Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Intended status: Informational July 9, 2012
Expires: January 10, 2013
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-48.txt
Abstract
For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected IP network routing
region and bounded by encapsulating border nodes. These virtual
topologies are manifested by tunnels that may span multiple IP and/or
sub-IP layer forwarding hops, and can introduce failure modes due to
packet duplication, packet reordering, source address spoofing and
traversal of links with diverse Maximum Transmission Units (MTUs).
This document specifies a Subnetwork Encapsulation and Adaptation
Layer (SEAL) that addresses these issues.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 10, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 7
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 9
4. SEAL Specification . . . . . . . . . . . . . . . . . . . . . . 10
4.1. VET Interface Model . . . . . . . . . . . . . . . . . . . 10
4.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 11
4.3. SEAL Header and Trailer Format . . . . . . . . . . . . . . 12
4.4. ITE Specification . . . . . . . . . . . . . . . . . . . . 14
4.4.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 14
4.4.2. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 15
4.4.3. Pre-Encapsulation . . . . . . . . . . . . . . . . . . 16
4.4.4. SEAL Encapsulation and Segmentation . . . . . . . . . 17
4.4.5. Outer Encapsulation . . . . . . . . . . . . . . . . . 18
4.4.6. Path Probing and ETE Reachability Verification . . . . 19
4.4.7. Processing ICMP Messages . . . . . . . . . . . . . . . 19
4.4.8. IPv4 Middlebox Reassembly Testing . . . . . . . . . . 20
4.4.9. Stateful MTU Determination . . . . . . . . . . . . . . 22
4.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 22
4.5. ETE Specification . . . . . . . . . . . . . . . . . . . . 22
4.5.1. Minimum Reassembly Buffer Requirements . . . . . . . . 22
4.5.2. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 23
4.5.3. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 23
4.5.4. Decapsulation and Re-Encapsulation . . . . . . . . . . 23
4.6. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 25
4.6.1. Generating SCMP Error Messages . . . . . . . . . . . . 25
4.6.2. Processing SCMP Error Messages . . . . . . . . . . . . 27
5. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 30
6. End System Requirements . . . . . . . . . . . . . . . . . . . 30
7. Router Requirements . . . . . . . . . . . . . . . . . . . . . 30
8. Nested Encapsulation Considerations . . . . . . . . . . . . . 30
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 31
10. Security Considerations . . . . . . . . . . . . . . . . . . . 31
11. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 32
12. Implementation Status . . . . . . . . . . . . . . . . . . . . 32
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 32
14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 33
14.1. Normative References . . . . . . . . . . . . . . . . . . . 33
14.2. Informative References . . . . . . . . . . . . . . . . . . 33
Appendix A. Reliability . . . . . . . . . . . . . . . . . . . . . 37
Appendix B. Integrity . . . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Transport Mode . . . . . . . . . . . . . . . . . . . 38
Appendix D. Historic Evolution of PMTUD . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 40
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1. Introduction
As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (including
tunnels of one form or another) over an actual network that supports
the Internet Protocol (IP) [RFC0791][RFC2460]. Those virtual
topologies have elements that appear as one hop in the virtual
topology, but are actually multiple IP or sub-IP layer hops. These
multiple hops often have quite diverse properties that are often not
even visible to the endpoints of the virtual hop. This introduces
failure modes that are not dealt with well in current approaches.
The use of IP encapsulation (also known as "tunneling") has long been
considered as the means for creating such virtual topologies.
However, the encapsulation headers often include insufficiently
provisioned per-packet identification values. This can present
issues for duplicate packet detection and detection of packet
reordering within the subnetwork. IP encapsulation also allows an
attacker to produce encapsulated packets with spoofed source
addresses even if the source address in the encapsulating header
cannot be spoofed. A denial-of-service vector that is not possible
in non-tunneled subnetworks is therefore presented.
Additionally, the insertion of an outer IP header reduces the
effective path MTU visible to the inner network layer. When IPv6 is
used as the encapsulation protocol, original sources expect to be
informed of the MTU limitation through IPv6 Path MTU discovery
(PMTUD) [RFC1981]. 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 IPv4 PMTUD
[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,
IP is ubiquitously deployed as the Layer 3 protocol. The primary
functions of IP are to provide for routing, addressing, and a
fragmentation and reassembly capability used to accommodate links
with diverse MTUs. While it is well known that the IP address space
is rapidly becoming depleted, there is a lesser-known but growing
consensus that other IP protocol limitations have already or may soon
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become problematic.
First, the Internet historically provided no means for discerning
whether the source addresses of IP packets are authentic. This
shortcoming is being addressed more and more through the deployment
of site border router ingress filters [RFC2827], however the use of
encapsulation provides a vector for an attacker to circumvent
filtering for the encapsulated packet even if filtering is correctly
applied to the encapsulation header. Secondly, the IP header does
not include a well-behaved identification value unless the source has
included a fragment header for IPv6 or unless the source permits
fragmentation for IPv4. These limitations preclude an efficient
means for routers to detect duplicate packets and packets that have
been re-ordered within the subnetwork.
For IPv4 encapsulation, when fragmentation is permitted the header
includes a 16-bit Identification field, meaning that at most 2^16
unique packets with the same (source, destination, protocol)-tuple
can be active in the Internet at the same time
[I-D.ietf-intarea-ipv4-id-update]. (When middleboxes such as Network
Address Translators (NATs) re-write the Identification field to
random values, the number of unique packets is even further reduced.)
Due to the escalating deployment of high-speed links, however, these
numbers have become too small by several orders of magnitude for high
data rate packet sources such as tunnel endpoints [RFC4963].
Furthermore, there are many well-known limitations pertaining to IPv4
fragmentation and reassembly - even to the point that it has been
deemed "harmful" in both classic and modern-day studies (see above).
In particular, IPv4 fragmentation raises issues ranging from minor
annoyances (e.g., in-the-network router fragmentation [RFC1981]) to
the potential for major integrity issues (e.g., mis-association of
the fragments of multiple IP packets during reassembly [RFC4963]).
As a result of these perceived limitations, a fragmentation-avoiding
technique for discovering the MTU of the forward path from a source
to a destination node was devised through the deliberations of the
Path MTU Discovery Working Group (PMTUDWG) during the late 1980's
through early 1990's (see Appendix D). In this method, the source
node provides explicit instructions to routers in the path to discard
the packet and return an ICMP error message if an MTU restriction is
encountered. However, this approach has several serious shortcomings
that lead to an overall "brittleness" [RFC2923].
In particular, site border routers in the Internet have been known 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
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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 PMTUD failures for both IPv4 and IPv6 in the Internet
today [TBIT][WAND][SIGCOMM].
The issues with both IP fragmentation and this "classical" PMTUD
method are exacerbated further when IP tunneling is used [RFC4459].
For example, an ingress tunnel endpoint (ITE) may be required to
forward encapsulated packets into the subnetwork on behalf of
hundreds, thousands, or even more original sources. If the ITE
allows IP fragmentation on the encapsulated packets, persistent
fragmentation could lead to undetected data corruption due to
Identification field wrapping and/or reassembly congestion at the
ETE. If the ITE instead uses classical IP PMTUD it must rely on ICMP
error messages coming from the subnetwork that may be suspect,
subject to loss due to filtering middleboxes, or insufficiently
provisioned for translation into error messages to be returned to the
original sources.
Although recent works have led to the development of a robust end-to-
end MTU determination scheme [RFC4821], they do not excuse tunnels
from accounting for the encapsulation overhead they add to packets.
Moreover, in current practice existing tunneling protocols mask the
MTU issues by selecting a "lowest common denominator" MTU that may be
much smaller than necessary for most paths and difficult to change at
a later date. Therefore, a new approach to accommodate tunnels over
links with diverse MTUs is necessary.
1.2. Approach
For the purpose of this document, a subnetwork is defined as a
virtual topology configured over a connected network routing region
and bounded by encapsulating border nodes. Example connected network
routing regions include Mobile Ad hoc Networks (MANETs), enterprise
networks and the global public Internet itself. Subnetwork border
nodes forward unicast and multicast packets over the virtual topology
across multiple IP and/or sub-IP layer forwarding hops that may
introduce packet duplication and/or traverse links with diverse
Maximum Transmission Units (MTUs).
