Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track October 18, 2011
Expires: April 20, 2012
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-32.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.
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This Internet-Draft will expire on April 20, 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
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9
4. SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 10
4.1. VET Interface Model . . . . . . . . . . . . . . . . . . . 10
4.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 10
4.3. SEAL Header Format . . . . . . . . . . . . . . . . . . . . 12
4.4. ITE Specification . . . . . . . . . . . . . . . . . . . . 14
4.4.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
4.4.2. Tunnel Interface Soft State . . . . . . . . . . . . . 15
4.4.3. Submitting Packets for Encapsulation . . . . . . . . . 17
4.4.4. Mid-Layer Encapsulation . . . . . . . . . . . . . . . 17
4.4.5. SEAL Segmentation . . . . . . . . . . . . . . . . . . 17
4.4.6. SEAL Encapsulation . . . . . . . . . . . . . . . . . . 18
4.4.7. Outer Encapsulation . . . . . . . . . . . . . . . . . 19
4.4.8. Sending SEAL Protocol Packets . . . . . . . . . . . . 19
4.4.9. Probing Strategy . . . . . . . . . . . . . . . . . . . 19
4.4.10. Processing ICMP Messages . . . . . . . . . . . . . . . 20
4.4.11. Black Hole Detection . . . . . . . . . . . . . . . . . 21
4.5. ETE Specification . . . . . . . . . . . . . . . . . . . . 21
4.5.1. Reassembly Buffer Requirements . . . . . . . . . . . . 21
4.5.2. Tunnel Interface Soft State . . . . . . . . . . . . . 22
4.5.3. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 22
4.5.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 22
4.5.5. Decapsulation and Delivery to Upper Layers . . . . . . 24
4.6. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 24
4.6.1. Generating SCMP Error Messages . . . . . . . . . . . . 25
4.6.2. Processing SCMP Error Messages . . . . . . . . . . . . 28
4.6.3. Generating and Processing Other SCMP Message Types . . 30
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 30
6. End System Requirements . . . . . . . . . . . . . . . . . . . 30
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 30
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 32
11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 32
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 33
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 34
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13.1. Normative References . . . . . . . . . . . . . . . . . . . 34
13.2. Informative References . . . . . . . . . . . . . . . . . . 34
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 38
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 38
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 39
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
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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 (e.g., 1Gbps Ethernet),
however, this number may soon become too small by several orders of
magnitude for high data rate packet sources such as tunnel endpoints
[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
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ranging from minor 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], this approach requires
tunnels to present a consistent MTU the same as for ordinary links on
the end-to-end path. Moreover, in current practice existing
tunneling protocols mask the MTU issues by selecting a "lowest common
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denominator" MTU that may be much smaller than necessary for most
paths and difficult to change at a later date. Due to these many
consideration, a new approach to accommodate tunnels over links with
diverse MTUs is necessary.
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 accommodates links with diverse
MTUs, protects against off-path denial-of-service attacks, and can be
configured to enable efficient duplicate packet detection through the
use of a minimal mid-layer encapsulation.
SEAL specifically treats tunnels that traverse the subnetwork as
ordinary links that must support network layer services. As for any
link, tunnels that use SEAL must provide suitable networking services
including best-effort datagram delivery, integrity and consistent
handling of packets of various sizes. As for any link whose media
cannot provide suitable services natively, tunnels that use SEAL
employ link-level adaptation functions to meet the legitimate
expectations of the network layer service. As this is essentially a
link level adaptation, SEAL is therefore permitted to alter packets
within the subnetwork as long as it restores them to their original
form when they exit the subnetwork. The mechanisms described within
this document are designed precisely for this purpose.
SEAL encapsulation provides extended identification fields as well as
a mid-layer segmentation and reassembly capability that allows
simplified cutting and pasting of packets. Moreover, SEAL provides
dynamic mechanisms to ensure a functional path MTU on the path from
the ITE to the ETE. This is in contrast to static approaches which
avoid MTU issues by selecting a lowest common denominator MTU value
that may be overly conservative for the vast majority of tunnel paths
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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:
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
mid-layer or outer encapsulations are added. Internet protocol
numbers that identify inner packets are found in the IANA Internet
Protocol registry [RFC3232].
mid-layer packet
a packet resulting from adding mid-layer encapsulating headers to
an inner packet.
outer IP packet
a packet resulting from adding an outer IP header (and possibly
other outer headers) to a mid-layer 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.).
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IP, IPvX, IPvY
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 sum of MHLEN and OHLEN
ITE - Ingress Tunnel Endpoint
MHLEN - the length of any mid-layer headers and trailers
MRU - Maximum Reassembly Unit
MTU - Maximum Transmission Unit
OHLEN - the length of any outer encapsulating headers and trailers
S_MRU - SEAL Maximum Reassembly Unit
S_MSS - SEAL Maximum Segment Size
SCMP - the SEAL Control Message Protocol
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.
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3. Applicability Statement
SEAL was originally motivated by the specific case of subnetwork
abstraction for Mobile Ad hoc Networks (MANETs), however it soon
became apparent that the domain of applicability also extends to
subnetwork abstractions over enterprise networks, ISP networks, SOHO
networks, the global public Internet itself, and any other connected
network routing region. SEAL along with the Virtual Enterprise
Traversal (VET) [I-D.templin-intarea-vet] tunnel virtual interface
abstraction are the functional building blocks for a new
Internetworking architecture based on Routing and Addressing in
Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
and the Internet Routing Overlay Network (IRON) [RFC6179].
SEAL provides a network sublayer for encapsulation of an inner
network layer packet within outer encapsulating headers. For
example, for IPvX in IPvY encapsulation (e.g., as IPv4/SEAL/IPv6),
the SEAL header appears as a subnetwork encapsulation as seen by the
inner IP layer. SEAL can also be used as a sublayer within a
transport layer protocol data payload (e.g., as IPv4/UDP/SEAL/IPv6
similar to Teredo [RFC4380]), 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.
SEAL supports a segmentation and reassembly capability for adapting
the network layer to the underlying subnetwork characteristics, where
the Egress Tunnel Endpoint (ETE) determines how much or how little
reassembly it is willing to support. In the limiting case, the ETE
can avoid reassembly altogether and act as a passive observer that
simply informs the Ingress Tunnel Endpoint (ITE) of any MTU
limitations and otherwise discards all packets that arrive as
multiple fragments. This mode is useful for determining an
appropriate MTU for tunneling between performance-critical routers
connected to high data rate subnetworks such as the Internet DFZ, for
unidirectional tunneling in which the ETE is stateless, and for other
uses in which reassembly would present too great of a burden for the
routers or end systems.
