One document matched: draft-templin-intarea-seal-61.txt
Differences from draft-templin-intarea-seal-60.txt
Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Obsoletes: rfc5320 (if approved) August 2, 2013
Intended status: Informational
Expires: February 3, 2014
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-intarea-seal-61.txt
Abstract
This document specifies a Subnetwork Encapsulation and Adaptation
Layer (SEAL). SEAL operates over virtual topologies configured over
connected IP network routing regions bounded by encapsulating border
nodes. These virtual topologies are manifested by tunnels that may
span multiple IP and/or sub-IP layer forwarding hops, where they may
incur packet duplication, packet reordering, source address spoofing
and traversal of links with diverse Maximum Transmission Units
(MTUs). SEAL addresses these issues through the encapsulation and
messaging mechanisms specified in this document.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on February 3, 2014.
Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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carefully, as they describe your rights and restrictions with respect
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.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Approach . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Differences with RFC5320 . . . . . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
4. Applicability Statement . . . . . . . . . . . . . . . . . . . 10
5. SEAL Tunnel Mode Specification . . . . . . . . . . . . . . . . 11
5.1. SEAL Tunnel Model . . . . . . . . . . . . . . . . . . . . 11
5.2. SEAL Model of Operation . . . . . . . . . . . . . . . . . 12
5.3. SEAL Header and Trailer Format . . . . . . . . . . . . . . 13
5.4. ITE Specification . . . . . . . . . . . . . . . . . . . . 15
5.4.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 15
5.4.2. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 16
5.4.3. SEAL Layer Pre-Processing . . . . . . . . . . . . . . 18
5.4.4. SEAL Encapsulation and Segmentation . . . . . . . . . 19
5.4.5. Outer Encapsulation . . . . . . . . . . . . . . . . . 20
5.4.6. Path Probing and ETE Reachability Verification . . . . 21
5.4.7. Processing ICMP Messages . . . . . . . . . . . . . . . 22
5.4.8. IPv4 Middlebox Reassembly Testing . . . . . . . . . . 23
5.4.9. Stateful MTU Determination . . . . . . . . . . . . . . 24
5.4.10. Detecting Path MTU Changes . . . . . . . . . . . . . . 25
5.5. ETE Specification . . . . . . . . . . . . . . . . . . . . 25
5.5.1. Reassembly Buffer Requirements . . . . . . . . . . . . 25
5.5.2. Tunnel Neighbor Soft State . . . . . . . . . . . . . . 25
5.5.3. IP-Layer Reassembly . . . . . . . . . . . . . . . . . 26
5.5.4. Decapsulation, SEAL-Layer Reassembly, and
Re-Encapsulation . . . . . . . . . . . . . . . . . . . 26
5.6. The SEAL Control Message Protocol (SCMP) . . . . . . . . . 28
5.6.1. Generating SCMP Error Messages . . . . . . . . . . . . 29
5.6.2. Processing SCMP Error Messages . . . . . . . . . . . . 30
6. SEAL Transport Mode Specification . . . . . . . . . . . . . . 32
7. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 33
8. End System Requirements . . . . . . . . . . . . . . . . . . . 33
9. Router Requirements . . . . . . . . . . . . . . . . . . . . . 33
10. Nested Encapsulation Considerations . . . . . . . . . . . . . 34
11. Reliability Considerations . . . . . . . . . . . . . . . . . . 34
12. Integrity Considerations . . . . . . . . . . . . . . . . . . . 34
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35
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14. Security Considerations . . . . . . . . . . . . . . . . . . . 35
15. Related Work . . . . . . . . . . . . . . . . . . . . . . . . . 36
16. Implementation Status . . . . . . . . . . . . . . . . . . . . 36
17. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 36
18. References . . . . . . . . . . . . . . . . . . . . . . . . . . 37
18.1. Normative References . . . . . . . . . . . . . . . . . . . 37
18.2. Informative References . . . . . . . . . . . . . . . . . . 38
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (manifested by
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 network layer hop, 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 (e.g.,
see [RFC2003][RFC2473]). Tunnels serve a wide variety of purposes,
including mobility, security, routing control, traffic engineering,
multihoming, etc., and will remain an integral part of the
architecture moving forward. However, the encapsulation headers
often include insufficiently provisioned per-packet identification
values. 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
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is rapidly becoming depleted, there is also a growing awareness that
other IP protocol limitations have already or may soon 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. Additionally, recent studies
have shown that the arrival of fragments at high data rates can cause
denial-of-service (DoS) attacks on performance-sensitive networking
gear, prompting some administrators to configure their equipment to
drop fragments unconditionally [I-D.taylor-v6ops-fragdrop].