This document introduces a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for tunneling inner network layer protocol packets over
IP subnetworks that connect Ingress and Egress Tunnel Endpoints
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(ITEs/ETEs) of border nodes. It provides a modular specification
designed to be tailored to specific associated tunneling protocols.
A transport-mode of operation is also possible, and described in
Appendix C.
SEAL provides a mid-layer encapsulation that accommodates links with
diverse MTUs, and allows routers in the subnetwork to perform
efficient duplicate packet and packet reordering detection. The
encapsulation further ensures data origin authentication, packet
header integrity and anti-replay in environments in which these
functions are necessary.
SEAL treats tunnels that traverse the subnetwork as ordinary links
that must support network layer services. Moreover, SEAL provides
dynamic mechanisms to ensure a maximal path MTU over the tunnel.
This is in contrast to static approaches which avoid MTU issues by
selecting a lowest common denominator MTU value that may be overly
conservative for the vast majority of tunnel paths and difficult to
change even when larger MTUs become available.
The following sections provide the SEAL normative specifications,
while the appendices present non-normative additional considerations.
2. Terminology and Requirements
The following terms are defined within the scope of this document:
subnetwork
a virtual topology configured over a connected network routing
region and bounded by encapsulating border nodes.
IP
used to generically refer to either Internet Protocol (IP)
version, i.e., IPv4 or IPv6.
Ingress Tunnel Endpoint (ITE)
a virtual interface over which an encapsulating border node (host
or router) sends encapsulated packets into the subnetwork.
Egress Tunnel Endpoint (ETE)
a virtual interface over which an encapsulating border node (host
or router) receives encapsulated packets from the subnetwork.
ETE Link Path
a subnetwork path from an ITE to an ETE beginning with an
underlying link of the ITE as the first hop. Note that, if the
ITE's interface connection to the underlying link assigns multiple
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IP addresses, each address represents a separate ETE link path.
inner packet
an unencapsulated network layer protocol packet (e.g., IPv4
[RFC0791], OSI/CLNP [RFC0994], IPv6 [RFC2460], etc.) before any
outer encapsulations are added. Internet protocol numbers that
identify inner packets are found in the IANA Internet Protocol
registry [RFC3232]. SEAL protocol packets that incur an
additional layer of SEAL encapsulation are also considered inner
packets.
outer IP packet
a packet resulting from adding an outer IP header (and possibly
other outer headers) to a SEAL-encapsulated inner packet.
packet-in-error
the leading portion of an invoking data packet encapsulated in the
body of an error control message (e.g., an ICMPv4 [RFC0792] error
message, an ICMPv6 [RFC4443] error message, etc.).
Packet Too Big (PTB) message
a control plane message indicating an MTU restriction (e.g., an
ICMPv6 "Packet Too Big" message [RFC4443], an ICMPv4
"Fragmentation Needed" message [RFC0792], etc.).
Don't Fragment (DF) bit
a bit that indicates whether the packet may be fragmented by the
network. The DF bit is explicitly included in the IPv4 header
[RFC0791] and may be set to '0' to allow fragmentation or '1' to
disallow fragmentation. The bit is absent from the IPv6 header
[RFC2460], but implicitly set to '1'.
The following abbreviations correspond to terms used within this
document and/or elsewhere in common Internetworking nomenclature:
ETE - Egress Tunnel Endpoint
HLEN - the length of the SEAL header plus outer headers
ICV - Integrity Check Vector
ITE - Ingress Tunnel Endpoint
MTU - Maximum Transmission Unit
SCMP - the SEAL Control Message Protocol
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SDU - SCMP Destination Unreachable message
SPP - SCMP Parameter Problem message
SPTB - SCMP Packet Too Big message
SEAL - Subnetwork Encapsulation and Adaptation Layer
TE - Tunnel Endpoint (i.e., either ingress or egress)
VET - Virtual Enterprise Traversal
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. When used
in lower case (e.g., must, must not, etc.), these words MUST NOT be
interpreted as described in [RFC2119], but are rather interpreted as
they would be in common English.
3. Applicability Statement
SEAL was originally motivated by the specific case of subnetwork
abstraction for Mobile Ad hoc Networks (MANETs), however the domain
of applicability also extends to subnetwork abstractions over
enterprise networks, ISP networks, SOHO networks, the global public
Internet itself, and any other connected network routing region.
SEAL, along with the Virtual Enterprise Traversal (VET)
[I-D.templin-intarea-vet] tunnel virtual interface abstraction, are
the functional building blocks for the Internet Routing Overlay
Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in
Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
architectures.
SEAL provides a network sublayer for encapsulation of an inner
network layer packet within outer encapsulating headers. SEAL can
also be used as a sublayer within a transport layer protocol data
payload, where transport layer encapsulation is typically used for
Network Address Translator (NAT) traversal as well as operation over
subnetworks that give preferential treatment to certain "core"
Internet protocols (e.g., TCP, UDP, etc.). The SEAL header is
processed the same as for IPv6 extension headers, i.e., it is not
part of the outer IP header but rather allows for the creation of an
arbitrarily extensible chain of headers in the same way that IPv6
does.
To accommodate MTU diversity, the Ingress Tunnel Endpoint (ITE) may
need to perform a small amount of fragmentation which the Egress
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Tunnel Endpoint (ETE) must reassemble. The ETE further acts as a
passive observer that informs the ITE of any packet size limitations.
This allows the ITE to return appropriate PMTUD feedback even if the
network path between the ITE and ETE filters ICMP messages.
SEAL further provides mechanisms to ensure data origin
authentication, packet header integrity, and anti-replay. The SEAL
framework is therefore similar to the IP Security (IPsec)
Authentication Header (AH) [RFC4301][RFC4302], however it provides
only minimal hop-by-hop authenticating services while leaving full
data integrity, authentication and confidentiality services as an
end-to-end consideration. While SEAL performs data origin
authentication, the origin site must also perform the necessary
ingress filtering in order to provide full source address
verification [I-D.ietf-savi-framework].
In many aspects, SEAL also very closely resembles the Generic Routing
Encapsulation (GRE) framework [RFC1701]. SEAL can therefore be
applied in the same use cases that are traditionally addressed by
GRE, and can also provide additional capabilities as described in
this document.
4. SEAL Specification
The following sections specify the operation of SEAL:
4.1. VET Interface Model
SEAL is an encapsulation sublayer used within VET non-broadcast,
multiple access (NBMA) tunnel virtual interfaces. Each VET interface
is configured over one or more underlying interfaces attached to
subnetwork links. The VET interface connects an ITE to one or more
ETE "neighbors" via tunneling across an underlying subnetwork, where
the tunnel neighbor relationship may be either unidirectional or
bidirectional.
A unidirectional tunnel neighbor relationship allows the near end ITE
to send data packets forward to the far end ETE, while the ETE only
returns control messages when necessary. A bidirectional tunnel
neighbor relationship is one over which both TEs can exchange both
data and control messages.
Implications of the VET unidirectional and bidirectional models are
discussed in [I-D.templin-intarea-vet].
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4.2. SEAL Model of Operation
SEAL-enabled ITEs encapsulate each inner packet in a SEAL header and
any outer header encapsulations as shown in Figure 1:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| | --> | |
~ Inner ~ --> ~ Inner ~
~ Packet ~ --> ~ Packet ~
| | --> | |
+--------------------+ +----------+---------+
Figure 1: SEAL Encapsulation
The ITE inserts the SEAL header according to the specific tunneling
protocol. For simple encapsulation of an inner network layer packet
within an outer IP header, the ITE inserts the SEAL header following
the outer IP header and before the inner packet as: IP/SEAL/{inner
packet}.
For encapsulations over transports such as UDP, the ITE inserts the
SEAL header following the outer transport layer header and before the
inner packet, e.g., as IP/UDP/SEAL/{inner packet}. In that case, the
UDP header is seen as an "other outer header" as depicted in
Figure 1.
SEAL supports both "nested" tunneling and "re-encapsulating"
tunneling. Nested tunneling occurs when a first tunnel is
encapsulated within a second tunnel, which may then further be
encapsulated within additional tunnels. Nested tunneling can be
useful, and stands in contrast to "recursive" tunneling which is an
anomalous condition incurred due to misconfiguration or a routing
loop. Considerations for nested tunneling are discussed in Section 4
of [RFC2473].
