Routing Overlay
Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in
Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
architectures.
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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.
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.
SEAL further ensures data origin authentication
[I-D.ietf-savi-framework], packet header integrity, and anti-replay.
The SEAL framework resembles a lightweight version of the IP Security
(IPsec) [RFC4301] Authentication Header (AH) [RFC4302], 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.
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 [RFC2473].
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.
4. SEAL Specification
The following sections specify the operation of SEAL:
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4.1. VET Interface Model
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.
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.
Implications of the VET unidirectional and bidirectional models are
discussed in [I-D.templin-intarea-vet].
4.2. SEAL Model of Operation
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 Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +--------------------+
| SEAL Trailer |
+--------------------+
Figure 1: SEAL Encapsulation
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.,
[RFC1070][RFC2003][RFC2473][RFC4213], etc.), the ITE inserts the SEAL
header between the inner packet and outer IP headers as: IP/SEAL/
{inner packet}.
For encapsulations over transports such as UDP (e.g., in the same
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manner as for [RFC4380]), 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 Figure 1.)
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, and need not be aligned on an
even byte boundary.
The following sections specify the SEAL header format and SEAL-
related operations of the ITE and ETE.
4.3. SEAL Header Format
The SEAL header is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|A|R|F|P|X| NEXTHDR | PREFLEN | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ICV1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ PREFIX (when present) ~
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Figure 2: SEAL Header Format
where the header fields are defined as:
VER (2)
a 2-bit version field. This document specifies Version 0 of the
SEAL protocol, i.e., the VER field encodes the value 0.
C (1)
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.
A (1)
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.
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R (1)
the "Redirects Permitted" bit. For data packets, set to 1 by the
ITE to inform the ETE that the source is accepting Redirects (see:
[I-D.templin-intarea-vet]).
F (1)
the "Fragmentation Needed" bit. Set to 1 if the ITE was obliged
to perform outer fragmentation before sending the packet.
P (1)
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: [I-D.templin-intarea-vet]), and for NULL
SEAL data packets used as probes (see: Section 4.4.6).
X (1)
the "Reserved" bit. Must be set to 0 for this version of the SEAL
specification.
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.
PREFLEN (8) an 8-bit field that encodes the length of the prefix to
be applied to the source address of inner packets.
LINK_ID (5)
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.
LEVEL (3)
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.
PKT_ID (32)
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.
ICV1 (32)
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
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packet network and transport layer headers are covered by the
integrity check.
PREFIX (variable)
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.
4.4. SEAL Trailer Format
The SEAL trailer is formatted as follows:
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SEAL Trailer Format
When present, the trailer includes a single 32-bit field formatted as
follows:
ICV22 (32)
a 32-bit packet body integrity check value. Present only when
F=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.
4.5. ITE Specification
4.5.1. Tunnel Interface Soft State
The ITE maintains a per-ETE integrity check vector (ICV) calculation
algorithm and a symmetric secret key to verify the ICV(s) in received
packets. The ITE also maintains a window of PKT_ID values for the
packets it has recently sent to this ETE. Finally, for each
underlying link of each ETE, the ITE maintains a boolean variable
"USE_MIN_MTU" initialized to "FALSE". (The ITE may maintain
USE_MIN_MTU as a per-ETE instead of a per-link value, but in that
case a lowest-common-denominator MTU value may be chosen.)
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4.5.2. Tunnel Interface MTU
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.
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 [RFC2923].
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.
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").
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.
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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.).
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.
4.5.3. Submitting Packets for Encapsulation
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 discards
the inner packet and treats it as an auditable indication of
excessive nesting.
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 "EMTU" to the MTU of the underlying link minus HLEN. If
EMTU is less than 1280, the ITE also sets the boolean variable
USE_MIN_MTU for this ETE link path to TRUE. The ITE then prepares
the inner packet for encapsulation according to its length.
For IPv4 inner packets with DF=0 in the IPv4 header, if the packet is
not the first fragment of a SEAL data packet (i.e., not a SEAL packet
with Offset=0 in the IPv4 header and with C=0 in the SEAL header) the
ITE fragments the packet into inner fragments no larger than the
minimum of EMTU and 512 bytes. The ITE then submits each inner
fragment for SEAL encapsulation as specified in Section 4.5.4.