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 network at the same time [RFC6864]. (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 (MTUDWG) during the late 1980's
through early 1990's which resulted in the publication of [RFC1191].
In this negative feedback-based 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
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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
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][RIPE].
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 positive
feedback-based 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
This document concerns subnetworks manifested through 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
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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
(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 specified in Section 6.
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 message 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 (including limited segmentation and reassembly) 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.
1.3. Differences with RFC5320
This specification of SEAL is descended from an experimental
independent RFC publication of the same name [RFC5320]. However,
this specification introduces a number of important differences from
the earlier publication.
First, this specification includes a protocol version field in the
SEAL header whereas [RFC5320] does not, and therefore cannot be
updated by future revisions. This specification therefore obsoletes
(i.e., and does not update) [RFC5320].
Secondly, [RFC5320] forms a 32-bit Identification value by
concatenating the 16-bit IPv4 Identification field with a 16-bit
Identification "extension" field in the SEAL header. This means that
[RFC5320] can only operate over IPv4 networks (since IPv6 headers do
not include a 16-bit version number) and that the SEAL Identification
value can be corrupted if the Identification in the outer IPv4 header
is rewritten. In contrast, this specification includes a 32-bit
Identification value that is independent of any identification fields
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found in the inner or outer IP headers, and is therefore compatible
with any inner and outer IP protocol version combinations.
Additionally, the SEAL segmentation and reassembly procedures defined
in [RFC5320] differ significantly from those found in this
specification. In particular, this specification defines an 8-bit
Offset field that allows for smaller segment sizes when SEAL
segmentation is necessary. In contrast, [RFC5320] includes a 3-bit
Segment field and performs reassembly through concatenation of
consecutive segments.
The SEAL header in this specification also includes an optional
Integrity Check Vector (ICV) that can be used to digitally sign the
SEAL header and the leading portion of the encapsulated inner packet.
This allows for a lightweight integrity check and a loose message
origin authentication capability. The header further includes new
control bits as well as a link identification and encapsulation level
field for additional control capabilities.
Finally, this version of SEAL includes a new messaging protocol known
as the SEAL Control Message Protocol (SCMP), whereas [RFC5320]
performs signalling through the use of SEAL-encapsulated ICMP
messages. The use of SCMP allows SEAL-specific departures from ICMP,
as well as a control messaging capability that extends to other
specifications, including Virtual Enterprise Traversal (VET)
[I-D.templin-intarea-vet].
2. Terminology
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.
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SEAL 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
IP addresses, each address represents a separate SEAL 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 further in-network fragmentation. The bit is absent from
the IPv6 header [RFC2460], but implicitly set to '1' because
fragmentation can occur only at IPv6 sources.
The following abbreviations correspond to terms used within this
document and/or elsewhere in common Internetworking nomenclature:
HLEN - the length of the SEAL header plus outer headers
ICV - Integrity Check Vector
MAC - Message Authentication Code
MTU - Maximum Transmission Unit
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SCMP - the SEAL Control Message Protocol
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
3. Requirements
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.
4. 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, SO/HO networks, the global public
Internet itself, and any other connected network routing region.
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. (However, note that TCP
encapsulation may not be appropriate for all use cases; particularly
those that require low delay and/or delay variance.) The SEAL header
is processed in a similar manner 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
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need to perform limited segmentation which the Egress Tunnel Endpoint
(ETE) reassembles. 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 message 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.
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, but goes beyond GRE to also provide additional capabilities
(e.,g., path MTU accommodation, message origin authentication, etc.)
as described in this document.
Finally, SEAL can be used in conjunction with existing tunnel types
such as IPsec and GRE. In that case, the SEAL header appears as an
outer layer of encapsulation while the IPsec, GRE, etc. headers are
seen as inner headers that immediately follow the SEAL header.
5. SEAL Tunnel Mode Specification
The following sections specify the operation of SEAL:
5.1. SEAL Tunnel Model
SEAL is an encapsulation sublayer used within point-to-point, point-
to-multipoint, and non-broadcast, multiple access (NBMA) tunnels.
Each SEAL path is configured over one or more underlying interfaces
attached to subnetwork links. The SEAL tunnel connects an ITE to one
or more ETE "neighbors" via encapsulation across an underlying
subnetwork, where the tunnel neighbor relationship may be
bidirectional, partially unidirectional or fully unidirectional.
