Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track June 2, 2010
Expires: December 4, 2010
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-14.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 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 4, 2010.
Copyright Notice
Copyright (c) 2010 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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Templin Expires December 4, 2010 [Page 1]
Internet-Draft SEAL June 2010
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Templin Expires December 4, 2010 [Page 2]
Internet-Draft SEAL June 2010
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 Protocol Specification . . . . . . . . . . . . . . . . . 10
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 10
4.2. SEAL Header Format . . . . . . . . . . . . . . . . . . . . 12
4.3. ITE Specification . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
4.3.2. Tunnel Interface Soft State . . . . . . . . . . . . . 15
4.3.3. Admitting Packets into the Tunnel . . . . . . . . . . 16
4.3.4. Mid-Layer Encapsulation . . . . . . . . . . . . . . . 17
4.3.5. SEAL Segmentation . . . . . . . . . . . . . . . . . . 17
4.3.6. Outer Encapsulation . . . . . . . . . . . . . . . . . 17
4.3.7. Probing Strategy . . . . . . . . . . . . . . . . . . . 18
4.3.8. Packet Identification . . . . . . . . . . . . . . . . 18
4.3.9. Sending SEAL Protocol Packets . . . . . . . . . . . . 19
4.3.10. Processing Raw ICMP Messages . . . . . . . . . . . . . 19
4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 19
4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 19
4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 20
4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 21
4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 22
4.5. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 22
4.5.1. Generating SCMP Messages . . . . . . . . . . . . . . . 22
4.5.2. Processing SCMP Messages . . . . . . . . . . . . . . . 25
4.6. Tunnel Endpoint Synchronization . . . . . . . . . . . . . 27
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 29
6. End System Requirements . . . . . . . . . . . . . . . . . . . 29
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 30
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 30
10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 31
11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 31
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
13.1. Normative References . . . . . . . . . . . . . . . . . . . 33
13.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 36
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 37
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 39
Templin Expires December 4, 2010 [Page 3]
Internet-Draft SEAL June 2010
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 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 (cited above). In particular, IPv4 fragmentation raises
Templin Expires December 4, 2010 [Page 4]
Internet-Draft SEAL June 2010
issues ranging from minor annoyances (e.g., in-the-network router
fragmentation) 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 made simple
since there is no way to authenticate the source of the messages
[I-D.ietf-tcpm-icmp-attacks]. 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].
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, ingress tunnel endpoints (ITEs) may be
required to forward encapsulated packets into the subnetwork on
behalf of hundreds, thousands, or even more original sources in the
end site. 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
denominator" MTU that may be much smaller than necessary for most
Templin Expires December 4, 2010 [Page 5]
Internet-Draft SEAL June 2010
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. Examples include the
global Internet interdomain routing core, Mobile Ad hoc Networks
(MANETs) and enterprise networks. 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
supports efficient duplicate packet detection through the use of a
minimal mid-layer encapsulation.
SEAL specifically treats tunnels that traverse the subnetwork as
unidirectional 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 introduces an extended Identification field for
packet identification and a mid-layer segmentation and reassembly
capability that allows simplified cutting and pasting of packets.
Moreover, SEAL senses in-the-network fragmentation as a "noise"
indication that packet sizing parameters are "out of tune" with
respect to the network path. As a result, SEAL can naturally tune
its packet sizing parameters to eliminate the in-the-network
Templin Expires December 4, 2010 [Page 6]
Internet-Draft SEAL June 2010
fragmentation. This approach is in contrast to existing tunneling
protocol practices which seek to avoid MTU issues by selecting a
"lowest common denominator" MTU that may be overly conservative for
many tunnels 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 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.).
Templin Expires December 4, 2010 [Page 7]
Internet-Draft SEAL June 2010
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 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
OHLEN - the length of the outer encapsulating headers and
trailers, including the outer IP header, the SEAL header and any
other outer headers and trailers.
PTB - a Packet Too Big message recognized by the inner network
layer, e.g., an ICMPv6 "Packet Too Big" message [RFC4443], an
ICMPv4 "Fragmentation Needed" message [RFC0792], etc.
S_MRU - the SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size
SCMP - the SEAL Control Message Protocol
SEAL_ID - an Identification value, randomly initialized and
monotonically incremented for each SEAL protocol packet
SEAL_PORT - a TCP/UDP service port number used for SEAL
SEAL_PROTO - an IPv4 protocol number used for SEAL
TE - Tunnel Endpoint (i.e., either ingress or egress)
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear 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.
Templin Expires December 4, 2010 [Page 8]
Internet-Draft SEAL June 2010
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 of enterprise networks, ISP networks, SOHO
networks, the interdomain routing core, and any other networking
scenario involving IP encapsulation. SEAL and its associated
technologies (including Virtual Enterprise Traversal (VET)
[I-D.templin-intarea-vet]) are functional building blocks for a new
Internetworking architecture based on Routing and Addressing in
Networks with Global Enterprise Recursion (RANGER)
[RFC5720][I-D.russert-rangers] and the Internet Routing Overlay
Network (IRON) [I-D.templin-iron].