For all other inner packets, if the packet is no larger than EMTU the
ITE submits it for SEAL encapsulation; otherwise the ITE processes it
further according to its size. If the packet is no larger than 1280
bytes, the ITE submits it for SEAL encapsulation. Otherwise, the ITE
sends a PTB message toward the source address of the inner packet.
To send the PTB message, the ITE first checks its forwarding tables
to discover the previous hop toward the source address of the inner
packet. If the previous hop is reached via the same tunnel
interface, the ITE sends an SCMP PTB (SPTB) message to the previous
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hop with the MTU field set to EMTU (see: Section 4.7.1). Otherwise,
the ITE prepares an ordinary PTB message appropriate to the inner
protocol version.
When preparing the PTB message, the ITE sets the MTU field to EMTU
the same as for an SPTB message. When the inner packet is an IPv4
packet that includes the first fragment 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.
After sending the (S)PTB message, the ITE discards the inner packet.
4.5.4. SEAL Encapsulation
The ITE next encapsulates the inner packet in a SEAL header formatted
as specified in Section 4.3. The ITE 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 [RFC2003], the value '41' for encapsulated IPv6 packets
[RFC2473][RFC4213], the value '80' for encapsulated OSI/CLNP packets
[RFC1070], etc.
The ITE then sets R=1 if redirects are permitted (see:
[I-D.templin-intarea-vet]) 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 4.6.4), 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.)
The ITE next sets C=0, P=0 and sets A=1 if an explicit
acknowledgement is required from the ETE (see: Section 4.5.6). The
ITE 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.
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. If the inner packet is no larger than
1280 and the variable USE_MIN_MTU for this ETE link path is TRUE, the
ITE then sets F=1; otherwise, it sets F=0.
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The ITE finally sets ICV1 and ICV2 (when F=1) to 0 and calculates the
ICVs over the packet beginning with the SEAL header and leading
portion of the inner packet. 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.
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 F=1, and if the packet includes more
than 128 bytes beginning with the SEAL header, the ITE next
calculates the ICV over the remainder of the packet beyond the
leading 128 bytes and places the result in the ICV2 field in the SEAL
trailer. The ITE then submits the packet for outer encapsulation.
4.5.5. Outer Encapsulation
Following SEAL encapsulation, the ITE next encapsulates the packet in
the requisite outer headers according to the specific encapsulation
format (e.g., [RFC1070], [RFC2003], [RFC2473], [RFC4213], 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).
When UDP encapsulation is used, the ITE sets the UDP header fields as
specified in Section 5.5.4 of [I-D.templin-intarea-vet]. The ITE
then performs outer IP header encapsulation as specified in Section
5.5.5 of [I-D.templin-intarea-vet]. If this process is entered via
re-encapsulation (see: Section 4.6.4), the ITE instead follows the
outer IP/UDP re-encapsulation procedures specified in Section 5.5.6
of [I-D.templin-intarea-vet].
When IPv4 is used as the outer encapsulation layer, the ITE sets DF=0
in the IPv4 header if the ETE correctly implements IP fragmentation
(see: Section 4.5.6); otherwise, it sets DF=1.
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.
Next, if the SEAL header has F=1 the ITE uses IP fragmentation if
necessary to fragment it into outer IP fragments that are no larger
than (EMTU + HLEN). During fragmentation, the ITE should fragment
the packet into fragments of approximately equal length, i.e.,
instead of causing the first fragment to be approximately MTU-sized.
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The ITE then sends each outer packet via the underlying link
corresponding to LINK_ID.
4.5.6. Probing Strategy
The ITE can perform a qualification exchange over an underlying link
to ensure that the subnetwork path to the ETE correctly delivers IP
fragments. This procedure could be employed, e.g., to determine
whether there are middleboxes on the path that violate the [RFC1812],
Section 5.2.6 requirement that: "A router MUST NOT reassemble any
datagram before forwarding it".
To perform this qualification, the ITE constructs a NULL SEAL data
packet to be used as a probe. The ITE sets (C=0; A=1; R=0; F=0; P=1)
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 to the value '4' for an IPv4
prefix, the value '41' for an IPv6 prefix , the value '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 to an appropriate value 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.
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. If the ETE
returns an SCMP PTB message with a non-zero MTU (see Section
4.7.1.1), then the subnetwork path correctly delivers IP fragments.
