Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Standards Track October 21, 2010
Expires: April 24, 2011
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-23.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.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on April 24, 2011.
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
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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 Protocol Specification . . . . . . . . . . . . . . . . . 10
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 10
4.2. SEAL Header Format . . . . . . . . . . . . . . . . . . . . 13
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. SEAL Encapsulation . . . . . . . . . . . . . . . . . . 18
4.3.7. Outer Encapsulation . . . . . . . . . . . . . . . . . 18
4.3.8. Sending SEAL Protocol Packets . . . . . . . . . . . . 19
4.3.9. Probing Strategy . . . . . . . . . . . . . . . . . . . 19
4.3.10. Processing ICMP Messages . . . . . . . . . . . . . . . 20
4.4. ETE Specification . . . . . . . . . . . . . . . . . . . . 20
4.4.1. Reassembly Buffer Requirements . . . . . . . . . . . . 20
4.4.2. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 21
4.4.3. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 22
4.4.4. Decapsulation and Delivery to Upper Layers . . . . . . 23
4.5. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 23
4.5.1. Generating SCMP Messages . . . . . . . . . . . . . . . 23
4.5.2. Processing SCMP Messages . . . . . . . . . . . . . . . 27
4.6. Tunnel Endpoint Synchronization . . . . . . . . . . . . . 30
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 32
6. End System Requirements . . . . . . . . . . . . . . . . . . . 32
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 33
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 33
9. Security Considerations . . . . . . . . . . . . . . . . . . . 33
10. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 34
11. SEAL Advantages over Classical Methods . . . . . . . . . . . . 34
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 36
13.1. Normative References . . . . . . . . . . . . . . . . . . . 36
13.2. Informative References . . . . . . . . . . . . . . . . . . 36
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 39
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 39
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 40
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 41
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 42
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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 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
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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 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].
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
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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
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 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 engages both tunnel endpoints in ensuring a functional
path MTU on the path from the ITE to the ETE. This is in contrast to
"stateless" approaches which seek to avoid MTU issues by selecting a
lowest common denominator MTU value that may be overly conservative
for the vast majority of tunnel paths and difficult to change even
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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 network layer message indicating an MTU restriction, e.g., an
ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
"Fragmentation Needed" message [RFC0792], an SCMP "Packet Too Big"
message (see: Section 4.5), 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 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 - the SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size
SCMP - the SEAL Control Message Protocol
SEAL_ID - a SEAL packet Identification/Nonce value
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)
THRESH - inner fragmentation threshold
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 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 Network
Address Translator (NAT) traversal as well as 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
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 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
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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 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 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 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:
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+--------------------+
~ 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:
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+------------------+ +------------------+
~ 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 that the TEs maintain as
either a monotonically-incrementing packet identification number or
as a static nonce to identify the tunnel. When the SEAL_ID is
maintained as a packet identifier, routers within the subnetwork can
use it for duplicate packet detection and the TEs can use it for SEAL
segmentation/reassembly. TEs can also use the SEAL_ID to detect off-
path attacks whether it is maintained as a packet identifier or a
nonce.
The following sections specify the SEAL header format and SEAL-
related operations of the ITE and ETE, respectively.
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4.2. SEAL Header Format
The SEAL header is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|A|I|R|F|M| 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.
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 data
packets for which it wishes to receive an explicit acknowledgement
from the ETE.
I (1)
the "Identifier" bit. Set to 1 if the SEAL_ID contains a
monotonically-incrementing packet identifier; set to 0 if the
SEAL_ID contains a static nonce.
R (1)
the "Redirects Permitted" bit. Set to 1 if the ITE is willing to
accept SCMP redirects (see: Section 4.5); set to 0 otherwise.
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.
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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.)
SEAL_ID (48)
a 48-bit Identification or nonce 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 point-to-(multi)point 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 the
inner network layer as the size for admission of inner packets into
the interface. Since point-to-multipoint tunnel 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 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 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 that use the
tunnel virtual interface directly, this option must be carefully
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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, however, the
inner layer must send a SEAL PTB message instead of a PTB of the
inner network layer (see: Section 4.3.3).
Note that when the tunnel interface sets a finite MTU the inner
network layer must be made aware of the SEAL protocol; this may not
be practical for some implementations. When the interface sets an
indefinite MTU, however, the inner network layer unconditionally
admits all packets into the interface without fragmentation. Once
the packet has been admitted into the interface, it transitions from
the inner network layer and becomes subject to SEAL layer processing.
In light of the above considerations, it is RECOMMENDED that the ITE
configure an indefinite MTU on the tunnel virtual interface such that
the inner network layer unconditionally admits all inner packets into
the tunnel and any necessary adaptations are performed by the SEAL
layer within the tunnel virtual interface as described in the
following sections.