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 UDP
data payload (e.g., as IPv4/UDP/SEAL/IPv6 similar to Teredo
[RFC4380]), where UDP encapsulation is typically used for operation
over subnetworks that give preferential treatment to the "core"
Internet protocols (i.e., TCP and UDP). 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
acts 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 tunnels between performance-
critical routers connected to high data rate subnetworks such as the
Internet DFZ, as well as 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
endpoints that use SEAL can transport packets that are much larger
than the underlying subnetwork links themselves can carry in a single
piece.
Templin Expires December 4, 2010 [Page 9]
Internet-Draft SEAL June 2010
SEAL tunnels may be configured over paths that include not only
ordinary physical links, but also virtual links that may include
other SEAL 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 Protocol Specification
The following sections specify the operation of the SEAL protocol.
4.1. Model of Operation
SEAL is an encapsulation sublayer that supports a multi-level
segmentation and reassembly capability for the transmission of
unicast and multicast packets across an underlying IP subnetwork with
heterogeneous links. First, the ITE can use IPv4 fragmentation to
fragment inner IPv4 packets before SEAL encapsulation if necessary.
Secondly, the SEAL layer itself provides a simple cutting-and-pasting
capability for mid-layer packets to avoid IP fragmentation on the
outer packet. Finally, ordinary IP fragmentation is permitted on the
outer packet after SEAL encapsulation and is used 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-in-
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:
Templin Expires December 4, 2010 [Page 10]
Internet-Draft SEAL June 2010
+--------------------+
~ 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-in-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:
Templin Expires December 4, 2010 [Page 11]
Internet-Draft SEAL June 2010
+------------------+ +------------------+
~ outer IPv6 hdr ~ ~ outer IPv6 hdr ~
+------------------+ +------------------+ +------------------+
~ other outer hdrs ~ ~ outer IPv6 hdr ~ ~ other outer hdrs ~
+------------------+ +------------------+ +------------------+
~ SEAL hdr (SEG=0) ~ ~ other outer hdrs ~ ~ SEAL hdr (SEG=2) ~
+------------------+ +------------------+ +------------------+
~ mid-layer hdrs ~ ~ SEAL hdr (SEG=1) ~ | inner IPv4 |
+------------------+ +------------------+ ~ Packet ~
| inner IPv4 | | inner IPv4 | | (Segment 2) |
~ Packet ~ ~ Packet ~ +------------------+
| (Segment 0) | | (Segment 1) | ~ mid-layer trails ~
+------------------+ +------------------+ +------------------+
~ 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 SEAL header itself is inserted 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 SEAL header is inserted between the inner packet and
outer IPvX headers as: IPvX/SEAL/{inner packet}.
o For encapsulations over transports such as UDP (e.g., [RFC4380]),
the SEAL header is inserted 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".
SEAL-encapsulated packets include a SEAL_ID to uniquely identify each
packet. Routers within the subnetwork use the SEAL_ID for duplicate
packet detection, and TEs use the SEAL_ID for SEAL segmentation/
reassembly and protection against off-path attacks. The following
sections specify the SEAL header format and SEAL-related operations
of the ITE and ETE, respectively.
4.2. SEAL Header Format
The SEAL header is formatted as follows:
Templin Expires December 4, 2010 [Page 12]
Internet-Draft SEAL June 2010
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|A|R|D|F|M|Z| NEXTHDR/SEG | SEAL_ID (bits 48 - 32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SEAL_ID (bits 31 - 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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.
A (1)
the "Acknowledgement Requested" bit. Set to 1 by the ITE in data
packets if it wishes to receive an explicit acknowledgement from
the ETE.
R (1)
the "Redirect" bit. Set to 0 unless otherwise specified in other
documents.
D (1)
the "Discard" bit. Set to 0 unless otherwise specified in other
documents.
F (1)
the "First Segment" bit. Set to 1 if this SEAL protocol 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 protocol packet
contains a non-final segment of a multi-segment mid-layer packet.
Z (1)
a 1-bit "Reserved" field. Set to zero by the ITE and ignored by
the ETE.
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.)
Templin Expires December 4, 2010 [Page 13]
Internet-Draft SEAL June 2010
SEAL_ID (48)
a 48-bit Identification field.
Setting of the various bits and fields of the SEAL header is
specified in the following sections.
4.3. ITE Specification
4.3.1. Tunnel Interface MTU
The ITE configures a tunnel virtual interface over one or more
underlying links that connect the border node to the subnetwork. The
tunnel interface must present a fixed MTU to Layer 3 as the size for
admission of inner packets into the tunnel. Since the tunnel
interface 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 the 1500 byte packets sent by end systems
incur additional encapsulation at an ITE, however, they may be
dropped silently since the network may not always deliver the
necessary PTBs [RFC2923].
The ITE should therefore set a tunnel virtual 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 set larger MTU values still, but should select a value
that is not so large as to cause excessive PTBs coming from within
the tunnel interface. 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 virtual
interface such that all inner packets are admitted into the interface
without regard to size. For ITEs that host applications, 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
Templin Expires December 4, 2010 [Page 14]
Internet-Draft SEAL June 2010
segment size (e.g., the size advertised in the TCP MSS option) based
on the per-neighbor MTU.