Re-encapsulating tunneling occurs when a packet arrives at a first
ETE, which then acts as an ITE to re-encapsulate and forward the
packet to a second ETE connected to the same subnetwork. In that
case each ITE/ETE transition represents a segment of a bridged path
between the ITE nearest the source and the ETE nearest the
destination. Combinations of nested and re-encapsulating tunneling
are also naturally supported by SEAL.
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The SEAL ITE considers each underlying interface as the ingress
attachment point to a subnetwork link path to the ETE. The ITE
therefore may experience different path MTUs on different ETE link
paths.
Finally, the SEAL ITE ensures that the inner network layer protocol
will see a minimum MTU of 1500 bytes over each ETE link path
regardless of the outer network layer protocol version, i.e., even if
a small amount of fragmentation and reassembly are necessary. This
is necessary to avoid path MTU "black holes" for the minimum MTU
configured by the vast majority of links in the Internet.
4.3. SEAL Header and Trailer Format
The SEAL header is formatted as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|VER|C|A|R|L|I|V|X|M| Offset | NEXTHDR | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Vector (ICV) (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SEAL Header Format
VER (2)
a 2-bit version field. This document specifies Version 0 of the
SEAL protocol, i.e., the VER field encodes the value 0.
C (1)
the "Control/Data" bit. Set to 1 by the ITE in SEAL Control
Message Protocol (SCMP) control messages, and set to 0 in ordinary
data packets.
A (1)
the "Acknowledgement Requested" bit. Set to 1 by the ITE in SEAL
data packets for which it wishes to receive an explicit
acknowledgement from the ETE.
R (1)
the "Redirects Permitted" bit. For data packets, set to 1 by the
ITE to inform the ETE that the source is accepting Redirects (see:
[I-D.templin-intarea-vet]).
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L (1)
the "Rate Limit" bit for IPv4 ETE link paths. Reserved for future
use for IPv6 ETE link paths.
I (1)
the "Identification Included" bit.
V (1)
the "ICV included" bit.
X (1) a 1-bit reserved field.
M (1) the "More Segments" bit. Set to 1 in a non-final segment and
set to 0 in the final segment of the SEAL packet.
Offset (6) a 6-bit Offset field. Set to 0 in the first segment of a
segmented SEAL packet. Set to an integral number of 32 byte
blocks in subsequent segments (e.g., an Offset of 10 indicates a
block that begins at the 320th byte in the packet).
NEXTHDR (8) an 8-bit field that encodes the next header Internet
Protocol number the same as for the IPv4 protocol and IPv6 next
header fields.
LINK_ID (5)
a 5-bit link identification value, set to a unique value by the
ITE for each link path over which it will send encapsulated
packets to the ETE (up to 32 link paths per ETE are therefore
supported). Note that, if the ITE's interface connection to the
underlying link assigns multiple IP addresses, each address
represents a separate ETE link path that must be assigned a
separate LINK_ID.
LEVEL (3)
a 3-bit nesting level; use to limit the number of tunnel nesting
levels. Set to an integer value up to 7 in the innermost SEAL
encapsulation, and decremented by 1 for each successive additional
SEAL encapsulation nesting level. Up to 8 levels of nesting are
therefore supported.
Identification (32)
an optional 32-bit per-packet identification field; present when
I==1. Set to a monotonically-incrementing 32-bit value for each
SEAL packet transmitted to this ETE, beginning with 0.
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Integrity Check Vector (ICV) (32)
an optional 32-bit header integrity check value; present when
V==1. Covers the leading 128 bytes of the packet beginning with
the SEAL header. The value 128 is chosen so that at least the
SEAL header as well as the inner packet network and transport
layer headers are covered by the integrity check.
4.4. ITE Specification
4.4.1. Tunnel Interface MTU
The tunnel interface must present a constant MTU value to the inner
network layer as the size for admission of inner packets into the
interface. Since VET NBMA tunnel virtual interfaces may support a
large set of ETE link paths that accept widely varying maximum packet
sizes, however, a number of factors should be taken into
consideration when selecting a tunnel interface MTU.
Due to the ubiquitous deployment of standard Ethernet and similar
networking gear, the nominal Internet cell size has become 1500
bytes; this is the de facto size that end systems have come to expect
will either be delivered by the network without loss due to an MTU
restriction on the path or a suitable ICMP Packet Too Big (PTB)
message returned. When large packets sent by end systems incur
additional encapsulation at an ITE, however, they may be dropped
silently within the tunnel since the network may not always deliver
the necessary PTBs [RFC2923]. The ITE should therefore set a tunnel
interface MTU of at least 1500 bytes.
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 contains
as much of the invoking packet as possible without the entire message
exceeding the network layer MINMTU size.
The ITE can alternatively set an indefinite MTU on the tunnel
interface such that all inner packets are admitted into the interface
regardless of their size. For ITEs that host applications that use
the tunnel 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 (sometimes known as "MSS clamping").
In light of the above considerations, the ITE should configure an
indefinite MTU on tunnel *router* interfaces so that subnetwork
adaptation is handled from within the interface. The ITE can instead
set a smaller MTU on tunnel *host* interfaces (e.g., the smallest MTU
among all of the underlying links minus the size of the encapsulation
headers) but should not set an MTU smaller than 1500 bytes.
4.4.2. Tunnel Neighbor Soft State
The tunnel virtual interface maintains a number of soft state
variables for each ETE and for each ETE link path.
When per-packet identification is required, the ITE maintains a per
ETE window of Identification values for the packets it has recently
sent to this ETE. The ITE then sets a variable "USE_ID" to TRUE, and
includes an Identification in each packet it sends to this ETE;
otherwise, it sets USE_ID to FALSE.
When data origin authentication and integrity checking is required,
the ITE also maintains a per ETE integrity check vector (ICV)
calculation algorithm and a symmetric secret key to calculate the ICV
in each packet it will send to this ETE. The ITE then sets a
variable "USE_ICV" to TRUE, and includes an ICV in each packet it
sends to this ETE; otherwise, it sets USE_ICV to FALSE.
For IPv4 ETE link paths, the ITE further maintains a variable
"RATE_LIMIT" initialized to FALSE. If the link path subsequently
exhibits unavoidable IPv4 fragmentation the ETE sets RATE_LIMIT to
TRUE.
For each ETE link path, the ITE must also account for encapsulation
header lengths. The ITE therefore maintains the per ETE link path
constant values "SHLEN" set to the length of the SEAL header, "THLEN"
set to the length of the outer encapsulating transport layer headers
(or 0 if outer transport layer encapsulation is not used), "IHLEN"
set to the length of the outer IP layer header, and "HLEN" set to
(SHLEN+THLEN+IHLEN). (The ITE must include the length of the
uncompressed headers even if header compression is enabled when
calculating these lengths.) In addition, the ITE maintains a per ETE
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link path variable "PATH_MTU" initialized to the maximum of 1500
bytes and the MTU of the underlying link minus HLEN. (Thereafter,
the ITE must not reduce PATH_MTU to a value smaller than 1500 bytes.)
The ITE may instead maintain the packet sizing variables and
constants as per ETE (rather than per ETE link path) values. In that
case, the values reflect the lowest-common-denominator size across
all of the ETE's link paths.
4.4.3. Pre-Encapsulation
For each inner packet admitted into the tunnel interface, if the
packet is itself a SEAL packet (i.e., one with the port number for
SEAL in the transport layer header or one with the protocol number
for SEAL in the IP layer header) and the LEVEL field of the SEAL
header contains the value 0, the ITE silently discards the packet.
Otherwise, the ITE sets a constant value 'MINMTU' to the minimum MTU
for the path over which encapsulated packets will travel. For IPv6
paths the ITE sets MINMTU=1280 (see: [RFC2460]) and for IPv4 paths
the ITE sets MINMTU=576 even though the true MINMTU for IPv4 is only
68 bytes (see: [RFC0791]). Note that the ITE may use larger initial
values for MINMTU (e.g., 1280 for IPv4) as long as the value does not
exceeed the ETE's minimum reassembly buffer size (see Section 4.5.1).
If this value proves too large, the ITE will receive feedback either
from the ETE or from a router on the path and will be able to reduce
its MINMTU to a smaller value.