4.3.2. Tunnel Interface Soft State
The ITE optionally maintains per-ETE 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:
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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 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 "Packet Too Big (PTB)" messages received (see Section 4.5).
o a SEAL Maximum Reassembly Unit (S_MRU). If the ITE is not
configured to use SEAL segmentation, it initializes S_MRU to the
static value 0. Otherwise, it initializes S_MRU to "infinity" and
decreases or increases S_MRU based on any SCMP PTB 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
Once an inner packet/fragment has been admitted into the tunnel
interface, it transitions from the inner network layer and becomes
subject to SEAL layer processing. The ITE then examines each packet
to determine whether it is too large for SEAL encapsulation, then
prepares the packet for admission into the tunnel according to
whether it is "fragmentable" (discussed in the next paragraph) or
"unfragmentable" (discussed in the following paragraph).
If the packet is a non-SEAL IPv4 packet with DF=0 in the IPv4 header
(*), and the packet is larger than a constant inner fragmentation
threshold value (THRESH), the ITE uses fragmentation to break the
packet into IPv4 fragments no larger than THRESH bytes then submits
each fragment for encapsulation separately. The THRESH value may be
maintained as per-ETE soft state or as a single value that records a
lowest common denominator value for all ETEs that are reached by the
tunnel. The ITE should use a "safe" estimate for THRESH that would
be highly unlikely to trigger additional fragmentation within the
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current tunnel or within any additional tunnels that may occur along
the path. In particular, it is RECOMMENDED that the ITE set THRESH
to 512 unless it can determine a more accurate safe value, e.g., via
probing.
For all other packets, if the packet is larger than (MAX(S_MRU,
S_MSS) - HLEN), the ITE drops it and sends a PTB message to the
source (**) with an MTU value of (MAX(S_MRU, S_MSS) - HLEN);
otherwise, it submits the packet for encapsulation. The ITE must
include the length of the uncompressed headers and trailers when
calculating HLEN if the tunnel interface is using header compression.
The ITE is also 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.
(*) In order to support nested encapsulations, inner SEAL-protocol
IPv4 packets with DF=0 must be treated as unfragmentable.
(**) When the ITE needs to drop a packet and send a PTB message, it
sends an SCMP PTB message if the packet itself is a SEAL encapsulated
packet (see: Section 4.5.1.1). Otherwise, it sends a PTB
corresponding to the inner network layer protocol packet.
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 encapsulation.
4.3.5. 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 SEAL 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. 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.
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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.
4.3.6. SEAL 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.)
For each segment, the ITE then sets C=0, sets R=1 if it is willing to
accept SCMP redirects (see Section 4.5) and sets A=1 if explicit
probing is necessary (see Section 4.3.9). The ITE then sets the I
flag and SEAL_ID values as follows.
The ITE maintains a randomly-initialized SEAL_ID value as per-ETE
soft state (e.g., in the neighbor cache). If the SEAL_ID is to be
used as a packet identifier, the ITE monotonically increments the
value for each successive SEAL protocol packet it sends to the ETE.
If the SEAL_ID is to be used as a tunnel identifier, the ITE instead
maintains SEAL_ID as a static value.
For each successive SEAL segment, the ITE writes the current SEAL_ID
value into the header field of the same name in the SEAL header. It
then sets I=1 if the SEAL_ID represents a packet identifier and I=0
if the SEAL_ID represents a tunnel identifier. The ITE must be
consistent in its setting of the I flag. For example, it must not
set I=1 in some packets and I=0 in others since this may result in
unpredictable behavior.
4.3.7. 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],
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[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).
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 4.3.9),
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 the packet will not be fragmented within the subnetwork
since IPv6 deprecates in-the-network fragmentation.
4.3.8. Sending SEAL Protocol Packets
Following outer encapsulation, the ITE sends each outer packet that
encapsulates a segment of the same mid-layer packet into the
subnetwork in canonical order, i.e., segment 0 first, followed by
segment 1, etc., and finally segment N-1.
4.3.9. Probing Strategy
When IPv4 is used as the outer encapsulation layer, the ITE should
perform a qualification exchange to determine whether the subnetwork
path to the ETE correctly implements IPv4 fragmentation. The
qualification exchange can be performed either as an initial probe or
in-band with real data packets, and should be repeated periodically
since the path to the ETE may change due to dynamic routing.