A bidirectional tunnel neighbor relationship is one over which both
TEs can exchange both data and control messages. A partially
unidirectional tunnel neighbor relationship allows the near end ITE
to send data packets forward to the far end ETE, while the far end
only returns control messages when necessary. Finally, a fully
unidirectional mode of operation is one in which the near end ITE can
receive neither data nor control messages from the far end ETE.
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Implications of the SEAL bidirectional and unidirectional models are
the same as discussed in [I-D.templin-intarea-vet].
5.2. 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
and the outer IP and transport layer headers are together seen as the
outer encapsulation headers.
For SEAL encapsulations that involve other tunnel types (e.g., GRE,
IPsec, etc.) the ITE inserts the SEAL header as a leading extension
to the other tunnel headers, i.e., the SEAL encapsulation appears as
part of the same tunnel and not a separate tunnel. For example, for
IPsec/ESP the ITE inserts the SEAL header as IP/SEAL/ESP-header/
{inner packet}/ESP-trailer. In that case, SEAL considers the length
of the inner packet only (i.e., and not the other tunnel headers and
trailers) when performing its packet size calculations. Also, the
other tunnel headers appear in only the first segment of a segmented
SEAL packet (i.e., they do not appear in non-initial segments) and
the other tunnel trailers (if any) appear only in the final segment.
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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 and avoiding recursive
tunneling are discussed in Section 4 of [RFC2473] as well as in
Section 10 of this document.
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. Considerations for re-encapsulating tunneling are
discussed in[I-D.templin-ironbis]. Combinations of nested and re-
encapsulating tunneling are also naturally supported by SEAL.
The SEAL ITE considers each underlying interface as the ingress
attachment point to a SEAL path to the ETE. The ITE therefore may
experience different path MTUs on different SEAL paths.
Finally, the SEAL ITE ensures that the inner network layer protocol
will see a minimum MTU of 1500 bytes over each SEAL path regardless
of the outer network layer protocol version, i.e., even if a small
amount of segmentation and reassembly are necessary. This is to
avoid path MTU "black holes" for the minimum MTU configured by the
vast majority of links in the Internet. Note that in some scenarios,
however, reassembly may place a heavy burden on the ETE. In that
case, the ITE can avoid invoking segmentation and instead report an
MTU smaller than 1500 bytes to the original source.
5.3. 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NEXTHDR |VER|C|P|I|V|R|M| Offset | LINK_ID |LEVEL|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Vector (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 2: SEAL Header Format
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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.
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.
P (1)
the "Probe" bit. Set to 1 by the ITE in SEAL probe packets for
which it wishes to receive an explicit acknowledgement from the
ETE.
I (1)
the "Identification Included" bit.
V (1)
the "Integrity Check Vector included" bit.
R (1)
the "Redirects Permitted" bit when used by VET (see:
[I-D.templin-intarea-vet]); reserved for future use in other
contexts.
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 (8) an 8-bit Offset field. Set to 0 in the first segment of
a segmented SEAL packet. Set to an integer multiple of 256 byte
blocks in subsequent segments (e.g., an Offset of 4 indicates a
block that begins at the 1024th byte in the packet).
LINK_ID (5)
a 5-bit link identification value, set to a unique value by the
ITE for each SEAL path over which it will send encapsulated
packets to the ETE (up to 32 SEAL 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 SEAL path that must be assigned a separate
LINK_ID.
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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 randomly-initialized 32-bit value that is
monotonically-incremented for each SEAL packet transmitted to this
ETE.
Integrity Check Vector (ICV) (variable)
an optional variable-length integrity check vector field; present
when V==1.
5.4. ITE Specification
5.4.1. Tunnel Interface MTU
The tunnel interface must present a stable MTU value to the inner
network layer as the size for admission of inner packets into the
interface. Since NBMA tunnel virtual interfaces may support a large
set of SEAL 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 and provide accommodations to
ensure that packets up to that size are successfully conveyed to the
ETE.
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 cleared (i.e, DF==0),
if the packet is larger than the tunnel interface MTU the inner IPv4
layer uses IPv4 fragmentation to break the packet into fragments no
larger than the tunnel interface MTU. The ITE then admits each
fragment into the interface as an independent packet.