Next, for non-atomic inner packets (i.e., a non-SEAL IPv4 packet with
DF==0 in the IP header), if the packet is larger than (MINMTU-HLEN)
bytes the ITE fragments the packet into N roughly equal-length
pieces, where N is minimized and each fragment is significantly
smaller than (MINMTU-HLEN) to allow for additional encapsulations in
the path. The ITE then submits each inner fragment for SEAL
encapsulation as specified in Section 4.4.4.
For atomic inner packets (i.e., for all other packets), if the packet
is larger than (MINMTU-HLEN) but no larger than 1500 bytes the ITE
must ensure that it will traverse the tunnel. For SEAL packets, the
ITE first sends an SCMP PTB (SPTB) message toward the previous hop
SEAL ITE (see: Section 4.6.1.1) with the MTU field set to (MINMTU-
HLEN) subject to rate limiting. For non-SEAL IPv6 packets, the ITE
sends an ICMPv6 PTB message with the MTU field set to (MINMTU-HLEN)
toward the original source subject to rate limiting. (For IPv4 SEAL
packets with DF==0, the ITE should set DF=1 and re-calculate the IPv4
header checksum before generating the PTB message in order to avoid
bogon filters.) After sending the (S)PTB message, the ITE submits
the packet for SEAL segmentation as specifed in Section 4.4.4.
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For atomic inner packets larger than 1500 bytes, if the packet is no
larger than PATH_MTU for the corresponding ETE link path, the ITE
submits it for SEAL encapsulation. Otherwise, the ITE drops the
packet and sends a PTB error message toward the source address of the
inner packet. For SEAL packets, the ITE sends an SPTB message to the
previous hop (see: Section 4.6.1.1) with the MTU field set to
PATH_MTU. Otherwise, the ITE sends an ordinary PTB message
appropriate to the inner protocol version with the MTU field set to
PATH_MTU. (For IPv4 SEAL packets with DF==0, the ITE should set DF=1
and re-calculate the IPv4 header checksum before generating the PTB
message in order to avoid bogon filters.) After sending the (S)PTB
message, the ITE discards the inner packet.
4.4.4. SEAL Encapsulation and Segmentation
For each inner packet/fragment submitted for SEAL encapsulation, the
ITE next encapsulates the packet in a SEAL header formatted as
specified in Section 4.3. The SEAL header includes an Identification
field when USE_ID is TRUE, followed by an ICV field when USE_ICV is
TRUE.
The ITE next sets C=0 in the SEAL header. The ITE also sets A=1 if
necessary for ETE reachability determination (see: Section 4.4.6) or
for stateful MTU determination (see Section 4.4.9). Otherwise, the
ITE sets A=0. Next, when RATE_LIMIT is TRUE the ITE sets L=1;
otherwise, it sets L=0. The ITE also sets X=0.
The ITE then sets R=1 if redirects are permitted (see:
[I-D.templin-intarea-vet]). (Note that if this process is entered
via re-encapsulation (see: Section 4.5.4), R is instead copied from
the SEAL header of the re-encapsulated packet. This implies that the
R value is propagated across a re-encapsulating chain of ITE/ETEs.)
The ITE then sets LINK_ID to the value assigned to the underlying ETE
link path, and sets NEXTHDR to the protocol number corresponding to
the address family of the encapsulated inner packet. For example,
the ITE sets NEXTHDR to the value '4' for encapsulated IPv4 packets
[RFC2003], '41' for encapsulated IPv6 packets [RFC2473][RFC4213],
'80' for encapsulated OSI/CLNP packets [RFC1070], etc.
Next, if the inner packet is not itself a SEAL packet the ITE sets
LEVEL to an integer value between 0 and 7 as a specification of the
number of additional layers of nested SEAL encapsulations permitted.
If the inner packet is a SEAL packet that is undergoing nested
encapsulation, the ITE instead sets LEVEL to the value that appears
in the inner packet's SEAL header minus 1. If the inner packet is
undergoing SEAL re-encapsulation, the ITE instead copies the LEVEL
value from the SEAL header of the packet to be re-encapsulated.
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Next, if the inner packet is no larger than (MINMTU-HLEN) or larger
than 1500, the ITE sets (M=0; Offset=0). Otherwise, the ITE breaks
the inner packet into a minimum number of non-overlapping segments
that are no larger than (MINMTU-HLEN) bytes then appends a clone of
the SEAL header from the first segment onto the head of each
additional segment. The ITE then sets (M=1; Offset=0) in the first
segment, sets (M=0/1; Offset=i) in the second segment, sets (M=0/1;
Offset=j) in the third segment (if needed), etc., then finally sets
(M=0; Offset=k) in the final segment (where i, j, k, etc. are the
number of 32 byte blocks that preceded this segment).
When USE_ID is FALSE, the ITE next sets I=0. Otherwise, the ITE sets
I=1 and writes a monotonically-increasing integer value for this ETE
in the Identification field beginning with 0 in the first packet
transmitted. (For SEAL packets that have been split into multiple
pieces, the ITE writes the same Identification value in each piece.)
When USE_ICV is FALSE, the ITE next sets V=0. Otherwise, the ITE
sets V=1 and calculates the packet header ICV value using an
algorithm agreed on by the ITE and ETE. When data origin
authentication is required, the algorithm uses a symmetric secret key
so that the ETE can verify that the ICV was generated by the ITE.
Beginning with the SEAL header, the ITE calculates the ICV over the
leading 128 bytes of the packet (or up to the end of the packet if
there are fewer than 128 bytes) and places result in the ICV field.
(For SEAL packets that have been split into two pieces, each piece
calculates its own ICV value.)
The ITE then adds the outer encapsulating headers as specified in
Section 4.4.5.
4.4.5. Outer Encapsulation
Following SEAL encapsulation, the ITE next encapsulates each segment
in the requisite outer transport (when necessary) and IP layer
headers. When a transport layer header is included, the ITE writes
the port number for SEAL in the transport destination service port
field and writes the protocol number of the transport protocol in the
outer IP header protocol field. Otherwise, the ITE writes the
protocol number for SEAL in the outer IP header protocol field.
The ITE then sets the other fields of the outer transport and IP
layer headers as specified in Sections 5.5.4 and 5.5.5
of[I-D.templin-intarea-vet]. If this process is entered via re-
encapsulation (see: Section 4.5.4), the ITE instead follows the re-
encapsulation procedures specified in Section 5.5.6 of
[I-D.templin-intarea-vet].
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For IPv4 ETE link paths, the ITE sets DF=0 in the IPv4 header to
allow the packet to be fragmented if it encounters a restricting
link. (For IPv6 link paths, the DF bit is implicitly set to 1.)
The ITE then sends each outer packet via the underlying link
corresponding to LINK_ID. For IPv4 ETE link paths with
RATE_LIMIT=TRUE, the ITE sends the packet subject to rate limiting so
that the IPv4 Identification value is not repeated within the IPv4
Maximum Segment Lifetime (i.e., 120 seconds) [RFC1122].
4.4.6. Path Probing and ETE Reachability Verification
All SEAL data packets sent by the ITE are considered implicit probes.
SEAL data packets will elicit an SCMP message from the ETE if it
needs to acknowledge a probe and/or report an error condition. SEAL
data packets may also be dropped by either the ETE or a router on the
path, which will return an ICMP message.
The ITE can also send an SCMP Router/Neighbor Solicitation message to
elicit an SCMP Router/Neighbor Advertisement response (see:
[I-D.templin-intarea-vet]) as verification that the ETE is still
reachable via a specific link path.
The ITE processes ICMP messages as specified in Section 4.4.7.
The ITE processes SCMP messages as specified in Section 4.6.2.
4.4.7. Processing ICMP Messages
When the ITE sends SEAL packets, it may receive ICMP error messages
[RFC0792][RFC4443] from an ordinary router within the subnetwork.
Each ICMP message includes an outer IP header, followed by an ICMP
header, followed by a portion of the SEAL data packet that generated
the error (also known as the "packet-in-error") beginning with the
outer IP header.
The ITE should process ICMPv4 Protocol Unreachable messages and
ICMPv6 Parameter Problem messages with Code "Unrecognized Next Header
type encountered" as a hint that the ETE does not implement the SEAL
protocol. The ITE can also process other ICMP messages that do not
include sufficient information in the packet-in-error as a hint that
the ETE link path may be failing. Specific actions that the ITE may
take in these cases are out of scope.