The inner network layer protocol consults the tunnel interface MTU
when admitting a packet into the interface. For 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
tunnel as an independent packet.
For all other inner packets, the ITE 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.).
Note that when the tunnel interface sets an indefinite MTU the ITE
unconditionally admits all packets into the interface without
fragmentation. In light of the above considerations, it is
RECOMMENDED that the ITE configure an indefinite MTU on the tunnel
virtual interface and adapt to any per-neighbor MTU limitations
within the tunnel virtual interface as described in the following
sections.
4.3.2. Tunnel Interface Soft State
For each ETE, the ITE maintains soft state within the tunnel
interface (e.g., in a neighbor cache) used to support inner
fragmentation and SEAL segmentation for packets admitted into the
tunnel interface. The soft state includes the following:
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 underlying interface MTU if the underlying interface MTU can
be determined (otherwise, the ITE initializes S_MSS to
"infinity"). The ITE decreases or increased S_MSS based on any
SCMP "MTU Report" messages received (see Section 4.5).
Templin Expires December 4, 2010 [Page 15]
Internet-Draft SEAL June 2010
o a SEAL Maximum Reassembly Unit (S_MRU). The ITE initializes S_MRU
to "infinity" and decreases or increases S_MRU based on any SCMP
MTU Report messages received (see Section 4.5). When
(S_MRU>(S_MSS*256)), the ITE uses (S_MSS*256) as the effective
S_MRU value.
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.
4.3.3. Admitting Packets into the Tunnel
After the ITE admits an inner packet/fragment into the tunnel
interface, it uses the following algorithm to determine whether the
packet can be accommodated and (if so) whether (further) inner IP
fragmentation is needed:
o if the inner packet is unfragmentable (e.g., an IPv6 packet, an
IPv4 packet with DF=1, etc.), and the packet is larger than
(MAX(S_MRU, S_MSS) - HLEN), the ITE drops the packet and sends a
PTB message to the original source with an MTU value of
(MAX(S_MRU, S_MSS) - HLEN); else,
o if the inner packet is fragmentable (e.g., an IPv4 packet with
DF=0), and the packet is larger than (foo) bytes, the ITE uses
inner fragmentation to break the packet into fragments no larger
than (foo) bytes; else,
o the ITE processes the packet without inner fragmentation.
In the above, the ITE must track whether the tunnel interface is
using header compression. If so, the ITE must include the length of
the uncompressed headers and trailers when calculating HLEN. Note
also in the above that the ITE is permitted to admit inner packets
into the tunnel that 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 4.4.1.
When the ITE uses inner fragmentation, it should use a "safe"
fragment size of (foo) bytes that would be highly unlikely to incur
an outer IP MTU restriction within the tunnel. If the ITE can
determine a larger fragment size (e.g., via probing), it should use
the larger size for inner fragmentation. In the absence of
deterministic information, it is RECOMMENDED that the ITE set (foo)
Templin Expires December 4, 2010 [Page 16]
Internet-Draft SEAL June 2010
to 1280.
4.3.4. Mid-Layer Encapsulation
After inner IP fragmentation (if necessary), the ITE next
encapsulates each inner packet/fragment in the MHLEN bytes of mid-
layer headers and trailers. The ITE then presents the mid-layer
packet for SEAL segmentation and outer encapsulation.
4.3.5. SEAL Segmentation
After mid-layer encapsulation, if the length of the resulting mid-
layer packet plus OHLEN is greater than S_MSS the ITE must
additionally perform SEAL segmentation. To do so, it breaks the mid-
layer packet into N segments (N <= 256) that are no larger than
(S_MSS - OHLEN) bytes each. Each segment, except the final one, MUST
be of equal length. 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.
Note that this SEAL segmentation ignores the fact that the mid-layer
packet may be unfragmentable outside of the subnetwork. This
segmentation 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.
4.3.6. Outer Encapsulation
Following SEAL segmentation, the ITE next encapsulates each segment
in a SEAL header formatted as specified in Section 4.2. For the
first segment, the ITE sets F=1, then sets NEXTHDR to the Internet
Protocol number of the encapsulated inner packet, and finally sets
M=1 if there are more segments or sets M=0 otherwise. For each non-
initial segment of an N-segment mid-layer packet (N <= 256), the ITE
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.)
The ITE next encapsulates each segment in the requisite outer headers
and trailers according to the specific encapsulation format (e.g.,
Templin Expires December 4, 2010 [Page 17]
Internet-Draft SEAL June 2010
[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 service port field (e.g., when IP/UDP encapsulation
is used). The ITE finally sets A=1 if probing is necessary as
specified in Section 4.3.7, sets the packet identification values as
specified in Section 4.3.8 and sends the packets as specified in
Section 4.3.9.