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For all other inner packets, the inner network layer admits the
packet if it is no larger than the tunnel interface MTU; otherwise,
it drops the packet and sends a PTB error message to the source with
the MTU value set to the tunnel interface MTU. The message contains
as much of the invoking packet as possible without the entire message
exceeding the network layer minimum MTU 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 (theoretical maximums are 64KB for IPv4 and
4GB for IPv6 [RFC2675]). For ITEs that host applications that use
the tunnel interface directly, this option must be carefully
coordinated with protocol stack upper layers since some upper layer
protocols (e.g., TCP) derive their packet sizing parameters from the
MTU of the outgoing interface and as such may select too large an
initial size. This is not a problem for upper layers that use
conservative initial maximum segment size estimates and/or when the
tunnel interface can reduce the upper layer's maximum segment size,
e.g., by reducing the size advertised in the MSS option of outgoing
TCP messages (sometimes known as "MSS clamping").
In light of the above considerations, the ITE SHOULD configure an
indefinite MTU on tunnel *router* interfaces so that SEAL performs
all subnetwork adaptation from within the interface as specified in
the following sections. The ITE MAY instead set a smaller MTU on
tunnel *host* interfaces; in that case, the RECOMMENDED MTU is the
maximum of 1500 bytes and the smallest MTU among all of the
underlying links minus the size of the encapsulation headers.
5.4.2. Tunnel Neighbor Soft State
The ITE maintains a number of soft state variables for each ETE and
for each SEAL path.
The ITE maintains a per ETE window of Identification values for the
packets it has recently sent to this ETE as welll as a per ETE window
of Identification values for the packets it has recently received
from 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 message origin authentication and integrity checking is
required, the ITE also includes an ICV in the packets it sends to the
ETE. The ICV format is shown in Figure 3:
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|F|Key|Algorithm| Message Authentication Code (MAC) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 3: Integrity Check Vector (ICV) Format
As shown in the figure, the ICV begins with a 1-octet control field
with a 1-bit (F)lag, a 2-bit Key identifier and a 5-bit Algorithm
identifier. The control octet is followed by a variable-length
Message Authentication Code (MAC). The ITE maintains a per ETE
algorithm and secret key to calculate the MAC in each packet it will
send to this ETE. (By default, the ITE sets the F bit and Algorithm
fields to 0 to indicate use of the HMAC-SHA-1 algorithm with a 160
bit shared secret key to calculate an 80 bit MAC per [RFC2104] over
the leading 128 bytes of the packet. Other values for F and
Algorithm are out of scope.) 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 each SEAL path, the ITE must also account for encapsulation
header lengths. The ITE therefore maintains the per SEAL 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.) When SEAL is used in conjunction with
another tunnel type such as GRE or IPsec, the length of the headers
associated with those tunnels is also included in the HLEN
calculation for the first segment only and the length of the
associated trailers is included in the HLEN calculation for the final
segment only.
The ITE maintains a per SEAL path variable "MAXMTU" initialized to
the maximum of 1500 bytes and the MTU of the underlying link minus
HLEN. The ITE further sets a variable 'MINMTU' to the minimum MTU
for the SEAL path over which encapsulated packets will travel. For
IPv6 paths, the ITE sets MINMTU=1280 per [RFC2460]. For IPv4 paths,
the ITE sets MINMTU=576 based on practical interpretation of
[RFC1122] even though the theoretical MINMTU for IPv4 is only 68
bytes [RFC0791].
The ITE can also set MINMTU to a larger value if there is reason to
believe that the minimum path MTU is larger, or to a smaller value if
there is reason to believe the MTU is smaller, e.g., if there may be
additional encapsulations on the path. If this value proves too
large, the ITE will receive PTB message feedback either from the ETE
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or from a router on the path and will be able to reduce its MINMTU to
a smaller value. (Note that since IPv4 links with MTUs smaller than
1280 are almost certainly low data rate, the ITE can instead set
MINMTU to 1280 the same as for IPv6. If this value proves too large,
standard IPv4 fragmentation and reassembly will provide short term
accommodation for the sizing constraints.)
The ITE may instead maintain the packet sizing variables and
constants as per ETE (rather than per SEAL path) values. In that
case, the values reflect the smallest MTU size across all of the SEAL
paths associated with this ETE.