For other ICMP messages, the ITE should use any outer header
information available as a first-pass authentication filter (e.g., to
determine if the source of the message is within the same
administrative domain as the ITE) and discards the message if first
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pass filtering fails.
Next, the ITE examines the packet-in-error beginning with the SEAL
header. If the value in the Identification field (if present) is not
within the window of packets the ITE has recently sent to this ETE,
or if the value in the SEAL header ICV field (if present) is
incorrect, the ITE discards the message.
Next, if the received ICMP message is a PTB the ITE sets the
temporary variable "PMTU" for this ETE link path to the MTU value in
the PTB message. If PMTU==0, the ITE consults a plateau table (e.g.,
as described in [RFC1191]) to determine PMTU based on the length
field in the outer IP header of the packet-in-error. For example, if
the ITE receives a PTB message with MTU==0 and length 4KB, it can set
PMTU=2KB. If the ITE subsequently receives a PTB message with MTU==0
and length 2KB, it can set PMTU=1792, etc. to a minimum value of
PMTU=(1500+HLEN). If the ITE is performing stateful MTU
determination for this ETE link path (see Section 4.4.9), the ITE
next sets PATH_MTU=MAX((PMTU-HLEN), 1500).
If the ICMP message was not discarded, the ITE then transcribes it
into a message to return to the previous hop. If the inner packet
was a SEAL data packet, the ITE transcribes the ICMP message into an
SCMP message. Otherwise, the ITE transcribes the ICMP message into a
message appropriate for the inner protocol version.
To transcribe the message, the ITE extracts the inner packet from
within the ICMP message packet-in-error field and uses it to generate
a new message corresponding to the type of the received ICMP message.
For SCMP messages, the ITE generates the message the same as
described for ETE generation of SCMP messages in Section 4.6.1. For
(S)PTB messages, the ITE writes (PMTU-HLEN) in the MTU field.
The ITE finally forwards the transcribed message to the previous hop
toward the inner source address.
4.4.8. IPv4 Middlebox Reassembly Testing
The ITE can perform a qualification exchange to ensure that the
subnetwork correctly delivers fragments to the ETE. This procedure
can be used, e.g., to determine whether there are middleboxes on the
path that violate the [RFC1812], Section 5.2.6 requirement that: "A
router MUST NOT reassemble any datagram before forwarding it".
The ITE should use knowledge of its topological arrangement as an aid
in determining when middlebox reassembly testing is necessary. For
example, if the ITE is aware that the ETE is located somewhere in the
public Internet, middlebox reassembly testing should not be
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necessary. If the ITE is aware that the ETE is located behind a NAT
or a firewall, however, then middlebox reassembly testing is
recommended.
The ITE can perform a middlebox reassembly test by selecting a data
packet to be used as a probe. While performing the test with real
data packets, the ITE should select only inner packets that are no
larger than (1500-HLEN) bytes for testing purposes. The ITE can also
construct a dummy probe packet instead of using ordinary SEAL data
packets.
To generate a dummy probe packet, the ITE creates a packet buffer
beginning with the same outer headers, SEAL header and inner network
layer header that would appear in an ordinary data packet, then pads
the packet with random data to a length that is at least 128 bytes
but no longer than (1500-HLEN) bytes. The ITE then writes the value
'0' in the inner network layer TTL (for IPv4) or Hop Limit (for IPv6)
field.
The ITE then sets (C=0; R=0) in the SEAL header of the probe packet
and sets the NEXTHDR field to the inner network layer protocol type.
(The ITE may also set A=1 if it requires a positive acknowledgement;
otherwise, it sets A=0.) Next, the ITE sets LINK_ID and LEVEL to the
appropriate values for this ETE link path, sets Identification and
I=1 (when USE_ID is TRUE), then finally calculates the ICV and sets
V=1(when USE_ICV is TRUE).
The ITE then encapsulates the probe packet in the appropriate outer
headers, splits it into two outer IPv4 fragments, then sends both
fragments over the same ETE link path.
The ITE should send a series of probe packets (e.g., 3-5 probes with
1sec intervals between tests) instead of a single isolated probe in
case of packet loss. If the ETE returns an SCMP PTB message with MTU
!= 0, then the ETE link path correctly supports fragmentation;
otherwise, the ITE enables stateful MTU determination for this ETE
link path as specified in Section 4.4.9.
(Examples of middleboxes that may perform reassembly include stateful
NATs and firewalls. Such devices could still allow for stateless MTU
determination if they gather the fragments of a fragmented IPv4 SEAL
data packet for packet analysis purposes but then forward the
fragments on to the final destination rather than forwarding the
reassembled packet.)
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4.4.9. Stateful MTU Determination
SEAL supports a stateless MTU determination capability, however the
ITE may in some instances wish to impose a stateful MTU limit on a
particular ETE link path. For example, when the ETE is situated
behind a middlebox that performs IPv4 reassembly (see: Section 4.4.8)
it is imperative that fragmentation be avoided. In other instances
(e.g., when the ETE link path includes performance-constrained
links), the ITE may deem it necessary to cache a conservative static
MTU in order to avoid sending large packets that would only be
dropped due to an MTU restriction somewhere on the path.
To determine a static MTU value, the ITE can send a series of dummy
probe packets of various sizes to the ETE with A=1 in the SEAL header
and DF=1 in the outer IP header. The ITE can then cache the size 'S'
of the largest packet for which it receives a probe reply from the
ETE by setting PATH_MTU=MAX((S-HLEN), 1500) for this ETE link path.
For example, the ITE could send probe packets of 4KB, followed by
2KB, followed by 1792 bytes, etc. While probing, the ITE processes
any ICMP PTB message it receives as a potential indication of probe
failure then discards the message.
4.4.10. Detecting Path MTU Changes
When stateful MTU determination is used, the ITE can periodically
reset PATH_MTU and/or re-probe the path to determine whether PATH_MTU
has increased. If the path still has a too-small MTU, the ITE will
receive a PTB message that reports a smaller size.
For IPv4 ETE link paths, when the path correctly implements
fragmentation and RATE_LIMIT is TRUE, the ITE can periodically reset
RATE_LIMIT=FALSE to determine whether the path still requires rate
limiting. If the ITE receives an SPTB message it should again set
RATE_LIMIT=TRUE.
4.5. ETE Specification
4.5.1. Minimum Reassembly Buffer Requirements
For IPv6, the ETE must configure a minimum reassembly buffer size of
1500 bytes for the reassembly of outer IPv6 packets (see: [RFC2460].
For IPv4, the ETE must also configure a minimum reassembly buffer
size of 1500 bytes for the reassembly of outer IPv4 packets, i.e.,
even though the true minimum reassembly size for IPv4 is only 576
bytes [RFC1122].
In addition to this outer reassembly buffer requirement, the ETE must
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further configure a minimum SEAL reassembly buffer size of (1500 +
HLEN) bytes for the reassembly of segmented SEAL packets (see:
Section 4.5.4).
4.5.2. Tunnel Neighbor Soft State
When data origin authentication and integrity checking is required,
the ETE maintains a per-ITE ICV calculation algorithm and a symmetric
secret key to verify the ICV. When per-packet identification is
required, the ETE also maintains a window of Identification values
for the packets it has recently received from this ITE.
When the tunnel neighbor relationship is bidirectional, the ETE
further maintains a per ETE link path mapping of outer IP and
transport layer addresses to the LINK_ID that appears in packets
received from the ITE.
4.5.3. IP-Layer Reassembly
The ETE should maintain conservative reassembly cache high- and low-
water marks. When the size of the reassembly cache exceeds this
high-water mark, the ETE should actively discard stale incomplete
reassemblies (e.g., using an Active Queue Management (AQM) strategy)
until the size falls below the low-water mark. The ETE should also
actively discard any pending reassemblies that clearly have no
opportunity for completion, e.g., when a considerable number of new
fragments have arrived before a fragment that completes a pending
reassembly arrives.
The ETE processes non-SEAL IP packets as specified in the normative
references, i.e., it performs any necessary IP reassembly then
discards the packet if it is larger than the reassembly buffer size
or delivers the (fully-reassembled) packet to the appropriate upper
layer protocol module.