When the ETE supports reassembly, the tunnel can be used to transport
packets that are too large to traverse the path without
fragmentation. In this mode, the ITE determines the tunnel MTU based
on the largest packet the ETE is capable of reassembling rather than
on the MTU of the smallest link in the path. Therefore, tunnel
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endpoints that use SEAL can transport packets that are much larger
than the underlying subnetwork links themselves can carry in a single
piece.
SEAL tunnels may be configured over paths that include not only
ordinary physical links, but also virtual links that may include
other tunnels. An example application would be linking two
geographically remote supercomputer centers with large MTU links by
configuring a SEAL tunnel across the Internet. A second example
would be support for sub-IP segmentation over low-end links, i.e.,
especially over wireless transmission media such as IEEE 802.15.4,
broadcast radio links in Mobile Ad-hoc Networks (MANETs), Very High
Frequency (VHF) civil aviation data links, etc.
Many other use case examples are anticipated, and will be identified
as further experience is gained.
4. SEAL Specification
The following sections specify the operation of SEAL:
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 enterprise network, or "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 TE
to send data packets forward to the far end TE, while the far end
only returns control messages when necessary. A bidirectional tunnel
neighbor relationship is one over which both TEs can exchange both
data and control messages.
Implications of the VET unidirectional and bidirectional models for
SEAL are discussed in [I-D.templin-intarea-vet].
4.2. SEAL Model of Operation
SEAL supports a multi-level segmentation and reassembly capability
for the transmission of unicast and multicast packets across an
underlying IP subnetwork which may include heterogeneous links.
First, the ITE can use IPv4 fragmentation to fragment inner IPv4
packets before SEAL encapsulation if necessary. Secondly, the SEAL
layer itself provides a simple cutting-and-pasting capability for
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mid-layer packets that can be used to avoid IP fragmentation on the
outer packet. Finally, ordinary IP fragmentation is permitted on the
outer packet after SEAL encapsulation, and allows the TEs to detect
and tune out any in-the-network fragmentation.
SEAL-enabled ITEs encapsulate each inner packet in any mid-layer
headers and trailers, segment the resulting mid-layer packet into
multiple segments if necessary, then append a SEAL header and any
outer encapsulations to each segment. As an example, for IPv6 within
IPv4 encapsulation a single-segment inner IPv6 packet encapsulated in
any mid-layer headers and trailers, followed by the SEAL header,
followed by any outer headers and trailers, followed by an outer IPv4
header would appear as shown in Figure 1:
+--------------------+
~ outer IPv4 header ~
+--------------------+
I ~ other outer hdrs ~
n +--------------------+
n ~ SEAL Header ~
e +--------------------+ +--------------------+
r ~ mid-layer headers ~ ~ mid-layer headers ~
+--------------------+ +--------------------+
I --> | | --> | |
P --> ~ inner IPv6 ~ --> ~ inner IPv6 ~
v --> ~ Packet ~ --> ~ Packet ~
6 --> | | --> | |
+--------------------+ +--------------------+
P ~ mid-layer trailers ~ ~ mid-layer trailers ~
a +--------------------+ +--------------------+
c ~ outer trailers ~
k Mid-layer packet +--------------------+
e after mid-layer encaps.
t Outer IPv4 packet
after SEAL and outer encaps.
Figure 1: SEAL Encapsulation - Single Segment
As a second example, for IPv4 within IPv6 encapsulation an inner IPv4
packet requiring three SEAL segments would appear as three separate
outer IPv6 packets, where the mid-layer headers are carried only in
segment 0 and the mid-layer trailers are carried in segment 2 as
shown in Figure 2:
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+------------------+ +------------------+ +------------------+
~ outer IPv6 hdr ~ ~ outer IPv6 hdr ~ ~ outer IPv6 hdr ~
+------------------+ +------------------+ +------------------+
~ other outer hdrs ~ ~ other outer hdrs ~ ~ other outer hdrs ~
+------------------+ +------------------+ +------------------+
~ SEAL hdr (SEG=0) ~ ~ SEAL hdr (SEG=1) ~ ~ SEAL hdr (SEG=2) ~
+------------------+ +------------------+ +------------------+
| mid-layer hdrs | | inner IPv4 | | inner IPv4 Packet|
~ plus inner IPv4 ~ ~ Packet Segment ~ ~ Segment plus ~
~ Packet Segment ~ ~ (Length = L) ~ ~ mid-layer trails ~
| (Length = L) | | | | (Len may be !=L) |
+------------------+ +------------------+ +------------------+
~ outer trailers ~ ~ outer trailers ~ ~ outer trailers ~
+------------------+ +------------------+ +------------------+
Segment 0 (includes Segment 1 (no mid- Segment 2 (includes
mid-layer hdrs) layer encaps) mid-layer trails)
Figure 2: SEAL Encapsulation - Multiple Segments
The ITE inserts the SEAL header according to the specific tunneling
protocol. Examples include the following:
o For simple encapsulation of an inner network layer packet within
an outer IPvX header (e.g., [RFC1070][RFC2003][RFC2473][RFC4213],
etc.), the ITE inserts the SEAL header between the inner packet
and outer IPvX headers as: IPvX/SEAL/{inner packet}.
o For encapsulations over transports such as UDP (e.g., in the same
manner as for [RFC4380]), the ITE inserts the SEAL header between
the outer transport layer header and the mid-layer packet, e.g.,
as IPvX/UDP/SEAL/{mid-layer packet}. (Here, the UDP header is
seen as an "other outer header" as depicted in Figure 2.)
The SEAL header includes per-neighbor, per-link and per-packet
identification values which routers within the subnetwork can use for
duplicate packet detection and both TEs can use for SEAL
segmentation/reassembly.
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:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|A|I|F|M|R| NEXTHDR/SEG | LINK_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NBR_ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PKT_ID (when necessary) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: 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.
I (1)
the "Identification Field Included" bit. Set to 1 if the SEAL
header includes a 32-bit packet Identification field (see below);
set to 0 otherwise.
F (1)
the "First Segment" bit. Set to 1 if this SEAL packet contains
the first segment (i.e., Segment #0) of a mid-layer packet.
M (1)
the "More Segments" bit. Set to 1 if this SEAL packet contains a
non-final segment of a multi-segment mid-layer packet.
R (1)
the "Redirect Requested" bit. Set to 1 by the ITE to inform the
ETE that a Redirect should be sent (see:
[I-D.templin-intarea-vet]).
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NEXTHDR/SEG (8) an 8-bit field. When 'F'=1, encodes the next header
Internet Protocol number the same as for the IPv4 protocol and
IPv6 next header fields. When 'F'=0, encodes a segment number of
a multi-segment mid-layer packet. (The segment number 0 is
reserved.)