5.4.3. SEAL Layer Pre-Processing
The SEAL layer is logically positioned between the inner and outer
network protocol layers, where the inner layer is seen as the (true)
network layer and the outer layer is seen as the (virtual) data link
layer. Each packet to be processed by the SEAL layer is either
admitted into the tunnel interface by the inner network layer
protocol as described in Section 5.4.1 or is undergoing re-
encapsulation from within the tunnel interface. The SEAL layer sees
the former class of packets as inner packets that include inner
network and transport layer headers, and sees the latter class of
packets as transitional SEAL packets that include the outer and SEAL
layer headers that were inserted by the previous hop SEAL ITE. For
these transitional packets, the SEAL layer re-encapsulates the packet
with new outer and SEAL layer headers when it forwards the packet to
the next hop SEAL ITE.
We now discuss the SEAL layer pre-processing actions for these two
classes of packets.
5.4.3.1. Inner Packet Pre-Processing
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, for non-SEAL IPv4 inner packets with DF==0 in the IP
header and IPv6 inner packets with a fragment header and with (MF=0;
Offset=0), if the packet is larger than (MINMTU-HLEN) the ITE uses IP
fragmentation to fragment 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 fragment for SEAL encapsulation
as specified in Section 5.4.4.
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For all other inner packets, if the packet is no larger than MAXMTU
for the corresponding SEAL path the ITE submits it for SEAL
encapsulation as specified in Section 5.4.4. Otherwise, the ITE
drops the packet and sends an ordinary PTB message appropriate to the
inner protocol version (subject to rate limiting) with the MTU field
set to MAXMTU. (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 PTB
message, the ITE discards the inner packet.
5.4.3.2. Transitional SEAL Packet Pre-Processing
For each transitional packet that is to be processed by the SEAL
layer from within the tunnel interface, the ITE sets aside the SEAL
encapsulation headers that were received from the previous hop.
Next, if the packet is no larger than MAXMTU for the next hop SEAL
path the ITE submits it for SEAL encapsulation as specified in
Section 5.4.4. Otherwise, the ITE drops the packet and sends an SCMP
Packet Too Big (SPTB) message to the previous hop subject to rate
limiting (see: Section 5.6.1.1) with the MTU field set to MAXMTU.
After sending the SPTB message, the ITE discards the packet.
5.4.4. 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 5.3. 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; P=0), sets LINK_ID to the value assigned to
the underlying SEAL 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.
Next, if the inner packet is no larger than (MINMTU-HLEN) or larger
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than 1500, the ITE sets (M=0; Offset=0). Otherwise, the ITE breaks
the inner packet into N approximately equal length non-overlapping
segments (where N is minimized and each segment is significantly
smaller than (MINMTU-HLEN) to allow for additional encapsulations in
the path). In this process, the ITE MUST ensure that each segment
except the final segment contains an integer multiple of 256 byte
blocks, and that the inner packet's network and transport layer
headers are included in the first segment. The ITE then appends a
clone of the SEAL header from the first segment onto the head of each
additional segment. The ITE MUST also include an Identification
field and set USE_ID=TRUE for each segment. The ITE then sets (M=1;
Offset=0) in the first segment, sets (M=0/1; Offset=O(1)) in the
second segment, sets (M=0/1; Offset=O(2)) in the third segment (if
needed), etc., then finally sets (M=0; Offset=O(n)) in the final
segment (where O(i) is the number of 256 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-incrementing integer value for this
ETE in the Identification field beginning with a randomly-initialized
value 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.) The monotonically-incrementing
requirement is to satisfy ETEs that use this value for anti-replay
purposes. The value is incremented modulo 2^32, i.e., it wraps back
to 0 when the previous value was (2^32 - 1).
When USE_ICV is FALSE, the ITE next sets V=0. Otherwise, the ITE
sets V=1, includes an ICV and calculates the MAC using HMAC-SHA-1
with a 160 bit secret key and 80 bit MAC field. Beginning with the
SEAL header, the ITE sets the ICV field to 0, calculates the MAC 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 the result in the MAC
field. (For SEAL packets that have been split into multiple pieces,
each piece calculates its own MAC.) The ITE then writes the value 0
in the F flag and 0x00 in the Algorithm field of the ICV control
octet (other values for these fields, and other MAC calculation
disciplines, are outside the scope of this document and may be
specified in future documents.)
The ITE then adds the outer encapsulating headers as specified in
Section 5.4.5.
5.4.5. 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 such as UDP or TCP is
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included, the ITE writes the port number for SEAL in the transport
destination service port field.
When UDP encapsulation is used, the ITE sets the UDP checksum field
to zero for IPv4 packets and also sets the UDP checksum field to zero
for IPv6 packets even though IPv6 generally requires UDP checksums.