4.3.7. Probing Strategy
All SEAL encapsulated packets sent by the ITE are considered implicit
probes. SEAL encapsulated packets that use IPv4 as the outer layer
of encapsulation will elicit SCMP PTB messages from the ETE (see:
Section 4.5) if any IPv4 fragmentation occurs in the path. SEAL
encapsulated packets that use IPv6 as the outer layer of
encapsulation may be dropped by an IPv6 router on the path to the ETE
which will also return an ICMPv6 PTB message to the ITE. The ITE can
then use the SEAL_ID 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 segment to be used as an explicit probe, where the probe can be
either an ordinary data packet or a NULL packet created by setting
the NEXTHDR field to a value of "No Next Header" (see Section 4.7 of
[RFC2460]). The probe will elicit a solicited SCMP Neighbor
Advertisement (NA) message from the ETE as an acknowledgement (see
Section 4.5.1.2).
Finally, the ITE MAY send "expendable" outer IP probe packets (see
Section 4.3.9) as explicit probes in order to detect increases in the
path MTU to the ETE. One possible strategy is to send expendable
packets with A=1 in the SEAL header and DF=1 in the IP header. In
all cases, the ITE MUST be conservative in its use of the A bit in
order to limit the resultant control message overhead.
4.3.8. Packet Identification
The ITE maintains a randomly-initialized SEAL_ID value as per-ETE
soft state (e.g., in the neighbor cache) and monotonically increments
it for each successive SEAL protocol packet it sends to the ETE. For
each successive SEAL protocol packet, the ITE writes the current
SEAL_ID value into the header field of the same name in the SEAL
header.
Templin Expires December 4, 2010 [Page 18]
Internet-Draft SEAL June 2010
4.3.9. Sending SEAL Protocol Packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
the header of each outer IPv4 packet to ensure that they will be
delivered to the ETE even if they are fragmented within the
subnetwork. (The ITE can instead set DF=1 for "expendable" outer
IPv4 packets (e.g., for NULL packets used as probes -- see Section
4.3.7), but these may be lost due to an MTU restriction). For outer
IPv6 packets, the "DF" bit is always implicitly set to 1; hence, they
will not be fragmented within the subnetwork.
The ITE sends each outer packet that encapsulates a segment of the
same mid-layer packet into the tunnel in canonical order, i.e.,
segment 0 first, followed by segment 1, etc., and finally segment
N-1.
4.3.10. Processing Raw ICMP Messages
The ITE may receive "raw" ICMP error messages [RFC0792][RFC4443] from
either the ETE or routers within the subnetwork that comprise an
outer IP header, followed by an ICMP header, followed by a portion of
the SEAL packet that generated the error (also known as the "packet-
in-error"). The ITE can use the SEAL_ID encoded in the packet-in-
error as a nonce 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; specific actions that the ITE
may take in this case are out of scope.
4.4. ETE Specification
4.4.1. Reassembly Buffer Requirements
The ETE SHOULD support IP-layer and SEAL-layer reassembly for inner
packets of at least 1280 bytes in length and MAY support reassembly
for larger inner packets; the ETE may instead support only a minimum-
sized reassembly buffer, but this may cause MTU underruns in some
environments. 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:
Templin Expires December 4, 2010 [Page 19]
Internet-Draft SEAL June 2010
o SHOULD configure an outer IP-layer reassembly buffer size of at
least (1280 + HELN) bytes.
o MUST configure a SEAL-layer reassembly buffer size (i.e., S_MRU)
that is no smaller than the IP-layer reassembly buffer size.
o MUST be capable of discarding inner packets that require IP-layer
or SEAL-layer reassembly and that are larger than (S_MRU - HLEN).
The ETE can maintain S_MRU either as a single value to be applied for
all ITEs, or as a per-ITE value. In that case, the ETE can manage
each per-ITE S_MRU value separately (e.g., to reduce congestion
caused by excessive segmentation from specific ITEs) but should seek
to maintain as stable a value as possible for each ITE.
Note that the ETE is permitted to accept inner packets that did not
undergo IP-layer and/or SEAL-layer reassembly even if they are larger
than (S_MRU - HELN) bytes. Hence, S_MRU is a maximum *reassembly*
size, and may be less than the ETE is able to receive without
reassembly.
4.4.2. IP-Layer Reassembly
The ETE submits unfragmented SEAL protocol IP packets for SEAL-layer
reassembly as specified in Section 4.4.3. 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 been received before a fragment that completes a
pending reassembly has arrived. Following successful IP-layer
reassembly, the ETE submits the reassembled packet for SEAL-layer
reassembly as specified in Section 4.4.3.
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, it sends an
SCMP PTB message back to the ITE (see Section 4.5.1.1). 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.
Templin Expires December 4, 2010 [Page 20]
Internet-Draft SEAL June 2010
4.4.3. SEAL-Layer Reassembly
Following IP reassembly (if necessary), if the mid-layer packet has
an incorrect value in the SEAL header the ETE discards the packet and
returns an SCMP "Parameter Problem" message (see Section 4.5.1.3).
Next, if the SEAL header has A=1, the ETE sends a solicited SCMP
Neighbor Advertisement (NA) message back to the ITE (see Section
4.5.1.2). The ETE next submits single-segment mid-layer packets for
decapsulation and delivery to upper layers as specified in Section
4.4.4. The ETE instead performs SEAL-layer reassembly for multi-
segment mid-layer packets as follows.