LINK_ID (16)
a 16-bit link identification value, set to a unique value by the
ITE for each underlying link over which it will send encapsulated
packets to the ETE. Used as a neighbor selector adjunct for the
NBR_ID.
NBR_ID (32)
a 32-bit neighbor identification value. Set to a random value by
the ETE in an initial exchange with the ITE, and used as a tunnel
neighbor selector in conjunction with the LINK_ID.
PKT_ID (32)
a 32-bit per-packet identification field. Present only when the I
bit is set to 1 (see above).
Setting of the various bits and fields of the SEAL header is
specified in the following sections.
4.4. ITE Specification
4.4.1. 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
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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.
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.
For all other inner packets, the inner network layer admits the
packet if it is no larger than the tunnel interface MTU; otherwise,
it drops the packet and sends a PTB error message to the source with
the MTU value set to the tunnel interface MTU. The message must
contain as much of the invoking packet as possible without the entire
message exceeding the network layer minimum MTU (e.g., 576 bytes for
IPv4, 1280 bytes for IPv6, etc.). For SEAL packets that would
undergo recursive encapsulation, however, the inner layer must send a
SEAL PTB message instead of a PTB of the inner network layer (see:
Section 4.4.3).
In light of the above considerations, the ITE SHOULD configure an
indefinite MTU on tunnel *router* interfaces, since these may be
required to carry recursively-nested SEAL encapsulations. The ITE
MAY instead set a finite MTU on tunnel *host* interfaces. Any
necessary tunnel adaptations are then performed by the SEAL layer
within the tunnel interface as described in the following sections.
4.4.2. Tunnel Interface Soft State
The ITE maintains per-ETE soft state within the tunnel interface,
e.g., in a neighbor cache. The soft state includes the following:
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o a Mid-layer Header Length (MHLEN); set to the length of any mid-
layer encapsulation headers and trailers that must be added before
SEAL segmentation.
o an Outer Header Length (OHLEN); set to the length of the outer IP,
SEAL and other outer encapsulation headers and trailers.
o a total Header Length (HLEN); set to MHLEN plus OHLEN.
o a SEAL Maximum Segment Size (S_MSS). The ITE initializes S_MSS to
the minimum MTU of the underlying interfaces if the underlying
interface MTUs can be determined (otherwise, the ITE initializes
S_MSS to "infinity"). The ITE decreases or increases S_MSS based
on any SCMP "Packet Too Big (PTB)" messages received (see Section
4.6).
o a SEAL Maximum Reassembly Unit (S_MRU). If the ITE is not
configured to use SEAL segmentation, it initializes S_MRU to the
constant value 0 and ignores any S_MRU values reported by the ETE.
Otherwise, the ITE initializes S_MRU to "infinity" (i.e., the
largest possible inner packet size) and decreases or increases
S_MRU based on any SCMP PTB messages received from the ETE (see
Section 4.6). When (S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256)
as the effective S_MRU value.
o a NBR_ID value that is coordinated with the ETE and used to fill
the SEAL header field of the same name for packets sent to this
ETE.
o one or more LINK_ID values that are coordinated with the ETE and
used to fill the SEAL header field of the same name for packets
sent to this ETE.
o for each LINK_ID, an outer IP address (and transport layer port
number when upper layer encapsulation is used) for use as the
destination addresses for each packet sent to this ETE.
o a PKT_ID value that is randomly-initialized and monotonically-
incremented for each packet sent to this ETE.
Note that S_MSS and S_MRU include the length of the outer and mid-
layer encapsulating headers and trailers (i.e., HLEN), since the ETE
must retain the headers and trailers during reassembly. Note also
that the ITE maintains S_MSS and S_MRU as 32-bit values such that
inner packets larger than 64KB (e.g., IPv6 jumbograms [RFC2675]) can
be accommodated when appropriate for a given subnetwork.
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4.4.3. Submitting Packets for Encapsulation
The ITE prepares each inner packet/fragment admitted into the tunnel
interface for encapsulation according to its length. For IPv4 inner
packets with DF=0 in the IPv4 header, if the packet is larger than
S_MSS bytes the ITE fragments it into IPv4 fragments no larger than
S_MSS then submits each fragment for encapsulation separately. For
packets no larger than S_MSS bytes, the ITE instead submits the
unfragmented packet for encapsulation.
For all other inner packets, if the packet is larger than (MAX(S_MRU,
S_MSS) - HLEN), the ITE discards it and sends a PTB message to the
source with an MTU value of (MAX(S_MRU, S_MSS) - HLEN); otherwise,
the ITE submits the packet for encapsulation. The ITE must include
the length of the uncompressed headers and trailers when calculating
HLEN even if the tunnel is using header compression. The ITE is also
permitted to submit inner packets for encapsulation if they can be
accommodated in a single SEAL segment (i.e., no larger than S_MSS)
even if they are larger than the ETE would be willing to reassemble
if fragmented (i.e., larger than S_MRU) - see: Section 4.5.1.
NB: A stateless MTU discovery capability is possible for paths in
which inner packet headers are transmitted in-the-clear, when the
inner network layer protocol does not permit in-the-network
fragmentation (e.g., IPv6), and when either the ITE or ETE sets S_MRU
to 0. In that case, the ITE submits each packet that is no larger
than the outgoing underlying interface for encapsulation and
statelessly processes any resulting SCMP PTB messages as described in
Section 4.6.2.1.
4.4.4. Mid-Layer Encapsulation
After inner IP fragmentation (if necessary), the ITE next
encapsulates each inner packet/fragment in the MHLEN bytes of any
mid-layer headers and trailers. The ITE then submits the mid-layer
packet for SEAL segmentation and encapsulation.
4.4.5. SEAL Segmentation
If the ITE is configured to use SEAL segmentation, it checks the
length of the resulting packet after mid-layer encapsulation to
determine whether segmentation is needed. If the length of the
resulting mid-layer packet plus OHLEN is larger than S_MSS but no
larger than S_MRU the ITE performs SEAL segmentation by breaking the
mid-layer packet into N segments (N <= 256) that are no larger than
(S_MSS - OHLEN) bytes each.
When the ITE performs SEAL segmentation, it MUST segment the mid-
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layer packet such that the first segment includes at least the mid-
layer headers. (When the inner packet header is available in-the-
clear, the first segment MUST also include the inner header.) Each
segment except the final one MUST be of equal length, and the first
byte of each segment MUST begin immediately after the final byte of
the previous segment, i.e., the segments MUST NOT overlap. The ITE
SHOULD generate the smallest number of segments possible, e.g., it
SHOULD NOT generate 6 smaller segments when the packet could be
accommodated with 4 larger segments.