If the ETE returns an SCMP PTB message with MTU=0, however, then a
middlebox in the subnetwork is reassembling the fragments before they
are delivered to the ETE [RFC1812].
In addition to any control plane probing, all SEAL data packets sent
from the ITE to the ETE are considered implicit probes. SEAL data
packets will elicit SPTB messages from the ETE if any outer IP
fragmentation occurs in the path. SEAL data packets that are too
large may also be dropped by a router on the path, which will return
an ICMP PTB message.
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 data packet to be used as an explicit probe. The probe will
elicit an SPTB message with MTU=0 from the ETE as an acknowledgement
(see Section 4.7.1.1). The ITE can also send an SCMP Neighbor
Solicitation (SNS) message to elicit an SCMP Neighbor Advertisement
(SNA) response from the ETE when there are no convenient data packets
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to use as explicit probes (see: [I-D.templin-intarea-vet].
The ITE processes ICMP PTB messages as specified in Section 4.5.7.
The ITE processes SCMP messages as specified in Section 4.7.2.
4.5.7. Processing ICMP Messages
When the ITE sends SEAL packets, it may receive raw ICMP error
messages[RFC0792][RFC4443] 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.
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.
For other ICMP messages, the ITE examines the packet-in-error
beginning with the SEAL header. If the value in the SEAL header ICV1
field is incorrect and/or the value in the PKT_ID field is not within
the window of packets the ITE has recently sent to this ETE, the ITE
discards the message.
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 deducts the
length of the outer IP headers and SEAL header of the packet-in-error
from the MTU value. If the resulting MTU value is less than 1280,
the ITE marks the ETE link path as USE_MIN_MTU and discards the ICMP
message.
Otherwise, 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 4.7.1) 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.
To transcribe the message, the ITE extracts the inner packet from
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within the ICMP message packet-in-error field and generates a new
SCMP/ICMP message corresponding to the type of the received ICMP
message.
4.6. ETE Specification
4.6.1. Tunnel Interface Soft State
The ETE maintains a per-ITE ICV calculation algorithm and a symmetric
secret key to verify the ICVs 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.
4.6.2. Reassembly Buffer Requirements
The ETE must maintain a minimum IP reassembly buffer size of 1500
bytes for both IPv4 [RFC0791] and IPv6 [RFC2460]. 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.
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.
4.6.3. IP-Layer Reassembly
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.
For each SEAL packet received, if the packet did not undergo outer IP
fragmentation the ETE submits it for decapsulation as specified in
Section 4.6.4. Otherwise, the ETE gathers the outer IP fragments of
the 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 examines the SEAL header within this (partially)
reassembled SEAL packet. If the PKT_ID value is not acceptable for
this ITE, or if the value in the ICV1 field is incorrect, the ETE
silently discards the packet. Otherwise, the ETE processes the
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packet further according to the F bit.
If the SEAL header has F=0, the ETE sends an SPTB message back to the
ITE (see Section 4.7.1.1) with MTU set to the size of the IP first
fragment.
Next, if the inner packet is larger than 1280 bytes the ETE silently
discards the packet regardless of the F bit setting.
Finally, if the SEAL header has F=1 and the packet contains more than
128 beginning with the SEAL header, the ETE verifies the ICV2 value
over the remainder of the packet and silently discards the packet if
the value is incorrect.
If the reassembled SEAL packet has not been discarded, the ETE
finally submits it for decapsulation.
4.6.4. Decapsulation and Re-Encapsulation
For each SEAL packet submitted for decapsulation, if the packet did
not undergo the integrity checks specified in Section 4.6.4 the ETE
examines the PKT_ID and ICV fields. If the PKT_ID is not within the
window of acceptable values from this ITE, or if an ICV field
includes an incorrect value, the ETE silently discards the packet.
Next, if the SEAL header has C=1 the ETE processes the packet as an
SCMP packet as specified in Section 4.7.2. Otherwise the ETE process
the packet as a SEAL data packet as follows.
If there is an incorrect value in a SEAL header field, the returns an
SCMP "Parameter Problem" (SPP) message (see Section 4.7.1.2) then
discards the packet. Otherwise, if the SEAL header has A=1 the ETE
next sends an SPTB message with MTU=0 back to the ITE (see Section
4.7.1.1) but does not discard the packet.