The ETE adds each segment of a multi-segment mid-layer packet to a
SEAL-layer pending-reassembly queue according to the (Source,
Destination, SEAL_ID)-tuple found in the outer IP and SEAL headers.
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
(see Section 4.5.1.3). 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
(Source, Destination, SEAL_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. If the ETE accepts a new SEAL
segment that would cause the reassembled outer packet to be larger
than S_MRU following reassembly, it schedules the reassembly
resources for garbage collection and sends an SCMP PTB message back
to the ITE (see Section 4.5.1.1).
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 (F=1, M=1, SEAL_ID=j) in the first
SEAL header, followed by (F=0, M=1, SEG=1, SEAL_ID=(j+1)) in the next
Templin Expires December 4, 2010 [Page 21]
Internet-Draft SEAL June 2010
SEAL header, followed by (F=0, M=1, SEG=2, SEAL_ID=(j+2)), etc., up
to (F=0, M=0, SEG=(N-1), SEAL_ID=(j + N-1)) in the final SEAL header.
(Note that modulo arithmetic based on the length of the SEAL_ID field
is used). Following successful SEAL-layer reassembly, the ETE
submits the reassembled mid-layer packet for decapsulation and
delivery to upper layers as specified in Section 4.4.4.
4.4.4. Decapsulation and Delivery to Upper Layers
Following any necessary IP- and SEAL-layer reassembly, the ETE
discards the outer headers and trailers and performs any mid-layer
transformations on the mid-layer packet. The ETE next discards the
mid-layer headers and trailers, and delivers the inner packet to the
upper-layer protocol indicated 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
4.3.9).
4.5. The SEAL Control Message Protocol (SCMP)
SEAL uses a companion SEAL Control Message Protocol (SCMP) based on
the same message format as the Internet Control Message Protocol for
IPv6 (ICMPv6) [RFC4443]. SCMP messages are further identified by the
NEXTHDR value '58' the same as for ICMPv6 messages, however the SCMP
message is *not* immediately preceded by an inner IPv6 header.
Instead, SCMP messages appear immediately following the SEAL header
which allows TEs to differentiate them from ordinary ICMPv6 messages.
Unlike ICMPv6 messages, SCMP messages are used only for the purpose
of conveying information between TEs, i.e., they are used only for
sharing control information within the tunnel and not beyond the
tunnel.
The following sections specify the generation and processing of SCMP
messages:
4.5.1. Generating SCMP Messages
SCMP messages may be generated by either ITEs or ETEs (i.e., by any
TE) using use the same message Type and Code values specified for
ordinary ICMPv6 messages in [RFC4443]. SCMP can also be used to
carry other message types and their associated options as specified
in other documents (e.g., [RFC4191][RFC4861]). The general format
for SCMP messages is shown in Figure 4:
Templin Expires December 4, 2010 [Page 22]
Internet-Draft SEAL June 2010
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 ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of invoking SEAL data |
~ packet as possible without the SCMP ~
| packet exceeding 576 bytes (*) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
(*) also known as the "packet-in-error"
Figure 4: SCMP Message Format
As for ordinary ICMPv6 messages, the SCMP message begins with a 4
byte header that includes 8-bit Type and Code fields followed by a
16-bit Checksum field followed by a variable-length Message Body.
The Message Body is followed by the leading portion of the invoking
SEAL data packet (i.e., the "packet-in-error") IFF the packet-in-
error would also be included in the corresponding ICMPv6 message.
The TE sets the Type and Code fields to the same values that would
appear in the corresponding ICMPv6 message and also formats the
message body the same as for the corresponding ICMPv6 message except
as otherwise specified.
If the SCMP message will include a packet-in-error, the TE then
includes as much of the leading portion of the invoking SEAL data
packet as possible beginning with the outer IP header and extending
to a length that would not cause the entire SCMP message following
encapsulation to exceed 576 bytes. The ITE finally calculates the
Checksum the same as specified for ICMPv4 messages [RFC0792] and does
not include a pseudo-header of the outer IP header since the SEAL_ID
gives sufficient assurance against mis-delivery. The TE then
encapsulates the SCMP message in the outer headers as shown in
Figure 5:
Templin Expires December 4, 2010 [Page 23]
Internet-Draft SEAL June 2010
+--------------------+
~ outer IPv4 header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
~ SCMP message header~ --> ~ SCMP message header~
+--------------------+ --> +--------------------+
~ SCMP message body ~ --> ~ SCMP message body ~
+--------------------+ --> +--------------------+
~ packet-in-error ~ --> ~ packet-in-error ~
+--------------------+ +--------------------+
~ outer trailers ~
SCMP Message +--------------------+
before encapsulation
SCMP Message
after encapsulation
Figure 5: SCMP Message Encapsulation
When a TE generates an SCMP message in response to a packet-in-error,
it sets the outer IP destination and source addresses of the SCMP
packet to the packet-in-error's source and destination addresses
(respectively). (If the destination address in the packet-in-error
was multicast, the TE instead sets the outer IP source address of the
SCMP packet to an address assigned to the underlying IP interface.)