This SEAL segmentation process ignores the fact that the mid-layer
packet may be unfragmentable outside of the subnetwork. The process
is a mid-layer (not an IP layer) operation employed by the ITE to
adapt the mid-layer packet to the subnetwork path characteristics,
and the ETE will restore the packet to its original form during
reassembly. Therefore, the fact that the packet may have been
segmented within the subnetwork is not observable outside of the
subnetwork.
4.4.6. SEAL Encapsulation
Following SEAL segmentation, the ITE next encapsulates each segment
in a SEAL header formatted as specified in Section 4.3.
For the first segment, the ITE then sets F=1, and sets M=1 if there
are more segments or sets M=0 otherwise. The ITE then sets NEXTHDR
to the Internet Protocol number corresponding to the encapsulated
inner packet. For example, the ITE sets NEXTHDR to the value '4' for
encapsulated IPv4 packets [RFC2003], the value '41' for encapsulated
IPv6 packets [RFC2473][RFC4213], the value '50' for encapsulated
IPsec/ESP payloads [RFC4301][RFC4303], the value '80' for
encapsulated OSI packets [RFC1070], etc.
For each non-initial segment of an N-segment mid-layer packet (N <=
256), the ITE instead sets (F=0; M=1; SEG=1) in the SEAL header of
the first non-initial segment, sets (F=0; M=1; SEG=2) in the next
non-initial segment, etc., and sets (F=0; M=0; SEG=N-1) in the final
segment. (Note that the value SEG=0 is not used, since the initial
segment encodes a NEXTHDR value and not a SEG value.)
For each segment (i.e., both initial and non-initial), the ITE then
sets (C=0; R=0) and also sets A=1 if an explicit acknowledgement is
required (see Section 4.4.9). The ITE then sets the NBR_ID and
LINK_ID fields in order to identify itself to the ETE.
Finally, for each SEAL segment of a multi-segment SEAL packet, the
ITE sets I=1 and includes the current PKT_ID value in a trailing 32-
bit Identification field in the SEAL header of each segment. For
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each SEAL packet that will be sent as a single segment, however, the
ITE MAY set I=0 and omit the trailing PKT_ID field. Whether or not
the PKT_ID field was included, the ITE then monotonically increments
the PKT_ID value (modulo 2^32) for the next SEAL packet to be sent to
the ETE. This allows the ETE to determine whether a large number of
SEAL packets have been received since an incomplete reassembly was
initiated.
4.4.7. Outer Encapsulation
Following SEAL encapsulation, the ITE next encapsulates each SEAL
segment in the requisite outer headers and trailers according to the
specific encapsulation format (e.g., [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). The ITE then writes
the outer IP address for this ETE in the destination address of the
outer IP header.
When IPv4 is used as the outer encapsulation layer, the ITE finally
sets the DF flag in the IPv4 header of each segment. If the path to
the ETE correctly implements IP fragmentation (see: Section 4.4.9),
the ITE sets DF=0; otherwise, it sets DF=1.
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.
4.4.8. Sending SEAL Protocol Packets
Following outer encapsulation, the ITE sends each outer packet that
encapsulates a segment of the same mid-layer packet over the same
underlying link in canonical order, i.e., segment 0 first, followed
by segment 1, etc., and finally segment N-1.
4.4.9. Probing Strategy
When IPv4 is used as the outer encapsulation layer, the ITE can
perform a qualification exchange over each underlying link to
determine whether each subnetwork path to the ETE correctly
implements IPv4 fragmentation. This procedure could be employed,
e.g., to determine whether there are any middleboxes on the path that
violate the [RFC1812], Section 5.2.6 requirement that: "A router MUST
NOT reassemble any datagram before forwarding it".
To perform this qualification, the ITE prepares a probe packet that
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is no larger than 576 bytes (e.g., a NULL packet with A=1 and
NEXTHDR="No Next Header" [RFC2460] in the SEAL header), then splits
the packet into two outer IPv4 fragments and sends both fragments to
the ETE over the same underlying link. If the ETE returns an SCMP
PTB message with Code=0 (see Section 4.6.1.1), then the subnetwork
path correctly implements IPv4 fragmentation. If the ETE returns an
SCMP PTB message with Code=2, however, then a middlebox in the
subnetwork is reassembling the IPv4 fragments before they are
delivered to the ETE (i.e., in violation of [RFC1812]).
In addition to any control plane probing, all SEAL encapsulated data
packets sent by the ITE are considered implicit probes. SEAL
encapsulated packets that use IPv4 as the outer layer of
encapsulation with DF=0 will elicit SCMP PTB messages from the ETE if
any IPv4 fragmentation occurs in the path. SEAL encapsulated packets
that use either IPv6 or IPv4 with DF=1 as the outer layer of
encapsulation may be dropped by a router on the path to the ETE which
will also return an ICMP PTB message to the ITE. If the message
includes enough information (see Section 4.4.10), the ITE can then
use the (NBR_ID, LINK_ID, PKT_ID)-tuple along with the destination
addresses within the packet-in-error to determine whether the PTB
message corresponds to one of its recent packet transmissions.
The ITE should also send explicit probes, periodically, to verify
that the ETE is still reachable. The ITE sets A=1 in the SEAL header
of a packet to be used as an explicit probe, where the probe can be
either an ordinary data packet segment or a NULL packet (see above).
The probe will elicit an SCMP PTB message with Code=2 from the ETE as
an acknowledgement (see Section 4.6.1.1).
4.4.10. Processing ICMP Messages
When the ITE sends outer IP packets, it may receive ICMP error
messages [RFC0792][RFC4443] from either the ETE or routers within the
subnetwork. The ICMP messages include an outer IP header, followed
by an ICMP header, followed by a portion of the outer IP packet that
generated the error (also known as the "packet-in-error"). The ITE
can use the (NBR_ID, LINK_ID, PKT_ID)-tuple along with the source and
destination addresses within the packet-in-error to confirm that the
ICMP message came from either the ETE or an on-path router, and can
use any additional information to determine whether to accept or
discard the message.
The ITE should specifically process raw ICMPv4 Protocol Unreachable
messages and ICMPv6 Parameter Problem messages with Code
"Unrecognized Next Header type encountered" as a hint that the ETE
does not implement the SEAL protocol. The ITE can also process other
raw ICMPv4 messages as a hint that the path to the ETE may be
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failing. Specific actions that the ITE may take in these cases are
out of scope.