Thee ETE next discards the outer headers and processes the inner
packet according to the header type indicated in the SEAL NEXTHDR
field. If the next hop toward the inner destination address is via a
different interface than the SEAL packet arrived on, the ETE discards
the SEAL header and delivers the inner packet either to the local
host or to the next hop interface if the packet is not destined to
the local host.
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 4.5.3 above.
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4.7. The SEAL Control Message Protocol (SCMP)
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) [RFC4443]. 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 Figure 4:
+--------------------+
~ 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
Figure 4: SCMP Message Encapsulation
The following sections specify the generation, processing and
relaying of SCMP messages.
4.7.1. Generating SCMP Error Messages
ETEs generate SCMP error messages in response to receiving certain
SEAL data packets using the format shown in Figure 5:
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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"
Figure 5: SCMP Error Message Format
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.
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 Figure 5. 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.
The ETE next encapsulates the SCMP message in the requisite SEAL
header, outer headers and SEAL trailer as shown in Figure 4. 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.
The ETE then sets (C=1; A=0; R=0; F=1; P=0; NEXTHDR=0) in the SEAL
header, then sets PREFLEN and LEVEL to the same values that appeared
in the SEAL data packet header. The ETE then writes the value 0 in
the LINK_ID field and writes a value from within the current window
for this ITE in the PKT_ID .
The ETE then calculates and sets the ICV1 and ICV2 fields the same as
specified for SEAL data packet encapsulation in Section 4.5.4 (the
ETE instead omits the SEAL trailer if the SCMP message includes fewer
than 128 bytes beyond the SEAL header). Next, the ETE encapsulates
the SCMP message in the requisite outer encapsulations the same as
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specified for SEAL data packets in Section 4.5.5 (while performing
outer fragmentation on the packet if necessary) then sends the
resulting SCMP packet to the ITE.
The following sections describe additional considerations for various
SCMP error messages:
4.7.1.1. Generating SCMP Packet Too Big (SPTB) Messages
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 F=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 outer IP first fragment (i.e., the
fragment with MF=1 and Offset=0) in the MTU field of the message.
The ETE also generates an SPTB message when it accepts a SEAL
protocol data packet with A=1 in the SEAL header. The ETE prepares
the SPTB message the same as above, except that it writes the value 0
in the MTU field. The message is therefore a control plane
acknowledgement of a data plane probe, and does not signify a packet
size restriction.
4.7.1.2. Generating Other SCMP Error Messages
An ETE generates an SCMP "Destination Unreachable" (SDU) message
under the same circumstances that an IPv6 system would generate an
ICMPv6 Destination Unreachable message.
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.
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
[I-D.templin-intarea-vet].
4.7.2. Processing SCMP Error Messages
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:
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4.7.2.1. Processing SCMP PTB Messages
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 4.7.1.1). If the SPTB message has
MTU=0, the ITE processes the message as confirmation that the ETE has
accepted the packet indicated by the packet-in-error; the ITE then
discards the SPTB message. If the SPTB message is the response to a
fragmented SNS message used for path qualification (see Section
4.5.6), the ITE processes the message as a confirmation that the path
supports IP fragmentation. Otherwise, the ITE processes the message
as an indication of a packet size limitation.
If the MTU value in the SPTB message is less 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_MIN_MTU for this ETE
link path to TRUE. The ITE then discards the SPTB message.
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 first fragments. In that case, the ITE
can consult a plateau table (e.g., as described in [RFC1191]) 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.
The ITE then checks its 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 4.5.7) and sends the resulting transcribed message to the
original source.
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 [I-D.templin-intarea-vet], 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
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message.
4.7.2.2. Processing Other SCMP Error Messages
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 4.7.2.1.
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.
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
[I-D.templin-intarea-vet].
5. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
6. End System Requirements
End systems are encouraged to implement end-to-end MTU assurance
(e.g., using Packetization Layer Path MTU Discovery per [RFC4821])
even if the subnetwork is using SEAL.
7. Router Requirements
Routers within the subnetwork are expected to observe the router
requirements found in the normative references, including the
implementation of IP fragmentation and reassembly [RFC1812][RFC2460]
as well as the generation of ICMP messages [RFC0792][RFC4443].
8. IANA Considerations
The IANA is instructed to allocate an IP protocol number for
'SEAL_PROTO' in the 'protocol-numbers' registry.