When a TE generates an SCMP message that is not due to a packet-in-
error, it sets the outer IP destination and source addresses of the
SCMP packet the same as for ordinary data packets. The TE finally
sets the NEXTHDR field in the SEAL header to the value '58' (i.e.,
the official IANA protocol number for the ICMPv6 protocol) and sends
the SCMP message to the tunnel far end.
4.5.1.1. Generating SCMP Packet Too Big (PTB) Messages
An ETE generates an SCMP Packet Too Big (PTB) message when it
receives the IP first fragment (i.e., one with MF=1 and Offset=0 in
the outer IP header) of a SEAL protocol packet that arrived as
multiple IP fragments, or when it discontinues reassembly of a SEAL
protocol packet that arrived as multiple IP fragments and/or multiple
SEAL segments and would exceed S_MRU following reassembly.
The ETE prepares an SCMP PTB message the same as for the
corresponding ICMPv6 PTB message, except that it writes the value 0
in the MTU field of the message if the PTB is generated as a result
of receiving an IP first fragment and writes the S_MRU value for this
ITE in the MTU field otherwise.
Templin Expires December 4, 2010 [Page 24]
Internet-Draft SEAL June 2010
4.5.1.2. Generating SCMP Neighbor Discovery Messages
An ITE generates an SCMP "Neighbor Solicitation" (NS) or "Router
Solicitation" (RS) message when it needs to solicit a response from
an ETE. An ETE generates a solicited SCMP "Neighbor Advertisement"
(NA) or "Router Advertisement" (RA) message when it receives an NS/RS
message, and also generates a solicited NA message when it receives a
SEAL protocol packet with A=1 in the SEAL header. Any TE may also
generate unsolicited NA/RA messages that are not triggered by a
specific solicitation event.
The TE generates NS/RS and NA/RA messages the same as described for
the corresponding IPv6 Neighbor Discovery (ND) messages (see:
[RFC4861]), except that for solicited NA/RA messages it also includes
a Redirected Header option formatted the same as for an IPv6 ND
Redirect message. The messages may also be used in conjunction with
the tunnel endpoint synchronization procedure specified in Section
4.6.
4.5.1.3. Generating Other SCMP Messages
An ETE generates an SCMP "Destination Unreachable - Communication
with Destination Administratively Prohibited" message when it is
operating in synchronized mode and receives a SEAL packet with a
SEAL_ID that is outside of the current window for this ITE (see:
Section 4.6). An ETE also generates an SCMP "Destination
Unreachable" message with an appropriate code under the same
circumstances that an IPv6 system would generate an ICMPv6
Destination Unreachable message using the same code. The SCMP
Destination Unreachable message is formatted the same as for ICMPv6
Destination Unreachable messages.
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 message formats used are
the same as for the corresponding ICMPv6 messages.
Generation of all other SCMP message types is outside the scope of
this document.
4.5.2. Processing SCMP Messages
An ITE processes any SCMP messages it receives as long as it can
verify that the message was sent from an on-path ETE. The ITE can
verify that the SCMP message came from an on-path ETE by checking
that the SEAL_ID in the encapsulated packet-in-error corresponds to
one of its recently-sent SEAL data packets.
Templin Expires December 4, 2010 [Page 25]
Internet-Draft SEAL June 2010
An ITE maintains a window of SEAL_IDs of packets that it has recently
sent to each ETE. For each SCMP message it receives, the ITE first
verifies that the SEAL_ID encoded in the packet-in-error is within
the window of packets that it has recently sent to the ETE. The ITE
then verifies that the Checksum in the SCMP message header is
correct. If the SEAL_ID is outside of the window and/or the checksum
is incorrect, the ITE discards the message; otherwise, it processes
the message the same as for ordinary ICMPv6 messages.
Any TE may also receive unsolicited SCMP messages from the tunnel far
end. When the TEs are synchronized, they can also check that the
SEAL_ID in the SEAL header of an SCMP message is within the window of
recently received packets from this tunnel far end (see Section 4.6).
Finally, TEs process SCMP messages as an indication that the tunnel
far end is responsive, i.e., in the same manner implied for IPv6
Neighbor Unreachability Detection "hints of forward progress" (see:
[RFC4861]).
4.5.2.1. Processing SCMP PTB Messages
An ITE may receive an SCMP PTB message after it sends a SEAL data
packet (see: Section 4.5.1.1). When the ITE receives an SCMP PTB
message, it examines the MTU field in the message. If the MTU field
is non-zero, the PTB was the result of a reassembly buffer
limitation; in that case, the ITE records the value in the MTU field
as the new S_MRU value for this ETE then (optionally) sends a
translated PTB message of the inner network layer protocol to the
original source with MTU set to (MAX(S_MRU, S_MSS) - HLEN). If the
MTU field is zero, however, the PTB was the result of an IP
fragmentation event; in that case, the ITE does not send back a
translated PTB message but determines a new S_MSS value according to
the length recorded in the IP header of the packet-in-error as
follows:
o If the length is no less than 1280, the ITE records the length as
the new S_MSS value.
o If the length is less than the current S_MSS value and also less
than 1280, 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 must search for a reduced S_MSS value
through an iterative searching strategy that parallels (Section 5 of
[RFC1191]). This searching strategy may require multiple iterations
in which the ITE sends SEAL data packets using a reduced S_MSS and
Templin Expires December 4, 2010 [Page 26]
Internet-Draft SEAL June 2010
receives additional SCMP MTU Report messages. During this process,
it is essential that the ITE reduce S_MSS based on the first SCMP MTU
Report message received under the current S_MSS size, and refrain
from further reducing S_MSS until SCMP MTU Report messages pertaining
to packets sent under the new S_MSS are received.