4.4.11. Black Hole Detection
In some subnetwork paths, ICMP error messages may be lost due to
filtering or may not contain enough information due to a router in
the path not observing the recommendations of [RFC1812]. The ITE can
use explicit probing as described in Section 4.4.9 to determine
whether the path to the ETE is silently dropping packets (also known
as a "black hole"). For example, when the ITE is obliged to set DF=1
in the outer headers of data packets it should send explicit probe
packets, periodically, in order to detect path MTU increases or
decreases.
4.5. ETE Specification
4.5.1. Reassembly Buffer Requirements
The ETE SHOULD support the minimum IP-layer reassembly requirements
specified for IPv4 (i.e., 576 bytes [RFC1812]) and IPv6 (i.e., 1500
bytes [RFC2460]). The ETE SHOULD also support SEAL-layer reassembly
for inner packets of at least 1280 bytes in length and MAY support
reassembly for larger inner packets. The ETE records the SEAL-layer
reassembly buffer size in a soft-state variable "S_MRU" (see: Section
4.5.2).
The ETE may instead omit the reassembly function altogether and set
S_MRU=0, but this may cause ITEs to experience tunnel MTU underruns
over some paths resulting in an unusable link. When reassembly is
supported, the ETE must retain the outer IP, SEAL and other outer
headers and trailers during both IP-layer and SEAL-layer reassembly
for the purpose of associating the fragments/segments of the same
packet, and must also configure a SEAL-layer reassembly buffer that
is no smaller than the IP-layer reassembly buffer. Hence, the ETE:
o SHOULD configure an outer IP-layer reassembly buffer of at least
the minimum specified for the outer IP protocol version.
o SHOULD configure a SEAL-layer reassembly buffer S_MRU size of at
least (1280 + HELN) bytes, and
o MUST be capable of discarding inner packets that require IP-layer
and/or SEAL-layer reassembly and that are larger than (S_MRU -
HLEN).
The ETE is permitted to accept inner packets that did not undergo IP-
layer and/or SEAL-layer reassembly even if they are larger than
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(S_MRU - HELN) bytes. Hence, S_MRU is a maximum *reassembly* size,
and may be less than the largest packet size the ETE is able to
receive when no reassembly is required.
4.5.2. Tunnel Interface Soft State
The ETE maintains a per-interface default S_MRU value to be applied
for all ITEs, and can optionally maintain individual per-ITE S_MRU
values that override the default.
The ETE may also maintain per-ITE soft state to associate (NBR_ID,
LINK_ID, PKT_ID)-tuples with the inner and/or mid-layer source
addresses used by ITEs, e.g., for ingress filtering purposes (see
Section 4.5.5).
4.5.3. IP-Layer Reassembly
The ETE submits unfragmented SEAL protocol IP packets for SEAL-layer
reassembly as specified in Section 4.5.4. The ETE instead performs
standard IP-layer reassembly for multi-fragment SEAL protocol IP
packets as follows.
The ETE should maintain conservative IP-layer reassembly cache high-
and low-water marks. When the size of the reassembly cache exceeds
this high-water mark, the ETE should actively discard incomplete
reassemblies (e.g., using an Active Queue Management (AQM) strategy)
until the size falls below the low-water mark. The ETE should also
actively discard any pending reassemblies that clearly have no
opportunity for completion, e.g., when a considerable number of new
fragments have arrived before a fragment that completes a pending
reassembly arrives. Following successful IP-layer reassembly, the
ETE submits the reassembled packet for SEAL-layer reassembly as
specified in Section 4.5.4.
When the ETE processes the IP first fragment (i.e., one with MF=1 and
Offset=0 in the IP header) of a fragmented SEAL packet for which the
(NBR_ID, LINK_ID, PKT_ID)-tuple belongs to a neighboring ITE, it
sends an SCMP PTB message with Code=0 back to the ITE (see Section
4.6.1.1). When the ETE processes an IP fragment that would cause the
reassembled outer packet to be larger than the IP-layer reassembly
buffer following reassembly, it discontinues the reassembly and
discards any further fragments of the same packet.
4.5.4. SEAL-Layer Reassembly
Following IP reassembly (if necessary), the ETE examines each SEAL
data packet (i.e., those with C=0 in the SEAL header) to determine
whether an SCMP error message is required. If the packet has an
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incorrect value in the SEAL header the ETE discards the packet and
returns an SCMP "Parameter Problem" message (see Section 4.6.1.2).
Next, if the SEAL header has A=1 and the packet did not arrive as
multiple outer IP fragments, the ETE sends an SCMP PTB message with
Code=2 back to the ITE (see Section 4.6.1.1). The ETE next submits
single-segment mid-layer packets for decapsulation and delivery to
upper layers (see Section 4.5.5). The ETE instead performs SEAL-
layer reassembly for multi-segment mid-layer packets with I=1 in the
SEAL header as follows.
The ETE adds each segment of a multi-segment mid-layer packet with
I=1 in the SEAL header to a SEAL-layer pending-reassembly queue
according to the (NBR_ID, LINK_ID, PKT_ID)-tuple found in the SEAL
header. The ETE performs SEAL-layer reassembly through simple in-
order concatenation of the encapsulated segments of the same mid-
layer packet from N consecutive SEAL segments. SEAL-layer reassembly
requires the ETE to maintain a cache of recently received segments
for a hold time that would allow for nominal inter-segment delays.
When a SEAL reassembly times out, the ETE discards the incomplete
reassembly and returns an SCMP "Time Exceeded" message to the ITE
using the cached SEAL first segment as the packet-in-error (see
Section 4.6.1.2). As for IP-layer reassembly, the ETE should also
maintain a conservative reassembly cache high- and low-water mark and
should actively discard any pending reassemblies that clearly have no
opportunity for completion, e.g., when a considerable number of new
SEAL packets have been received before a packet that completes a
pending reassembly has arrived.
If the ETE receives a SEAL packet for which a segment with the same
(NBR_ID, LINK_ID, PKT_ID)-tuple is already in the queue, it must
determine whether to accept the new segment and release the old, or
drop the new segment. If accepting the new segment would cause an
inconsistency with other segments already in the queue (e.g.,
differing segment lengths), the ETE drops the segment that is least
likely to complete the reassembly. When the ETE receives a SEAL
segment would cause the size of the reassembled packet to exceed
S_MRU, the ETE schedules the reassembly resources for garbage
collection and sends an SCMP PTB message with Code=1 back to the ITE
(see Section 4.6.1.1).