Further considerations for setting the UDP checksum field for IPv6
packets are discussed in [RFC6935][RFC6936].
The ITE then sets the outer IP layer headers the same as specified
for ordinary IP encapsulation (e.g., [RFC1070][RFC2003], [RFC2473],
[RFC4213], etc.) except that for ordinary SEAL packets the ITE copies
the "TTL/Hop Limit", "Type of Service/Traffic Class" and "Congestion
Experienced" values in the inner network layer header into the
corresponding fields in the outer IP header. For transitional SEAL
packets undergoing re-encapsulation, the ITE instead copies the "TTL/
Hop Limit", "Type of Service/Traffic Class" and "Congestion
Experienced" values in the outer IP header of the received packet
into the corresponding fields in the outer IP header of the packet to
be forwarded (i.e., the values are transferred between outer headers
and *not* copied from the inner network layer header).
The ITE also sets the IP protocol number to the appropriate value for
the first protocol layer within the encapsulation (e.g., UDP, TCP,
SEAL, etc.). When IPv6 is used as the outer IP protocol, the ITE
then sets the flow label value in the outer IPv6 header the same as
described in [RFC6438]. When IPv4 is used as the outer IP protocol,
the ITE instead sets DF=0 in the IPv4 header to allow the packet to
be fragmented if it encounters a restricting link (for IPv6 SEAL
paths, the DF bit is absent but implicitly set to 1).
The ITE finally sends each outer packet via the underlying link
corresponding to LINK_ID.
5.4.6. Path Probing and ETE Reachability Verification
All SEAL data packets sent by the ITE are considered implicit probes
that detect in-the-network fragmentation, while explicit probe
packets can be constructed to probe the path MTU and/or verify ETE
reachability. These probes will elicit an SCMP message from the ETE
if it needs to send an acknowledgement and/or report an error
condition. The probe packets may also be dropped by either the ETE
or a router on the path, which may or may not result in an ICMP
message being returned to the ITE.
To generate an explicit probe packet, the ITE creates a duplicate of
an actual data packet and uses the duplicate as a probe.
(Alternatively, the ITE can create a packet buffer beginning with the
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same outer headers, SEAL header and inner network layer headers that
would appear in an ordinary data packet, then pad the packet with
random data.) The ITE then sets (C=0; P=1) in the SEAL header of the
probe packet, and also sets DF=1 in the outer IP header when IPv4 is
used.
The ITE sends periodic explicit probes to determine whether SEAL
segmentation is still necessary (see Section 5.4.4). In particular,
if a probe packet of 1500 bytes (i.e., a packet that becomes (1500+
HLEN) bytes after encapsulation) succeeds the ITE is assured that the
path MTU is large enough so that the segmentation/reassembly process
can be suspended. This probing discipline can therefore be
considered as Packetization Layer Path MTU Discovery (PLPMTUD)
[RFC4821] applied to tunnels, which operates independently of any
application of PLPMTUD between end systems.
Note that the explicit probe size of 1500 bytes is chosen since probe
packets smaller than this size may be fragmented by another ITE
further down the path. For example, a successful probe for a packet
size of 1400 bytes does not guarantee that fragmentation is not
occurring at another ITE.
While probing, the ITE processes ICMP messages as specified in
Section 5.4.7 and processes SCMP messages as specified in Section
5.6.2.
5.4.7. Processing ICMP Messages
When the ITE sends SEAL packets, it may receive ICMP error messages
[RFC0792][RFC4443] from a router on the path to the ETE. 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"). Note that the ITE may
receive an ICMP 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 the message the same
as if it originated from an ordinary router.
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 SEAL.
The ITE can optionally ignore other ICMP messages that do not include
sufficient information in the packet-in-error, or process them as a
hint that the SEAL path to the ETE may be failing. The ITE then
discards these types of messages.
For other ICMP messages, the ITE first examines the SEAL data packet
within the packet-in-error field. If the IP source and/or
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destination addresses are invalid, or if the value in the SEAL header
Identification field (if present) is not within the window of packets
the ITE has recently sent to this ETE, or if the MAC 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 SEAL 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 SEAL path (see Section 5.4.9), the ITE next
sets MAXMTU=MAX((PMTU-HLEN), 1500).