4.5.2.2. Processing SCMP Neighbor Discovery Messages
An ETE may received NS/RS messages from an ITE as an the initial leg
in a neighbor discovery exchange. An ITE may receive both solicited
and unsolicited NA/RA messages from an ETE, where solicited NA/RA
messages are distinguished by their inclusion of a Redirected header
option (see: Section 4.5.1.2).
The TE processes NS/RS and NA/RA messages the same as described for
the corresponding IPv6 Neighbor Discovery (ND) messages (see:
[RFC4861]). The messages may also be used in conjunction with the
tunnel endpoint synchronization procedure specified in Section 4.6.
4.5.2.3. Processing Other SCMP Messages
An ITE may receive an SCMP "Destination Unreachable - Communication
with Destination Administratively Prohibited" message after it sends
a SEAL data packet. The ITE processes this message as an indication
that it needs to (re)synchronize with the ETE (see: Section 4.6). An
ITE may also receive an SCMP "Destination Unreachable" message with
an appropriate code under the same circumstances that an IPv6 host
would receive an ICMPv6 Destination Unreachable message.
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.
Processing of all other SCMP message types is outside the scope of
this document.
4.6. Tunnel Endpoint Synchronization
The SEAL ITE maintains a per-ETE window of SEAL_IDs of its recently-
sent packets, but by default the SEAL ETE does not retain inter-
packet state. When closer synchronization is required, SEAL Tunnel
Endpoints (TEs) can exchange initial sequence numbers in a procedure
Templin Expires December 4, 2010 [Page 27]
Internet-Draft SEAL June 2010
that parallels IPv6 neighbor discovery and the TCP 3-way handshake.
When the TEs are synchronized, the ETE can also maintain a per-ITE
window of SEAL_IDs of its recently-received packets.
When an initiating TE ("TE(A)") needs to synchronize with a new
tunnel far end ("TE(B)"), it first chooses a randomly-initialized 48-
bit SEAL_ID value that it would like TE(B) to use (i.e.,
"SEAL_ID(B)"). TE(A) then creates a neighbor cache entry for TE(B)
and records SEAL_ID(B) in the neighbor cache entry. Next, TE(A)
creates an SCMP NS or RS message that includes a Nonce option (see:
[RFC3971], Section 5.3). TE(A) then writes the value SEAL_ID(B) in
the Nonce option, writes the value 0 in the SEAL_ID field of the SEAL
header and sends the NS/RS message to TE(B).
When TE(B) receives an NS/RS message with a Nonce option and with the
value 0 in the SEAL_ID of the SEAL header, it considers the message
as a potential synchronization request. TE(B) first extracts the
value SEAL_ID(B) from the Nonce option then chooses a randomly-
initialized 48-bit SEAL_ID value that it would like TE(A) to use
(i.e., "SEAL_ID(A)"). TE(B) then stores the tuple (ip_src,
SEAL_ID(A), SEAL_ID(B)) in a minimal temporary fast path data
structure, where "ip_src" is the outer IP source address of the SCMP
message. (For efficiency and security purposes, the data structure
should be indexed, e.g., by a secret hash of the -tuple). TE(B) then
creates a solicited SCMP NA or RA message that includes a Nonce
option. It then writes the value SEAL_ID(A) in the Nonce option,
writes the value SEAL_ID(B) in the SEAL_ID field of the SEAL header
and sends the NA/RA message back to TE(A).
When TE(A) receives the NA/RA, it considers the message as a
potential synchronization acknowledgement. TE(A) first verifies that
the value encoded in the SEAL_ID of the SEAL header matches the
SEAL_ID(B) in the neighbor cache entry. If the values match, TE(A)
extracts SEAL_ID(A) from the nonce option and records it in the
neighbor cache entry; otherwise, it drops the packet. If instead
TE(A) does not receive a timely NA/RA response, it retransmits the
initial NS/RS message for a total of 3 tries before giving up the
same as for ordinary IPv6 neighbor discovery.
After TE(A) receives the synchronization acknowledgement, it begins
sending either unsolicited NA/RA messages or ordinary data packets
back to TE(B) using SEAL_ID(A) as the initial sequence number. When
TE(B) receives these packets, it first checks its neighbor cache to
see if there is a matching neighbor cache entry. If there is a
neighbor cache entry, and the SEAL_ID in the header of the packet is
within the window of the SEAL_ID recorded in the neighbor cache
entry, TE(B) accepts the packet. If the SEAL_ID in the packet is
newer than the SEAL_ID in the neighbor cache entry, TE(B) also
Templin Expires December 4, 2010 [Page 28]
Internet-Draft SEAL June 2010
updates the neighbor cache value. If there is no neighbor cache
entry, TE(B) instead checks the fast path cache to see if the packet
is a match for an in-progress synchronization event. If there is a
fast path cache entry with a SEAL_ID(A) that is within the window of
the SEAL_ID in the packet header, TE(B) accepts the packet and also
creates a new neighbor cache entry with the tuple (ip_src,
SEAL_ID(A), SEAL_ID(B)). If there is no matching fast path cache
entry, TE(B) instead simply discards the packet.