To perform this qualification, the ITE prepares a probe packet that
is no larger than 576 bytes (e.g., a NULL packet with A=1 and
NEXTHDR="No Next Header" [RFC2460] in the SEAL header), then splits
the packet into two outer IPv4 fragments and sends both fragments to
the ETE. If the ETE returns an SCMP PTB message with Code=1 (see
Section 4.5.1.1), then the subnetwork path correctly implements IPv4
fragmentation and data packets can be sent with DF=0 in the outer
header to enable the preferred method of probing. If the ETE returns
an SCMP PTB message with Code=3, however, the ITE is obliged to set
DF=1 since a middlebox in the network is reassembling the IPv4
fragments before they are delivered to the ETE.
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
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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 4.3.10), 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 segment or a NULL packet (see above).
The probe will elicit an SCMP PTB message from the ETE as an
acknowledgement (see Section 4.5.1).
4.3.10. 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 SEAL_ID encoded in the SEAL header within 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.
In some subnetwork paths, ICMPv4 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 4.3.9 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.
4.4. 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 records the reassembly buffer size
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in a soft-state variable "S_MRU". The ETE may instead omit the
reassembly function altogether and set S_MRU=0, but this may cause
tunnel MTU underruns in some environments. 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 both an outer IP-layer reassembly buffer and 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
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.
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
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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.
4.4.3. SEAL-Layer Reassembly
Following IP reassembly (if necessary), the ETE examines each mid-
layer data packet (i.e., one with C=0 in the SEAL header) packet) to
determine whether an SCMP error message is required. If the mid-
layer data 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). Next, if the SEAL header has A=1, the ETE sends
an SCMP Neighbor Advertisement (SNA) message back to the ITE (see
Section 4.5.1). The ETE next submits single-segment mid-layer
packets for decapsulation and delivery to upper layers (see Section
4.4.4). 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 (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). 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. When the ETE processes the first
SEAL segment (i.e., one with F=1 and M=1 in the SEAL header) of a
segmented SEAL packet that would exceed 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).
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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
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,
where 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.
The ETE must not perform SEAL-layer reassembly for multi-segment mid-
layer packets with I=0 in the SEAL header. The ETE instead silently
drops all segments with I=0; F=0 in the SEAL header and uses any
segments with I=0; F=1; M=1 in the SEAL header to send an SCMP PTB
message back to the ITE. The ETE may also set S_MRU=0 in order to
disable SEAL layer reassembly altogether.
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]. Each SCMP message is embedded within an
SCMP packet which begins with the same outer header format as would
be used for outer encapsulation of a SEAL data packet (see: Section
4.3.7). 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 the same message Type and Code values specified for
ordinary ICMPv6 messages in [RFC4443]. SCMP is also used to carry
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other ICMPv6 message types and their associated options as specified
in other documents (e.g., [RFC4191][RFC4861], etc.). The general
format for SCMP messages is shown in Figure 4:
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
TEs generate solicitation messages (e.g., an SCMP echo request, an
SCMP router/neighbor solicitation, a SEAL data packet with A=1, etc.)
for the purpose of triggering an SCMP response. TEs generate
solicited SCMP messages (e.g., an SCMP echo reply, and SCMP router/
neighbor advertisement, etc.) in response to explicit solicitations,
and generate SCMP error messages in response to errored SEAL data
packets. As for ICMP, TEs must not generate SCMP error message in
response to other SCMP messages.
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 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.
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. If
the SCMP message will include a packet-in-error, the TE 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 packet following outer
encapsulation to exceed 576 bytes (see: Figure 5).
The TE then calculates the SCMP message Checksum the same as
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specified for ICMPv6 messages except that it does not prepend a
pseudo-header of the outer IP header since the SEAL_ID already gives
sufficient assurance against mis-delivery. (The Checksum calculation
procedure is therefore identical to that used for ICMPv4 [RFC0792].)
The TE then encapsulates the SCMP message in the outer headers as
shown in Figure 5:
+--------------------+
~ 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 Packet
after encapsulation
Figure 5: SCMP Message Encapsulation
When a TE generates an SCMP message in response to an SCMP
solicitation or an errored SEAL data packet (i.e., a soliciting
packet), it sets the outer IP destination and source addresses of the
SCMP packet to the soliciting packet's source and destination
addresses (respectively). (If the destination address in the
solicitation was multicast, the TE instead sets the outer IP source
address of the SCMP packet to an address assigned to the underlying
IP interface.) The TE then sets the SEAL_ID and I flag in the SEAL
header of the SCMP packet to the same values that appeared in the
soliciting or errored packet.
When a TE generates an unsolicited SCMP message, it sets the outer IP
destination and source addresses of the SCMP packet the same as it
would for ordinary SEAL data packets. The TE then sets the SEAL_ID
and I flag in the SEAL header of the SCMP packet to the same values
that it would use to send an ordinary SEAL data packet.