For SEAL packets, the ITE performs any necessary IP reassembly then
submits the packet for SEAL decapsulation as specified in Section
4.5.4. (Note that if the packet is larger than the reassembly buffer
size, the ITE still returns the leading portion of the (partially)
reassembled packet.)
4.5.4. Decapsulation and Re-Encapsulation
For each SEAL packet accepted for decapsulation, when I==1 the ETE
first examines the Identification field. If the Identification is
not within the window of acceptable values for this ITE, the ETE
silently discards the packet.
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Next, if V==1 the ETE verifies the ICV value (with the ICV field
itself reset to 0) and silently discards the packet if the value is
incorrect.
Next, if the packet arrived as multiple IPv4 fragments and L ==0, the
ETE sends an SPTB message back to the ITE with MTU set to the size of
the largest fragment received minus HLEN (see: Section 4.6.1.1).
Next, if the packet arrived as multiple IP fragments and the inner
packet is larger than 1500 bytes, the ETE silently discards the
packet; otherwise, it continues to process the packet.
Next, if there is an incorrect value in a SEAL header field (e.g., an
incorrect "VER" field value), the ETE discards the packet. If the
SEAL header has C==0, the ETE also returns an SCMP "Parameter
Problem" (SPP) message (see Section 4.6.1.2).
Next, if the SEAL header has C==1, the ETE processes the packet as an
SCMP packet as specified in Section 4.6.2. Otherwise, the ETE
continues to process the packet as a SEAL data packet.
Next, if the SEAL header has (M==1 || Offset!==0) the ETE checks to
see if the other segments of this already-segmented SEAL packet have
arrived, i.e., by looking for additional segments that have the same
outer IP source address, destination address, source transport port
number (if present) and SEAL Identification value. If the other
segments have already arrived, the ETE discards the SEAL header and
other outer headers from the non-initial segments and appends them
onto the end of the first segment. Otherwise, the ETE caches the
segment for at most 60 seconds while awaiting the arrival of its
partners.
Next, if the SEAL header in the (reassembled) packet has A==1, the
ETE sends an SPTB message back to the ITE with MTU=0 (see: Section
4.6.1.1).
Finally, the ETE discards the outer headers and processes the inner
packet according to the header type indicated in the SEAL NEXTHDR
field. If the inner (TTL / Hop Limit) field encodes the value 0, the
ETE silently discards the packet. Otherwise, if the next hop toward
the inner destination address is via a different interface than the
SEAL packet arrived on, the ETE discards the SEAL header and delivers
the inner packet either to the local host or to the next hop
interface if the packet is not destined to the local host.
If the next hop is on the same interface the SEAL packet arrived on,
however, the ETE submits the packet for SEAL re-encapsulation
beginning with the specification in Section 4.4.3 above and without
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decrementing the value in the inner (TTL / Hop Limit) field. In this
process, the packet remains within the tunnel (i.e., it does not exit
and then re-enter the tunnel); hence, the packet is not discarded if
the LEVEL field in the SEAL header contains the value 0.
4.6. The SEAL Control Message Protocol (SCMP)
SEAL provides a companion SEAL Control Message Protocol (SCMP) that
uses the same message types and formats as for the Internet Control
Message Protocol for IPv6 (ICMPv6) [RFC4443]. As for ICMPv6, each
SCMP message includes a 32-bit header and a variable-length body.
The ITE encapsulates the SCMP message in a SEAL header and outer
headers as shown in Figure 3:
+--------------------+
~ outer IP header ~
+--------------------+
~ other outer hdrs ~
+--------------------+
~ SEAL Header ~
+--------------------+ +--------------------+
| SCMP message header| --> | SCMP message header|
+--------------------+ +--------------------+
| | --> | |
~ SCMP message body ~ --> ~ SCMP message body ~
| | --> | |
+--------------------+ +--------------------+
SCMP Message SCMP Packet
before encapsulation after encapsulation
Figure 3: SCMP Message Encapsulation
The following sections specify the generation, processing and
relaying of SCMP messages.
4.6.1. Generating SCMP Error Messages
ETEs generate SCMP error messages in response to receiving certain
SEAL data packets using the format shown in Figure 4:
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type-Specific Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| As much of the invoking SEAL data packet as possible |
~ (beginning with the SEAL header) without the SCMP ~
| packet exceeding MINMTU bytes (*) |
(*) also known as the "packet-in-error"
Figure 4: SCMP Error Message Format
The error message includes the 32-bit SCMP message header, followed
by a 32-bit Type-Specific Data field, followed by the leading portion
of the invoking SEAL data packet beginning with the SEAL header as
the "packet-in-error". The packet-in-error includes as much of the
invoking packet as possible extending to a length that would not
cause the entire SCMP packet following outer encapsulation to exceed
MINMTU bytes.
When the ETE processes a SEAL data packet for which the
Identification and ICV values are correct but an error must be
returned, it prepares an SCMP error message as shown in Figure 4.
The ETE sets the Type and Code fields to the same values that would
appear in the corresponding ICMPv6 message [RFC4443], but calculates
the Checksum beginning with the SCMP message header using the
algorithm specified for ICMPv4 in [RFC0792].
The ETE next encapsulates the SCMP message in the requisite SEAL and
outer headers as shown in Figure 3. During encapsulation, the ETE
sets the outer destination address/port numbers of the SCMP packet to
the values associated with the ITE and sets the outer source address/
port numbers to its own outer address/port numbers.
The ETE then sets (C=1; A=0; R=0; L=0; X=0; M=0; Offset=0) in the
SEAL header, then sets I, V, NEXTHDR and LEVEL to the same values
that appeared in the SEAL header of the data packet. If the neighbor
relationship between the ITE and ETE is unidirectional, the ETE next
sets the LINK_ID field to the same value that appeared in the SEAL
header of the data packet. Otherwise, the ETE sets the LINK_ID field
to the value it would use in sending a SEAL packet to this ITE.
When I==1, the ETE next sets the Identification field to an
appropriate value for the ITE. If the neighbor relationship between
the ITE and ETE is unidirectional, the ETE sets the Identification
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field to the same value that appeared in the SEAL header of the data
packet. Otherwise, the ETE sets the Identification field to the
value it would use in sending the next SEAL packet to this ITE.
When V==1, the ETE then calculates and sets the ICV field the same as
specified for SEAL data packet encapsulation in Section 4.4.4.
Finally, the ETE sends the resulting SCMP packet to the ITE the same
as specified for SEAL data packets in Section 4.4.5.
The following sections describe additional considerations for various
SCMP error messages:
4.6.1.1. Generating SCMP Packet Too Big (SPTB) Messages
An ETE generates an SCMP "Packet Too Big" (SPTB) message when it
receives a SEAL data packet that arrived as multiple outer IPv4
fragments and for which L==0. The ETE prepares the SPTB message the
same as for the corresponding ICMPv6 PTB message, and writes the
length of the largest outer IP fragment received minus HLEN in the
MTU field of the message.
The ETE also generates an SPTB message when it accepts a SEAL
protocol data packet with A==1 in the SEAL header. The ETE prepares
the SPTB message the same as above, except that it writes the value 0
in the MTU field.
4.6.1.2. Generating Other SCMP Error Messages
An ETE generates an SCMP "Destination Unreachable" (SDU) message
under the same circumstances that an IPv6 system would generate an
ICMPv6 Destination Unreachable message.
An ETE generates an SCMP "Parameter Problem" (SPP) message when it
receives a SEAL packet with an incorrect value in the SEAL header.
TEs generate other SCMP message types using methods and procedures
specified in other documents. For example, SCMP message types used
for tunnel neighbor coordinations are specified in VET
[I-D.templin-intarea-vet].
4.6.2. Processing SCMP Error Messages
An ITE may receive SCMP messages with C==1 in the SEAL header after
sending packets to an ETE. The ITE first verifies that the outer
addresses of the SCMP packet are correct, and (when I==1) that the
Identification field contains an acceptable value. The ITE next
verifies that the SEAL header fields are set correctly as specified
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in Section 4.6.1. When V==1, the ITE then verifies the ICV value.
The ITE next verifies the Checksum value in the SCMP message header.
If any of these values are incorrect, the ITE silently discards the
message; otherwise, it processes the message as follows:
4.6.2.1. Processing SCMP PTB Messages
After an ITE sends a SEAL data packet to an ETE, it may receive an
SPTB message with a packet-in-error containing the leading portion of
the packet (see: Section 4.6.1.1). For IP SPTB messages with MTU==0,
the ITE processes the message as confirmation that the ETE received a
SEAL data packet with A==1 in the SEAL header. The ITE then discards
the message.