After all segments are gathered, the ETE reassembles the packet by
concatenating the segments encapsulated in the N consecutive SEAL
packets beginning with the initial segment (i.e., SEG=0) and followed
by any non-initial segments 1 through N-1. That is, for an N-segment
mid-layer packet, reassembly entails the concatenation of the SEAL-
encapsulated packet segments with the same value in the
Identification field and with (F=1, M=1) in the first SEAL header,
followed by (F=0, M=1, SEG=1) in the next SEAL header, followed by
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(F=0, M=1, SEG=2), etc., up to (F=0, M=0, SEG=(N-1)) in the final
SEAL header. Following successful SEAL-layer reassembly, the ETE
submits the reassembled mid-layer packet for decapsulation and
delivery to upper layers as specified in Section 4.5.5.
The ETE must not perform SEAL-layer reassembly for multi-segment mid-
layer packets with I=0 in the SEAL header. The ETE instead drops all
segments with (I=0 && (F=0 || M=1)) in the SEAL header and sends an
SCMP Parameter Problem message back to the ITE.
4.5.5. Decapsulation and Delivery to Upper Layers
Following any necessary IP- and SEAL-layer reassembly, the ETE
performs ingress filtering on the mid-layer and/or inner source
addresses if necessary (e.g., via a Reverse-Path Forwarding (RPF)
lookup) to determine whether they are correct for the (NBR_ID,
LINK_ID, PKT_ID)-tuple encoded in the SEAL header. (When the outer
source address and/or port number for the ITE is known, they are also
included in the ingress filtering check.) If ingress filtering
determines that the source addresses are incorrect, the ETE silently
drops the packet.
Following ingress filtering, the ETE performs any mid-layer
transformations on the mid-layer packet and delivers the inner packet
to the upper-layer protocol identified either in the SEAL NEXTHDR
field or the next header field of the mid-layer packet (i.e., if the
packet included mid-layer encapsulations). The ETE instead silently
discards the inner packet if it was a NULL packet (see Section
4.4.9).
4.6. The SEAL Control Message Protocol (SCMP)
SEAL provides a companion SEAL Control Message Protocol (SCMP) that
uses the same basic message types and formats as for the Internet
Control Message Protocol for IPv6 (ICMPv6) [RFC4443]. The SCMP
message begins with a 4 byte header that includes 8-bit Type and Code
fields followed by a 16-bit Checksum field. The header is then
followed by a variable-length Message Body, as shown in Figure 4:
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Message Body ~
| |
Figure 4: SCMP Message Format
When the TE prepares an SCMP message, it sets the Type and Code
fields to the same values that would appear in the corresponding
ICMPv6 message, then formats the Message Body the same as for the
corresponding ICMPv6 message. The TE then calculates the SCMP
message Checksum the same as specified for ICMPv6, except that it
does not prepend a pseudo-header of the outer IP header. The TE then
encapsulates the SCMP message in the outer headers and trailers as
shown in Figure 5:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
~ SCMP message header~ --> ~ SCMP message header~
+--------------------+ --> +--------------------+
~ SCMP message body ~ --> ~ SCMP message body ~
+--------------------+ --> +--------------------+
~ outer trailers ~
SCMP Message +--------------------+
before encapsulation
SCMP Packet
after encapsulation
Figure 5: SCMP Message Encapsulation
The following sections specify the generation and processing of SCMP
messages.
4.6.1. Generating SCMP Error Messages
TEs generate SCMP error messages in response to receiving certain
SEAL data packets using the format shown in Figure 6:
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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 | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ Type-Specific Data (4/8 bytes) ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking SEAL data packet as possible |
~ possible (beginning immediately after the SEAL header) ~
| without the SCMP packet exceeding 576 bytes (*) |
(*) also known as the "packet-in-error"
Figure 6: SCMP Error Message Format
The error message includes the 4 byte SCMP message header, followed
by a 4 or 8 byte Type-Specific Data field, followed by the leading
portion of the invoking SEAL data packet (beginning immediately after
the SEAL header) as the "packet-in-error". The packet-in-error
includes as much of the leading portion of the invoking SEAL data
packet as possible beginning immediately after the SEAL header and
extending to a length that would not cause the entire SCMP packet
following outer encapsulation to exceed 576 bytes.
When an ETE processes a SEAL data packet that passes ingress
filtering (see: Section 4.5.5) but for which an error must be
returned, it prepares an SCMP error message as shown in Figure 6.
The ETE sets the Type and Code fields in the SCMP header according to
the appropriate error message type, fills out the Type-Specific Data
field, includes the packet-in-error, then calculates and sets the
Checksum.
The ETE next encapsulates the SCMP message in the requisite outer
headers as shown in Figure 5. During encapsulation, the ETE sets the
outer destination address/port numbers of the SCMP packet to the
outer source address/port numbers of the original data packet and
sets the outer source address/port numbers of the SCMP message to the
outer destination address/port numbers of the data packet. The ETE
then sets the SCMP packet's SEAL header fields to the same values
that appeared in the SEAL header of the data packet except that it
sets (A=0; C=1). The ETE then sends the resulting SCMP packet to the
source of the original SEAL data packet.
NB: A simplified implementation of this method entails creating a
copy of the original data packet, inserting the SCMP message header
and Type-Specific Data fields between the SEAL header and mid-layer
headers, truncating the resulting message to 576 bytes, then
rewriting the header fields as described above.
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The following sections describe additional considerations for various
SCMP error messages:
4.6.1.1. Generating SCMP Packet Too Big (PTB) Messages
An ETE generates an SCMP "Packet Too Big" (PTB) message under one of
the following cases:
o Case 0: when it receives the outer first fragment (e.g., an IP
packet with MF=1 and Offset=0 in the outer header) of a SEAL
protocol data packet that arrived as multiple outer fragments, or:
o Case 1: when it receives a SEAL protocol data packet that encodes
a segment that would cause the size of the reassembled packet to
exceed S_MRU, or:
o Case 2: when it receives a SEAL protocol data packet with A=1 in
the SEAL header that did not arrive as multiple outer fragments
(i.e., one that does not also match Case 0).
The ETE prepares an SCMP PTB message the same as for the
corresponding ICMPv6 PTB message, except that the "Type-Specific
Data" field is 8 bytes in length and encodes a 4-byte MSS field
followed by a 4-byte MRU field as shown in Figure 7:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MSS (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MRU (4 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: SCMP PTB Message Type-Specific Data Field Format
For case 0 above, the ETE writes the value in the outer header length
field into the MSS field of the Type-Specific Data; for cases 1 and
2, the ETE instead writes the value 0 into the MSS field. For all
cases, the ETE then writes the S_MRU value for this ITE in the MRU
field of the Type-Specific Data (i.e., even if the value is 0).
The ETE then writes the value '2' in the Type field (i.e., the same
value that would appear in an ICMPv6 PTB message) then writes the
value 0, 1 or 2 in the Code field of the PTB message according to
whether the reason for generating the message was due to the
corresponding case number from the list of cases above. The ETE
finally copies the leading portion of the SEAL data packet (beginning
immediately after the SEAL header) into the packet-in-error field and
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calculates the Checksum the same as for any SCMP message.