By maintaining the fast path cache, each TE is able to mitigate
buffer exhaustion attacks that may be launched by off-path attackers
[RFC4987]. The TE will receive positive confirmation that the
synchronization request came from an on-path tunnel far end after it
receives a stream of in-window packets as the "third leg" of this
three-way handshake as described above. The TEs should maintain
neighbor cache entries as long as they receive hints of forward
progress from the tunnel far end, but should delete the neighbor
cache entries after a nominal stale time (e.g., 30 seconds). The TEs
should also purge fast-path cache entries for which no window
synchronization messages are received within a nominal stale time
(e.g., 5 seconds).
After synchronization is complete, when a TE receives a SEAL packet
it checks in its neighbor cache to determine whether the SEAL_ID is
within the current window, and discards any packets that are outside
the window. Since packets may be lost or reordered, and since SEAL
presents only a best effort (i.e., and not reliable) link model, the
TE should set a coarse-grained window size (e.g., 32768) and accept
any packet with a SEAL_ID that is within the window.
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].
Templin Expires December 4, 2010 [Page 29]
Internet-Draft SEAL June 2010
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.
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, resulting
in a stream of SCMP messages returned to a victim ITE. The SEAL_ID
in the encapsulated segment of the spoofed IP first fragment provides
mitigation for the ITE to detect and discard spurious SCMP messages.
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 SEAL_ID of the packet-in-error. Therefore,
when an ITE receives an SCMP message it can unambiguously associate
Templin Expires December 4, 2010 [Page 30]
Internet-Draft SEAL June 2010
it with the SEAL data packet that triggered the error. When the TEs
are synchronized, the ETE can also detect off-path spoofing attacks.
Security issues that apply to tunneling in general are discussed in
[I-D.ietf-v6ops-tunnel-security-concerns].
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
Templin Expires December 4, 2010 [Page 31]
Internet-Draft SEAL June 2010
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.
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 an Identification field
that is 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, 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.
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.
Templin Expires December 4, 2010 [Page 32]
Internet-Draft SEAL June 2010
13. References
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-00 (work in progress),
March 2010.
[I-D.ietf-lisp]
Farinacci, D., Fuller, V., Meyer, D., and D. Lewis,
"Locator/ID Separation Protocol (LISP)",
draft-ietf-lisp-07 (work in progress), April 2010.
[I-D.ietf-tcpm-icmp-attacks]
Gont, F., "ICMP attacks against TCP",
draft-ietf-tcpm-icmp-attacks-12 (work in progress),
Templin Expires December 4, 2010 [Page 33]
Internet-Draft SEAL June 2010
March 2010.
[I-D.ietf-v6ops-tunnel-security-concerns]
Hoagland, J., Krishnan, S., and D. Thaler, "Security
Concerns With IP Tunneling",
draft-ietf-v6ops-tunnel-security-concerns-02 (work in
progress), March 2010.
[I-D.russert-rangers]
Russert, S., Fleischman, E., and F. Templin, "Operational
Scenarios for IRON and RANGER", draft-russert-rangers-02
(work in progress), March 2010.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-12 (work in progress), May 2010.
[I-D.templin-iron]
Templin, F., "The Internet Routing Overlay Network
(IRON)", draft-templin-iron-01 (work in progress),
April 2010.
[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.
[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.
Templin Expires December 4, 2010 [Page 34]
Internet-Draft SEAL June 2010
[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.
[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.
[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.
Templin Expires December 4, 2010 [Page 35]
Internet-Draft SEAL June 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.
Templin Expires December 4, 2010 [Page 36]
Internet-Draft SEAL June 2010
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 a Reassembly Repot, and SEAL segmentation
and reassembly uses a much longer ID 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 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.
Section specifies the operation of SEAL in "tunnel mode", i.e., when
there are both an inner and outer IP layer with a SEAL encapsulation
layer between. However, the SEAL protocol can also be used in a
Templin Expires December 4, 2010 [Page 37]
Internet-Draft SEAL June 2010
"transport mode" of operation within a subnetwork region in which the
inner-layer corresponds to a transport layer protocol (e.g., UDP,
TCP, etc.) instead of an inner IP layer.
For example, two TCP endpoints connected to the same subnetwork
region can negotiate the use of transport-mode SEAL for a connection
by inserting a 'SEAL_OPTION' TCP option during the connection
establishment phase. If both TCPs agree on the use of SEAL, their
protocol messages will be carried as TCP/SEAL/IPv4 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
Templin Expires December 4, 2010 [Page 38]
Internet-Draft SEAL June 2010
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)
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
Templin Expires December 4, 2010 [Page 39]