For SPTB messages with MTU != 0, the ITE processes the message as an
indication of a packet size limitation as follows. If the inner
packet is itself a SEAL packet, and the inner packet length is less
than 1500, the ITE reduces its MINMTU value for this ITE. If the
inner packet is a non-SEAL IPv4 packet and the inner packet length is
less than 1500, the ITE instead sets RATE_LIMIT=1. For all other
cases, if the inner packet length is larger than 1500 and the MTU
value is not substantially less than 1500 bytes, the value is likely
to reflect the true MTU of the restricting link on the path to the
ETE; otherwise, a router on the path may be generating runt
fragments.
In that case, the ITE can consult a plateau table (e.g., as described
in [RFC1191]) to rewrite the MTU value to a reduced size. For
example, if the ITE receives an IPv4 SPTB message with MTU==256 and
inner packet length 4KB, it can rewrite the MTU to 2KB. If the ITE
subsequently receives an IPv4 SPTB message with MTU==256 and inner
packet length 2KB, it can rewrite the MTU to 1792, etc., to a minimum
of 1500 bytes. If the ITE is performing stateful MTU determination
for this ETE link path, it then writes the new MTU value minus HLEN
in PATH_MTU.
The ITE then checks its forwarding tables to discover the previous
hop toward the source address of the inner packet. If the previous
hop is reached via the same tunnel interface the SPTB message arrived
on, the ITE relays the message to the previous hop. In order to
relay the message, the first writes zero in the Identification and
ICV fields of the SEAL header within the packet-in-error. The ITE
next rewrites the outer SEAL header fields with values corresponding
to the previous hop and recalculates the ICV using the ICV
calculation parameters associated with the previous hop. Next, the
ITE replaces the SPTB's outer headers with headers of the appropriate
protocol version and fills in the header fields as specified in
Sections 5.5.4-5.5.6 of [I-D.templin-intarea-vet], where the
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destination address/port correspond to the previous hop and the
source address/port correspond to the ITE. The ITE then sends the
message to the previous hop the same as if it were issuing a new SPTB
message. (Note that, in this process, the values within the SEAL
header of the packet-in-error are meaningless to the previous hop and
therefore cannot be used by the previous hop for authentication
purposes.)
If the previous hop is not reached via the same tunnel interface, the
ITE instead transcribes the message into a format appropriate for the
inner packet (i.e., the same as described for transcribing ICMP
messages in Section 4.4.7) and sends the resulting transcribed
message to the original source. (NB: if the inner packet within the
SPTB message is an IPv4 SEAL packet with DF==0, the ITE should set
DF=1 and re-calculate the IPv4 header checksum while transcribing the
message in order to avoid bogon filters.) The ITE then discards the
SPTB message.
Note that the ITE may receive an SPTB message from another ITE that
is at the head end of a nested level of encapsulation. The ITE has
no security associations with this nested ITE, hence it should
consider this SPTB message the same as if it had received an ICMP PTB
message from an ordinary router on the path to the ETE. That is, the
ITE should examine the packet-in-error field of the SPTB message and
only process the message if it is able to recognize the packet as one
it had previously sent.
4.6.2.2. Processing Other SCMP Error Messages
An ITE may receive an SDU message with an appropriate code under the
same circumstances that an IPv6 node would receive an ICMPv6
Destination Unreachable message. The ITE either transcribes or
relays the message toward the source address of the inner packet
within the packet-in-error the same as specified for SPTB messages in
Section 4.6.2.1.
An ITE may receive an SPP message when the ETE receives a SEAL packet
with an incorrect value in the SEAL header. The ITE should examine
the SEAL header within the packet-in-error to determine whether a
different setting should be used in subsequent packets, but does not
relay the message further.
TEs process other SCMP message types using methods and procedures
specified in other documents. For example, SCMP message types used
for tunnel neighbor coordinations are specified in VET
[I-D.templin-intarea-vet].
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5. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
6. End System Requirements
End systems are encouraged to implement end-to-end MTU assurance
(e.g., using Packetization Layer PMTUD per [RFC4821]) even if the
subnetwork is using SEAL.
7. Router Requirements
Routers within the subnetwork are expected to observe the router
requirements found in the normative references, including the
implementation of IP fragmentation and reassembly [RFC1812][RFC2460]
as well as the generation of ICMP messages [RFC0792][RFC4443].
8. Nested Encapsulation Considerations
SEAL supports nested tunneling for up to 8 layers of encapsulation.
In this model, the SEAL ITE has a tunnel neighbor relationship only
with ETEs at its own nesting level, i.e., it does not have a tunnel
neighbor relationship with other ITEs, nor with ETEs at other nesting
levels.
Therefore, when an ITE 'A' within an inner nesting level needs to
return an error message to an ITE 'B' within an outer nesting level,
it generates an ordinary ICMP error message the same as if it were an
ordinary router within the subnetwork. 'B' can then perform message
validation as specified in Section 4.4.7, but full message origin
authentication is not possible.
Since ordinary ICMP messages are used for coordinations between ITEs
at different nesting levels, nested SEAL encapsulations should only
be used when the ITEs are within a common administrative domain
and/or when there is no ICMP filtering middlebox such as a firewall
or NAT between them. An example would be a recursive nesting of
mobile networks, where the first network receives service from an
ISP, the second network receives service from the first network, the
third network receives service from the second network, etc.
NB: As an alternative, the SCMP protocol could be extended to allow
ITE 'A' to return an SCMP message to ITE 'B' rather than return an
ICMP message. This would conceptually allow the control messages to
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pass through firewalls and NATs, however it would give no more
message origin authentication assurance than for ordinary ICMP
messages. It was therefore determined that the complexity of
extending the SCMP protocol was of little value within the context of
the anticipated use cases for nested encapsulations.
9. IANA Considerations
The IANA is instructed to allocate a User Port number for "SEAL" in
the 'port-numbers' registry for the TCP and UDP protocols.
The IANA is further instructed to allocate an IP protocol number for
"SEAL" in the "protocol-numbers" registry.
Considerations for port and protocol number assignments appear in
[RFC2780][RFC5226][RFC6335].
10. Security Considerations
SEAL provides a segment-by-segment data origin authentication and
anti-replay service across the (potentially) multiple segments of a
re-encapsulating tunnel. It further provides a segment-by-segment
integrity check of the headers of encapsulated packets, but does not
verify the integrity of the rest of the packet beyond the headers
unless fragmentation is unavoidable. SEAL therefore considers full
message integrity checking, authentication and confidentiality as
end-to-end considerations in a manner that is compatible with
securing mechanisms such as TLS/SSL [RFC5246].
An amplification/reflection/buffer overflow attack is possible when
an attacker sends IP fragments with spoofed source addresses to an
ETE in an attempt to clog the ETE's reassembly buffer and/or cause
the ETE to generate a stream of SCMP messages returned to a victim
ITE. The SCMP message ICV, Identification, as well as the inner
headers of the packet-in-error, provide mitigation for the ETE to
detect and discard SEAL segments with spoofed source addresses.
The SEAL header is sent in-the-clear the same as for the outer IP and
other outer headers. In this respect, the threat model is no
different than for IPv6 extension headers. Unlike IPv6 extension
headers, however, the SEAL header can be protected by an integrity
check that also covers the inner packet headers.
Security issues that apply to tunneling in general are discussed in
[RFC6169].
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11. 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.
Section 3 of [RFC4459] describes inner and outer fragmentation at the
tunnel endpoints as alternatives for accommodating the tunnel MTU.
Section 4 of [RFC2460] specifies a method for inserting and
processing extension headers between the base IPv6 header and
transport layer protocol data. The SEAL header is inserted and
processed in exactly the same manner.
IPsec/AH is [RFC4301][RFC4301] is used for full message integrity
verification between tunnel endpoints, whereas SEAL only ensures
integrity for the inner packet headers. The AYIYA proposal
[I-D.massar-v6ops-ayiya] uses similar means for providing message
authentication and integrity.