NB: Unlike cases 0 and 1 above, case 2 does not necessarily signify
packet loss. Instead, it is a control plane acknowledgement of a
data plane probe. Also, if the ETE generates a Case 0 SCMP PTB
message it MUST NOT also generate a Case 2 PTB message on behalf of
the same SEAL segment.
4.6.1.2. Generating Other SCMP Error Messages
An ETE generates an SCMP "Destination Unreachable" message under the
same circumstances that an IPv6 system would generate an ICMPv6
Destination Unreachable message. The ETE formats the SCMP
Destination Unreachable message the same as for ICMPv6 Destination
Unreachable messages, i.e., it writes the value '1' in the Type
field, writes the same Code and Type-Specific Data values, and
includes the leading portion of the SEAL data packet (beginning
immediately after the SEAL header) in the packet-in-error field.
An ETE generates an SCMP "Parameter Problem" message when it receives
a SEAL packet with an incorrect value in the SEAL header, and
generates an SCMP "Time Exceeded" message when it garbage collects an
incomplete SEAL data packet reassembly. The ETE formats the messages
the same as for the corresponding ICMPv6 messages, i.e., it includes
the same Type, Code and Type-Specific Data values, and includes the
leading portion of the SEAL data packet (beginning immediately after
the SEAL header) in the packet-in-error field.
4.6.2. Processing SCMP Error Messages
For each SCMP error message it receives, the TE first verifies that
the (NBR_ID, LINK_ID, PKT_ID)-tuple in the SEAL header as well as the
outer addresses of the SCMP packet are correct, then verifies that
the Checksum in the SCMP message header is correct. If the
identifying information and/or checksum are incorrect, the TE
discards the message; otherwise, it processes the message as follows:
4.6.2.1. Processing SCMP PTB Messages
An ITE may receive an SCMP PTB message with a packet-in-error
containing the leading portion of the mid-layer packet after it sends
a SEAL data packet to an ETE (see: Section 4.6.1.1) . The ITE first
records the value in the PTB message MRU field as the new S_MRU value
for this ETE. If the PTB message has Code=2 in the SCMP message
header, the ITE processes the message as both a response to an
explicit probe request and an indication that the tunnel neighbor is
responsive, i.e., in the same manner implied for IPv6 Neighbor
Unreachability Detection "hints of forward progress" (see:
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[RFC4861]). If the PTB has Code=0 /1 in the SCMP header, however,
the ITE processes the message as an indication of a packet size
limitation as follows.
If the PTB has Code =0, the ITE also examines the value in the PTB
message MSS field to determine a new S_MSS value. If the MSS value
is no less than 1280, the ITE records the value as the new S_MSS. If
the MSS value is less than the current S_MSS value and also less than
1280, however, the ITE can discern that IP fragmentation is occurring
but it cannot determine the true MTU of the restricting link due to
the possibility that a router on the path is generating runt first
fragments. In this latter case, the ITE searches for a reduced S_MSS
value through an iterative searching strategy that parallels the IPv4
Path MTU Discovery "plateau table" procedure described in Sections 5
and 7 of [RFC1191]. This searching strategy may entail multiple
iterations in which the ITE sends additional SEAL data packets using
a reduced S_MSS and receives additional SCMP PTB messages, but the
process should quickly converge. During this process, it is
essential that the ITE reduce S_MSS based on the first SCMP PTB
message received under the current S_MSS size, and refrain from
further reducing S_MSS until SCMP PTB messages pertaining to packets
sent under the new S_MSS are received.
For each PTB with Code=0/1, If the inner packet headers in the
packet-in-error are available in-the-clear, the inner packet is not
an IPv4 packet with DF=0, and the inner packet is larger than
(MAX(S_MRU, S_MSS) - HLEN), the ITE can send a transcribed PTB
message appropriate for the inner packet to the original source with
MTU set to (MAX(S_MRU, S_MSS) - HLEN).
NB: A stateless MTU discovery capability is possible for paths in
which inner packet headers are transmitted in-the-clear, when the
inner network layer protocol does not permit in-the-network
fragmentation (e.g., IPv6), and when either the ITE or ETE sets S_MRU
to 0. In that case, when the ITE receives an SCMP PTB with Code=0 it
can send a transcribed PTB message back to the source. If the SCMP
PTB reports an MSS size no less than 1280, the ETE writes the MSS
value into the MTU field of the transcribed PTB message; otherwise,
it consults a plateau table to report a reduced size as described
above. For example, if the ITE receives an SCMP PTB message with
Code=0, MSS=256, and inner header length 1500, it can send a
transcribed PTB message listing an MTU of 1400 back to the source.
If the ITE subsequently receives an SCMP PTB message with Code=0,
MSS=256, and inner header length 1400, it can send a transcribed PTB
message listing an MTU of 1300 back to the source, etc.
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4.6.2.2. Processing Other SCMP Error Messages
An ITE may receive other SCMP "Destination Unreachable" messages with
an appropriate code under the same circumstances that an IPv6 node
would receive an ICMPv6 Destination Unreachable message. The ITE
processes the message the same as for the corresponding ICMPv6
Destination Unreachable messages.
An ITE may receive an SCMP "Parameter Problem" message when the ETE
receives a SEAL packet with an incorrect value in the SEAL header.
The ITE should examine the incorrect SEAL header field setting to
determine whether a different setting should be used in subsequent
packets.
.An ITE may receive an SCMP "Time Exceeded" message when the ETE
garbage collects an incomplete SEAL data packet reassembly. The ITE
should consider the message as an indication of congestion.
4.6.3. Generating and Processing Other SCMP Message Types
TEs generate and process other SCMP message types using methods and
procedures outside the scope of this document. For example, SCMP
message types used for tunnel neighbor coordinations are specified in
[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
SEAL provides robust mechanisms for returning PTB messages; however,
end systems that send unfragmentable IP packets larger than 1500
bytes are strongly encouraged to implement their own end-to-end MTU
assurance, e.g., using Packetization Layer Path MTU Discovery per
[RFC4821].
7. Router Requirements
IPv4 routers within the subnetwork are strongly encouraged to
implement IPv4 fragmentation such that the first fragment is the
largest and approximately the size of the underlying link MTU, i.e.,
they should avoid generating runt first fragments.
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IPv6 routers within the subnetwork are required to generate the
necessary PTB messages when they drop outer IPv6 packets due to an
MTU restriction.
8. IANA Considerations
The IANA is instructed to allocate an IP protocol number for
'SEAL_PROTO' in the 'protocol-numbers' registry.