The ITE then transcribes the ICMP message into a message appropriate
for the SEAL data packet within the packet-in-error. If the previous
hop toward the inner source address within the SEAL data packet is
reached via the same SEAL interface, the ITE transcribes the message
into an SCMP message the same as described for ETE generation of SCMP
messages in Section 5.6.1, i.e., it copies the SEAL data packet
within the packet-in-error into the packet-in-error field of the new
message. Otherwise, the ITE seeks beyond the SEAL header within the
packet-in-error and transcribes the inner packet into a message
appropriate for the inner protocol version (e.g., ICMPv4 for IPv4,
ICMPv6 for IPv6, etc.).
The ITE finally forwards the transcribed message to the previous hop
toward the inner source address.
5.4.8. 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".
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 SEAL data
packet for packet analysis purposes but then forward the fragments on
to the final destination rather than forwarding the reassembled
packet. (This process is often referred to as "Virtual Fragmentation
Reassembly" (VFR)).
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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
necessary. If the ITE is aware that the ETE is located behind a NAT
or a firewall, however, then reassembly testing can be used to detect
middleboxes that do not conform to specifications.
The ITE can perform a middlebox reassembly test by sending explicit
probe packets. The ITE should only send probe packets that are no
larger than (1500-HLEN) before encapsulation since the most an
ordinary node can be expected to reassemble is 1500 bytes. To
generate a probe, the ITE either creates a clone of an ordinary data
packet or 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. The ITE then pads the probe packet with
random data to a length that is at least 128 bytes but no longer than
(1500-HLEN) bytes.
The ITE then sets (C=0; P=1) in the SEAL header of the probe packet
and sets the NEXTHDR field to the inner network layer protocol type.
Next, the ITE sets LINK_ID and LEVEL to the appropriate values for
this SEAL 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 IP fragments, then sends both
fragments over the same SEAL 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 SEAL path correctly supports fragmentation; otherwise,
the ITE enables stateful MTU determination for this SEAL path as
specified in Section 5.4.9.
5.4.9. 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 SEAL path. For example, when the ETE is situated behind a
middlebox that performs IPv4 reassembly in violation of the specs
(see: Section 5.4.8) it is imperative that fragmentation be avoided.
In other instances (e.g., when the SEAL 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.
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To determine a static MTU value, the ITE sends a series of probe
packets of various sizes to the ETE with (C=0; P=1) in the SEAL
header and DF=1 in the outer IP header. The ITE then caches the size
'S' of the largest packet for which it receives a probe reply from
the ETE by setting MAXMTU=MAX((S-HLEN), 1500) for this SEAL 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.
5.4.10. Detecting Path MTU Changes
When stateful MTU determination is used, the ITE SHOULD periodically
reset MAXMTU and/or re-probe the path to determine whether MAXMTU has
increased. If the path still has a too-small MTU, the ITE will
receive a PTB message that reports a smaller size.
5.5. ETE Specification
5.5.1. Reassembly Buffer Requirements
For IPv6, the ETE MUST configure a minimum reassembly buffer size of
(1500 + HLEN) bytes for the reassembly of outer IPv6 packets, i.e.,
even though the true minimum reassembly size for IPv6 is only 1500
bytes [RFC2460]. For IPv4, the ETE also MUST configure a minimum
reassembly buffer size of (1500 + HLEN) 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
further MUST configure a minimum SEAL reassembly buffer size of (1500
+ HLEN) bytes for the reassembly of segmented SEAL packets (see:
Section 5.5.4).
Note that the value "HLEN" may be variable and initially unknown to
the ETE, and would typically range from a few bytes to a few tens of
bytes or even more. It is therefore RECOMMENDED that the ETE
configure slightly larger minimum IP/SEAL reassembly buffer sizes of
2048 bytes (2KB).
5.5.2. Tunnel Neighbor Soft State
When message origin authentication and integrity checking is
required, the ETE maintains a per-ITE MAC calculation algorithm and a
symmetric secret key to verify the MAC. The ETE also maintains a
window of Identification values for the packets it has recently
received from this ITE as well as a window of Identification values
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for the packets it has recently sent to this ITE.
When the tunnel neighbor relationship is bidirectional, the ETE
further maintains a per SEAL path mapping of outer IP and transport
layer addresses to the LINK_ID that appears in packets received from
the ITE.
5.5.3. IP-Layer Reassembly
The ETE reassembles fragmented IP packets that are explicitly
addressed to itself. For IP fragments that are received via a SEAL
tunnel, 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 ETE performs any necessary IP reassembly then
submits the packet for SEAL decapsulation as specified in Section
5.5.4. (Note that if the packet is larger than the reassembly buffer
size, the ETE still examines the leading portion of the (partially)
reassembled packet during decapsulation.)