The concepts of path MTU determination through the report of
fragmentation and extending the IPv4 Identification field were first
proposed in deliberations of the TCP-IP mailing list and the Path MTU
Discovery Working Group (MTUDWG) during the late 1980's and early
1990's. An historical analysis of the evolution of these concepts,
as well as the development of the eventual PMTUD mechanism, appears
in Appendix D of this document.
12. Implementation Status
An early implementation of the first revision of SEAL [RFC5320] is
available at: http://isatap.com/seal/pre-rfc5320.txt
13. 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.
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Discussions with colleagues following the publication of [RFC5320]
have provided useful insights that have resulted in significant
improvements to this, the Second Edition of SEAL.
Path MTU determination through the report of fragmentation was first
proposed by Charles Lynn on the TCP-IP mailing list in 1987.
Extending the IP identification field was first proposed by Steve
Deering on the MTUDWG mailing list in 1989.
14. References
14.1. 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.
[RFC1122] Braden, R., "Requirements for Internet Hosts -
Communication Layers", STD 3, RFC 1122, October 1989.
[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.
14.2. 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.
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[I-D.ietf-intarea-ipv4-id-update]
Touch, J., "Updated Specification of the IPv4 ID Field",
draft-ietf-intarea-ipv4-id-update-05 (work in progress),
May 2012.
[I-D.ietf-savi-framework]
Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt,
"Source Address Validation Improvement Framework",
draft-ietf-savi-framework-06 (work in progress),
January 2012.
[I-D.massar-v6ops-ayiya]
Massar, J., "AYIYA: Anything In Anything",
draft-massar-v6ops-ayiya-02 (work in progress), July 2004.
[I-D.templin-aero]
Templin, F., "Asymmetric Extended Route Optimization
(AERO)", draft-templin-aero-08 (work in progress),
February 2012.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-33 (work in progress),
December 2011.
[I-D.templin-ironbis]
Templin, F., "The Internet Routing Overlay Network
(IRON)", draft-templin-ironbis-10 (work in progress),
December 2011.
[MTUDWG] "IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November
1989 - February 1995.".
[RFC0994] International Organization for Standardization (ISO) and
American National Standards Institute (ANSI), "Final text
of DIS 8473, Protocol for Providing the Connectionless-
mode Network Service", RFC 994, March 1986.
[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.
[RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum
options", RFC 1146, March 1990.
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[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701, October 1994.
[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.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
Values In the Internet Protocol and Related Headers",
BCP 37, RFC 2780, March 2000.
[RFC2827] Ferguson, P. and D. Senie, "Network Ingress Filtering:
Defeating Denial of Service Attacks which employ IP Source
Address Spoofing", BCP 38, RFC 2827, May 2000.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3232] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced by
an On-line Database", RFC 3232, January 2002.
[RFC3366] Fairhurst, G. and L. Wood, "Advice to link designers on
link Automatic Repeat reQuest (ARQ)", BCP 62, RFC 3366,
August 2002.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
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[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[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.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5320] Templin, F., "The Subnetwork Encapsulation and Adaptation
Layer (SEAL)", RFC 5320, February 2010.
[RFC5445] Watson, M., "Basic Forward Error Correction (FEC)
Schemes", RFC 5445, March 2009.
[RFC5720] Templin, F., "Routing and Addressing in Networks with
Global Enterprise Recursion (RANGER)", RFC 5720,
February 2010.
[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and
Addressing in Networks with Global Enterprise Recursion
(RANGER) Scenarios", RFC 6139, February 2011.
[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
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Concerns with IP Tunneling", RFC 6169, April 2011.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, August 2011.
[SIGCOMM] Luckie, M. and B. Stasiewicz, "Measuring Path MTU
Discovery Behavior", November 2010.
[TBIT] Medina, A., Allman, M., and S. Floyd, "Measuring
Interactions Between Transport Protocols and Middleboxes",
October 2004.
[TCP-IP] "Archive/Hypermail of Early TCP-IP Mail List,
http://www-mice.cs.ucl.ac.uk/multimedia/misc/tcp_ip/, May
1987 - May 1990.".
[WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and
Debugging Path MTU Discovery Failures", October 2005.
Appendix A. Reliability
Although a SEAL tunnel may span an arbitrarily-large subnetwork
expanse, the IP layer sees the tunnel as a simple link that supports
the IP service model. Links with high bit error rates (BERs) (e.g.,
IEEE 802.11) use Automatic Repeat-ReQuest (ARQ) mechanisms [RFC3366]
to increase packet delivery ratios, while links with much lower BERs
typically omit such mechanisms. Since SEAL tunnels may traverse
arbitrarily-long paths over links of various types that are already
either performing or omitting ARQ as appropriate, it would therefore
be inefficient to require the tunnel endpoints to also perform ARQ.
Appendix B. Integrity
The SEAL header includes an integrity check field that covers the
SEAL header and at least the inner packet headers. This provides for
header integrity verification on a segment-by-segment basis for a
segmented re-encapsulating tunnel path.
Fragmentation and reassembly schemes must also consider packet-
splicing errors, e.g., when two fragments from the same packet are
concatenated incorrectly, when a fragment from packet X is
reassembled with fragments from packet Y, etc. The primary sources
of such errors include implementation bugs and wrapping IPv4 ID
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fields.
In particular, the IPv4 16-bit ID field can wrap with only 64K
packets with the same (src, dst, protocol)-tuple alive in the system
at a given time [RFC4963]. When the IPv4 ID field is re-written by a
middlebox such as a NAT or Firewall, ID field wrapping can occur with
even fewer packets alive in the system.
When outer IPv4 fragmentation is unavoidable, SEAL institutes rate
limiting so that the number of packets admitted into the tunnel by
the ITE does not exceed the number of unique packets that may be
alive within the Internet.
Appendix C. Transport Mode
SEAL can also be used in "transport-mode", e.g., when the inner layer
comprises upper-layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL (e.g.,
by inserting an unspecified 'SEAL_OPTION' TCP option during
connection establishment) for the carriage of protocol data
encapsulated as IP/SEAL/TCP. In this sense, the "subnetwork" becomes
the entire end-to-end path between the TCP peers and may potentially
span the entire Internet.
If both TCPs agree on the use of SEAL, their protocol messages will
be carried as IP/SEAL/TCP and the connection will be serviced by the
SEAL protocol using TCP (instead of an encapsulating tunnel endpoint)
as the transport layer protocol. The SEAL protocol for transport
mode otherwise observes the same specifications as for Section 4.
Appendix D. Historic Evolution of PMTUD
The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
and numerous proposals in the late 1980's through early 1990. The
initial problem was posed by Art Berggreen on May 22, 1987 in a
message to the TCP-IP discussion group [TCP-IP]. The discussion that
followed provided significant reference material for [FRAG]. An IETF
Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
with charter to produce an RFC. Several variations on a very few
basic proposals were entertained, including:
1. Routers record the PMTUD estimate in ICMP-like path probe
messages (proposed in [FRAG] and later [RFC1063])
2. The destination reports any fragmentation that occurs for packets
received with the "RF" (Report Fragmentation) bit set (Steve
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Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal)
3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 (straw
RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)
4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
1990)
5. Fragmentation avoidance by setting "IP_DF" flag on all packets
and retransmitting if ICMPv4 "fragmentation needed" messages
occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191]
by Mogul and Deering).
Option 1) seemed attractive to the group at the time, since it was
believed that routers would migrate more quickly than hosts. Option
2) was a strong contender, but repeated attempts to secure an "RF"
bit in the IPv4 header from the IESG failed and the proponents became
discouraged. 3) was abandoned because it was perceived as too
complicated, and 4) never received any apparent serious
consideration. Proposal 5) was a late entry into the discussion from
Steve Deering on Feb. 24th, 1990. The discussion group soon
thereafter seemingly lost track of all other proposals and adopted
5), which eventually evolved into [RFC1191] and later [RFC1981].
In retrospect, the "RF" bit postulated in 2) is not needed if a
"contract" is first established between the peers, as in proposal 4)
and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
Feb 19. 1990. These proposals saw little discussion or rebuttal, and
were dismissed based on the following the assertions:
o routers upgrade their software faster than hosts
o PCs could not reassemble fragmented packets
o Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets
o Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)
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.
Templin Expires January 10, 2013 [Page 39]
Internet-Draft SEAL July 2012
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
Templin Expires January 10, 2013 [Page 40]