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
Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-
overlapping. This condition is naturally enforced due to the fact
that each consecutive SEAL segment begins at offset 0 with respect to
the previous SEAL segment.
An amplification/reflection attack is possible when an attacker sends
IP first fragments with spoofed source addresses to an ETE in an
attempt to generate a stream of SCMP messages returned to a victim
ITE. The (NBR_ID, LINK_ID, PKT_ID)-tuple as well as the mid-layer
and inner headers of the packet provide mitigation for the ETE to
detect and discard SEAL segments with spoofed source addresses.
The SEAL header is sent in-the-clear (outside of any IPsec/ESP
encapsulations) the same as for the outer IP and other outer headers.
In this respect, the threat model is no different than for IPv6
extension headers. As for IPv6 extension headers, the SEAL header is
protected only by L2 integrity checks and is not covered under any L3
integrity checks.
SCMP messages carry the (NBR_ID, LINK_ID, PKT_ID)-tuple as well as
the mid-layer and inner headers of the packet-in-error. Therefore,
when an ITE receives an SCMP message it can unambiguously associate
it with the SEAL data packet that triggered the error.
Security issues that apply to tunneling in general are discussed in
[RFC6169].
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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, which is in
part the same approach used by SEAL. SEAL could therefore be
considered as a fully functioned manifestation of the method
postulated by that informational reference.
Section 3 of [RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU;
however, the SEAL protocol specifies a mid-layer segmentation and
reassembly capability that is distinct from both inner and outer
fragmentation.
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.
The concepts of path MTU determination through the report of
fragmentation and extending the IP Identification field were first
proposed in deliberations of the TCP-IP mailing list and the Path MTU
Discovery Working Group (MTUDWG) during the late 1980's and early
1990's. SEAL supports a report fragmentation capability using bits
in an extension header (the original proposal used a spare bit in the
IP header) and supports ID extension through a 16-bit field in an
extension header (the original proposal used a new IP option). A
historical analysis of the evolution of these concepts, as well as
the development of the eventual path MTU discovery mechanism for IP,
appears in Appendix D of this document.
11. SEAL Advantages over Classical Methods
The SEAL approach offers a number of distinct advantages over the
classical path MTU discovery methods [RFC1191] [RFC1981]:
1. Classical path MTU discovery always results in packet loss when
an MTU restriction is encountered. Using SEAL, IP fragmentation
provides a short-term interim mechanism for ensuring that packets
are delivered while SEAL adjusts its packet sizing parameters.
2. Classical path MTU may require several iterations of dropping
packets and returning PTB messages until an acceptable path MTU
value is determined. Under normal circumstances, SEAL determines
the correct packet sizing parameters in a single iteration.
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3. Using SEAL, ordinary packets serve as implicit probes without
exposing data to unnecessary loss. SEAL also provides an
explicit probing mode not available in the classic methods.
4. Using SEAL, ETEs encapsulate SCMP error messages in outer and
mid-layer headers such that packet-filtering network middleboxes
will not filter them the same as for "raw" ICMP messages that may
be generated by an attacker.
5. The SEAL approach ensures that the tunnel either delivers or
deterministically drops packets according to their size, which is
a required characteristic of any IP link.
6. Most importantly, all SEAL packets have Identification values
that are sufficiently long to be used for duplicate packet
detection purposes and to associate ICMP error messages with
actual packets sent without requiring per-packet state; hence,
SEAL avoids certain denial-of-service attack vectors open to the
classical methods.
12. 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.
13. References
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13.1. 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.
13.2. 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.templin-aero]
Templin, F., "Asymmetric Extended Route Optimization
(AERO)", draft-templin-aero-02 (work in progress),
September 2011.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-25 (work in progress),
September 2011.
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[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.
[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.
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[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.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, 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.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
[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.
[RFC6179] Templin, F., "The Internet Routing Overlay Network
(IRON)", RFC 6179, March 2011.
[SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU
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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. Since SEAL supports segmentation at a layer
below IP, SEAL therefore presents a case in which the link unit of
loss (i.e., a SEAL segment) is smaller than the end-to-end
retransmission unit (e.g., a TCP segment).
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 to perform ARQ.
When the SEAL ITE has knowledge that the tunnel will traverse a
subnetwork with non-negligible loss due to, e.g., interference, link
errors, congestion, etc., it can solicit Segment Reports from the ETE
periodically to discover missing segments for retransmission within a
single round-trip time. However, retransmission of missing segments
may require the ITE to maintain considerable state and may also
result in considerable delay variance and packet reordering.
SEAL may also use alternate reliability mechanisms such as Forward
Error Correction (FEC). A simple FEC mechanism may merely entail
gratuitous retransmissions of duplicate data, however more efficient
alternatives are also possible. Basic FEC schemes are discussed in
[RFC5445].
The use of ARQ and FEC mechanisms for improved reliability are for
further study.
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Appendix B. Integrity
Each link in the path over which a SEAL tunnel is configured is
responsible for link layer integrity verification for packets that
traverse the link. As such, when a multi-segment SEAL packet with N
segments is reassembled, its segments will have been inspected by N
independent link layer integrity check streams instead of a single
stream that a single segment SEAL packet of the same size would have
received. Intuitively, a reassembled packet subjected to N
independent integrity check streams of shorter-length segments would
seem to have integrity assurance that is no worse than a single-
segment packet subjected to only a single integrity check steam,
since the integrity check strength diminishes in inverse proportion
with segment length. In any case, the link-layer integrity assurance
for a multi-segment SEAL packet is no different than for a multi-
fragment IPv6 packet.
Fragmentation and reassembly schemes must also consider packet-
splicing errors, e.g., when two segments from the same packet are
concatenated incorrectly, when a segment from packet X is reassembled
with segments from packet Y, etc. The primary sources of such errors
include implementation bugs and wrapping IP ID fields. In terms of
implementation bugs, the SEAL segmentation and reassembly algorithm
is much simpler than IP fragmentation resulting in simplified
implementations. In terms of wrapping ID fields, when IPv4 is used
as the outer IP protocol, the 16-bit IP ID field can wrap with only
64K packets with the same (src, dst, protocol)-tuple alive in the
system at a given time [RFC4963] increasing the likelihood of
reassembly mis-associations. However, SEAL ensures that any outer
IPv4 fragmentation and reassembly will be short-lived and tuned out
as soon as the ITE receives an SCMP PTB message, and SEAL
segmentation and reassembly uses a much longer Identification field.
Therefore, reassembly mis-associations of IP fragments nor of SEAL
segments should be prohibitively rare.
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
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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
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
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Internet-Draft SEAL October 2011
"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)
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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