For all SCMP messages, the TE then sets the other flag bits in the
SEAL header to C=1, A=0, R=0, F=1, and M=0. It next sets the
NEXTHDR/SEG to an arbitrary value and sends the SCMP packet to the
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tunnel far end.
4.5.1.1. Generating SCMP Packet Too Big (PTB) Messages
An ETE generates an SCMP PTB message under one of the following
cases:
o Case 1: 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:
o Case 2: when it receives the SEAL first segment (i.e., one with
F=1 and M=1 in the SEAL header) of a SEAL protocol packet that
arrived as multiple SEAL segments and the size of the reassembled
packet would exceed S_MRU, or:
o Case 3: when it receives a SEAL protocol data packet with A=1 in
the SEAL header.
It is essential that the ETE only send a single SCMP PTB message
according to the priority order listed above even if a received IP
packet matches more than one of the above cases. For example, if the
ETE receives the IP first fragment of a SEAL protocol packet that
also includes a SEAL header with F=1, M=1 and A=1, the case used for
generating the SCMP PTB is the first of these three.
The ETE prepares an SCMP PTB message the same as for the
corresponding ICMPv6 PTB message, except that it writes the S_MRU
value for this ITE in the MTU field (i.e, even if the S_MRU value is
0). The packet-in-error field includes the leading portion of the IP
packet or fragment that triggered a matching condition from the list
above.
Finally, the ETE writes the value 1, 2 or 3 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.
4.5.1.2. Generating SCMP Neighbor Discovery Messages
An ITE generates an SCMP "Neighbor Solicitation" (SNS) or "Router
Solicitation" (SRS) message when it needs to solicit a response from
an ETE. An ETE generates a solicited SCMP "Neighbor Advertisement"
(SNA) or "Router Advertisement" (SRA) message when it receives an
SNS/SRS message. Any TE may also generate unsolicited SNA/SRA
messages that are not triggered by a specific solicitation event, but
these may be discarded by the tunnel far-end.
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The TE generates SNS, SNA, SRS and SRA messages the same as described
for the corresponding IPv6 Neighbor Discovery (ND) messages (see:
[RFC4861]). These messages may also be used in conjunction with the
tunnel endpoint synchronization procedure specified in Section 4.6.
4.5.1.3. Generating SCMP Redirect Messages
An ETE generates an SCMP "Redirect" message when it receives a SEAL
data packet with R=1 in the SEAL header and needs to inform the ITE
of a better next hop. The ETE generates SCMP Redirect messages the
same as described in [RFC4861], except that it includes Route
Information Options (RIOs) [RFC4191] to inform the ITE of a better
next hop for an entire IP prefix instead of only a single
destination. The SCMP Redirect message therefore supports network
redirection instead of host redirection.
4.5.1.4. 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 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 solicited and error SCMP message it receives as
long as it can verify that the corresponding SCMP packet was sent
from an on-path ETE. The ITE can verify that the SCMP packet came
from an on-path ETE by checking that the SEAL_ID in the SEAL header
of the packet corresponds to one of its recently-sent SEAL data
packets or SCMP request packets.
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An ITE maintains a window of SEAL_IDs of packets that it has recently
sent to each ETE. For each solicited and error SCMP message it
receives, the ITE first verifies that the SEAL_ID is within the
window 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 (e.g., SNS, SRS,
SNA, etc.) from the tunnel far end. The TE sends SCMP response
messages in response to solicitations, but does not otherwise process
the unsolicited SCMP messages as an indication of tunnel far end
liveness.
Finally, TEs process solicited and error 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 to an ETE (see: Section 4.5.1). The packet-in-error within
the PTB message consists of the encapsulating IP/*/SEAL headers
followed by the inner packet in the form in which the ITE received it
prior to SEAL encapsulation.
The ITE first verifies that the PTB message corresponds to a packet
that it sent to the ETE (see Section 4.5.2). Next, if the PTB
message has Code=3 in the SCMP header the ITE processes the message
as a response to an explicit probe request and discards the message.
If the PTB has Code=2 or Code=1 in the SCMP header, however, the ITE
processes the message as an indication of an MTU limitation.
if the PTB has Code =1, the ITE first verifies that the outer IP
header in the packet-in-error encodes an IP first fragment, then
examines the outer IP header length field to determine a new S_MSS
value 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.