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The IANA is instructed to allocate a Well-Known Port number for
'SEAL_PORT' in the 'port-numbers' registry.
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.
9. Security Considerations
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 [RFC5246].
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.
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.
Security issues that apply to tunneling in general are discussed in
[RFC6169].
10. Related Work
Section 3.1.7 of [RFC2764] 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.
Section 3 of [RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU.
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Section 4 of [RFC2460] specifies a method for inserting and
processing extension headers between the base IPv6 header and
transport layer protocol data. The SEAL header is inserted and
processed in exactly the same manner.
IPsec/AH is [RFC4301][RFC4301] is used for full message integrity
verification between tunnel endpoints, whereas SEAL only ensures
integrity for the inner packet headers. The AYIYA proposal
[I-D.massar-v6ops-ayiya] uses similar means for providing full
message authentication and integrity.
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.
11. Acknowledgments
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.
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.
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.
12. References
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12.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[RFC3971] Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
Neighbor Discovery (SEND)", RFC 3971, March 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
12.2. Informative References
[FOLK] Shannon, C., Moore, D., and k. claffy, "Beyond Folklore:
Observations on Fragmented Traffic", December 2002.
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
October 1987.
[I-D.ietf-intarea-ipv4-id-update]
Touch, J., "Updated Specification of the IPv4 ID Field",
draft-ietf-intarea-ipv4-id-update-04 (work in progress),
September 2011.
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-05 (work in progress),
July 2011.
[I-D.massar-v6ops-ayiya]
Massar, J., "AYIYA: Anything In Anything",
draft-massar-v6ops-ayiya-02 (work in progress), July 2004.
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[I-D.templin-aero]
Templin, F., "Asymmetric Extended Route Optimization
(AERO)", draft-templin-aero-04 (work in progress),
October 2011.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-27 (work in progress),
October 2011.
[I-D.templin-ironbis]
Templin, F., "The Internet Routing Overlay Network
(IRON)", draft-templin-ironbis-06 (work in progress),
October 2011.
[MTUDWG] "IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November
1989 - February 1995.".
[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, July 1988.
[RFC1070] Hagens, R., Hall, N., and M. Rose, "Use of the Internet as
a subnetwork for experimentation with the OSI network
layer", RFC 1070, February 1989.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, February 2000.
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[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, March 2007.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963, July 2007.
[RFC4987] Eddy, W., "TCP SYN Flooding Attacks and Common
Mitigations", RFC 4987, August 2007.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
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[RFC5720] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720,
February 2010.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and
Addressing in Networks with Global Enterprise Recursion
(RANGER) Scenarios", RFC 6139, February 2011.
[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169, April 2011.
[SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU
Discovery Behavior", November 2010.
[TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
October 2004.
[TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List,
http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May
1987 - May 1990.".
[WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and
Debugging Path MTU Discovery Failures", October 2005.
Appendix A. Reliability
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 [RFC3366]
to increase packet delivery ratios, while links with much lower BERs
typically omit such mechanisms. Since SEAL tunnels may traverse
arbitrarily-long paths over links of various types that are already
either performing or omitting ARQ as appropriate, it would therefore
often be inefficient to also require the tunnel endpoints to also
perform ARQ.
Appendix B. Integrity
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
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packet also contains a trailer with a secondary ICV that covers the
remainder of the packet.
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.
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 [RFC4963] increasing the likelihood of reassembly mis-
associations
SEAL avoids reassembly mis-associations through the use of extended
ICVs, and also discards any reassembled packets larger than 1280
bytes.
Appendix C. Transport Mode
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.
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.
Appendix D. Historic Evolution of PMTUD
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 [TCP-IP]. The discussion that
followed provided significant reference material for [FRAG]. An IETF
Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
with charter to produce an RFC. Several variations on a very few
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basic proposals were entertained, including:
1. Routers record the PMTUD estimate in ICMP-like path probe
messages (proposed in [FRAG] and later [RFC1063])
2. 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)
3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw
RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)
4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
1990)
5. 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 [RFC1191]
by Mogul and Deering).
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 [RFC1191] and later [RFC1981].
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:
o routers upgrade their software faster than hosts
o PCs could not reassemble fragmented packets
o Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets
o Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)
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o the 16-bit IP_ID field could wrap around and disrupt reassembly at
high packet arrival rates
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.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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