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In this latter case, the ITE may need to search for a reduced S_MSS
value through an iterative searching strategy that parallels the IPv4
Path MTU Discovery "plateau table" procedure in a similar fashion as
described in Section 5 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 both Code=1 and Code=2 PTB messages, the ITE next records the
value in the MTU field of the SCMP PTB message as the new S_MRU value
for this ETE and examines the inner packet within the packet-in-
error. If the inner packet was unfragmentable (see: Section 4.3.3)
and larger than (MAX(S_MRU, S_MSS) - HLEN), the ITE then sends a
transcribed PTB message appropriate for the inner packet to the
original source with MTU set to (MAX(S_MRU, S_MSS) - HLEN). (In the
case of nested SEAL encapsulations, the transcribed PTB message will
itself be an SCMP PTB message). If the inner packet is fragmentable,
however, the ITE instead reduces its inner fragmentation THRESH
estimate to a size no larger than S_MSS for this ETE (see: Section
4.3.3) and does not send a transcribed PTB. In that case, some
fragmentable packets may be silently discarded but future
fragmentable packets will subsequently undergo inner fragmentation
based on this new THRESH estimate.
The ITE may alternatively avoid stateful caching of per-ETE S_MSS
values by treating all ETE S_MRU values as zero (i.e., by disabling
SEAL-layer segmentation) and implementing stateless MTU discovery.
In that case, the ITE sends all SEAL-encapsulated packets as single
segments. If the ITE receives an SCMP PTB message from the ETE with
a too-small length value in the outer IP header, it can send a
translated PTB message back to the source listing a slightly smaller
MTU size than the length value in the inner IP header. For example,
if the ITE receives an SCMP PTB message with outer IP length 256 and
inner IP length 1500, it can send a PTB message listing an MTU of
1400 back to the source. If the ITE subsequently receives an SCMP
PTB message with outer IP length 256 and inner IP length 1400, it can
send a PTB message listing an MTU of 1300 back to the source, etc.
Actual plateau table values for this "step-down" MTU determination
procedure are up to the implementation, which may consult Section 7
of [RFC1191] for non-normative example guidance.
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4.5.2.2. Processing SCMP Neighbor Discovery Messages
An ETE may receive SNS/SRS messages from an ITE as the initial leg in
a neighbor discovery exchange. An ITE may also receive both
solicited and unsolicited SNA/SRA messages from an ETE.
The TE processes SNS/SRS and SNA/SRA 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 SCMP Redirect Messages
An ITE may receive SCMP redirect messages after sending a SEAL data
packet with R=1 in the SEAL header to an ETE. The ITE processes any
RIO options in the SCMP redirect message and updates its Forwarding
Information Base (FIB) accordingly.
4.5.2.4. 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 the message as an indication
that it needs to (re)synchronize with the ETE (see: Section 4.6).
An ITE may receive an SCMP "Destination Unreachable" message 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.
Processing of all other SCMP message types is outside the scope of
this document.
4.6. Tunnel Endpoint Synchronization
SEAL ITEs that maintain state retain a per-ETE window of SEAL_IDs of
recently-sent packets, but by default the SEAL ETE does not retain
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inter-packet state. When closer synchronization is required, SEAL
TEs can exchange initial SEAL_IDs in a procedure 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 SNS or SRS 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 SNS/SRS message to TE(B).
When TE(B) receives an SNS/SRS 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 SNA or SRA 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 SNA/SRA message back to TE(A).
When TE(A) receives the SNA/SRA, 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 SNA/SRA response, it retransmits the
initial SNS/SRS 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 SNA/SRA 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
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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
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.
When the ITE sends SEAL packets with I=0, a constant SEAL_ID nonce
value instead of an incrementing sequence number is used.
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
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[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.
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.
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SCMP messages carry the SEAL_ID 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. 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]:
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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.
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, 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.
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
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Deering on the MTUDWG mailing list in 1989.
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-v6ops-tunnel-security-concerns]
Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns With IP Tunneling",
draft-ietf-v6ops-tunnel-security-concerns-03 (work in
progress), October 2010.
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[I-D.russert-rangers]
Russert, S., Fleischman, E., and F. Templin, "RANGER
Scenarios", draft-russert-rangers-05 (work in progress),
July 2010.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-16 (work in progress),
July 2010.
[I-D.templin-iron]
Templin, F., "The Internet Routing Overlay Network
(IRON)", draft-templin-iron-13 (work in progress),
October 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.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, December 1998.
[RFC2675] Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
RFC 2675, August 1999.
[RFC2764] Gleeson, B., Heinanen, J., Lin, A., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private
Networks", RFC 2764, February 2000.
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[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[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.
[TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
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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.
Appendix B. Integrity
Each link in the path over which a SEAL tunnel is configured is
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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
"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.
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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
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
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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
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