5.5.4. Decapsulation, SEAL-Layer Reassembly, 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.
Next, if V==1 the ETE SHOULD verify the MAC value (with the MAC field
itself reset to 0) and silently discard the packet if the value is
incorrect.
Next, if the packet arrived as multiple IP fragments, 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 5.6.1.1).
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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 5.6.1.2).
Next, if the SEAL header has C==1, the ETE processes the packet as an
SCMP packet as specified in Section 5.6.2. 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 all other
segments have already arrived, the ETE discards the SEAL header and
other outer headers from the non-initial segments and appends the
segments onto the end of the first segment according to their offset
value. Otherwise, the ETE caches the new segment for at most 60
seconds while awaiting the arrival of its partners. During this
process, the ETE discards any segments that are overlapping with
respect to segments that have already been received, and also
discards any segments that have M==1 in the SEAL header but do not
contain an integer multiple of 256 byte blocks. The ETE further
SHOULD manage the SEAL reassembly cache the same as described for the
IP-Layer Reassembly cache in Section 5.5.3, i.e., it SHOULD perform
an early discard for any pending reassemblies that have low
probability of completion.
Next, if the SEAL header in the (reassembled) packet has P==1, the
ETE drops the packet unconditionally and sends an SPTB message back
to the ITE with MTU=0 (see: Section 5.6.1.1) if it has not already
sent an SPTB message based on IP fragmentation. (Note that the ETE
therefore sends only a single SPTB message for a probe packet that
also experienced IP fragmentation, i.e., it does not send multiple
SPTB messages.)
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 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.
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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 5.4.3 above and without
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.
5.6. 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]. The SCMP messaging
protocol operates over bidirectional and partially unidirectional
tunnels. (For fully unidirectional tunnels, SEAL must operate
without the benefit of SCMP meaning that steady-state fragmentation
and reassembly may be necessary in extreme cases. In that case, the
ITE must select a conservative MINMTU to ensure that IPv4
fragmentation is avoided in order to avoid reassembly errors at high
data rates [RFC4963].)
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 4:
+--------------------+
~ 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 4: SCMP Message Encapsulation
The following sections specify the generation, processing and
relaying of SCMP messages.
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5.6.1. Generating SCMP Error Messages
ETEs generate SCMP error messages in response to receiving certain
SEAL data packets using the format shown in Figure 5:
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 5: 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 5.
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 4. 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; P=0; M=0; Offset=0) in the SEAL header, then
sets I, V, NEXTHDR, LINK_ID and LEVEL to the same values that
appeared in the SEAL header of the data packet.
When I==1, the ETE next sets the Identification field to the next
Identification value scheduled for this ITE, then increments the next
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Identification value.
When V==1, the ETE then prepares the ICV field the same as specified
for SEAL data packet encapsulation in Section 5.4.4.
Finally, the ETE sends the resulting SCMP packet to the ITE the same
as specified for SEAL data packets in Section 5.4.5.
The following sections describe additional considerations for various
SCMP error messages:
5.6.1.1. Generating SCMP Packet Too Big (SPTB) Messages
An ETE generates an SPTB message when it receives a SEAL data packet
that arrived as multiple outer IP fragments. 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 probe
packet (i.e., one with P==1 in the SEAL header) that arrived as an
unfragmented outer IP packet. The ETE prepares the SPTB message the
same as above, except that it writes the value 0 in the MTU field.
5.6.1.2. Generating Other SCMP Error Messages
An ETE generates an SCMP "Destination Unreachable" (SDU) message
under the same conditions 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].
5.6.2. 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
in Section 5.6.1. When V==1, the ITE then verifies the ICV. 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;
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otherwise, it processes the message as follows:
5.6.2.1. Processing SCMP PTB Messages
After an ITE sends a SEAL packet to an ETE, it may receive an SPTB
message with a packet-in-error containing the leading portion of the
packet (see: Section 5.6.1.1). For SPTB messages with MTU==0, the
ITE processes the message as confirmation that the ETE received an
unfragmented SEAL probe packet with P==1 in the SEAL header. Based
on the size of the probe, the ITE then suspends the segmentation/
reassembly process and/or caches the size of the probe as a stateful
MTU limitation for this ETE. 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 no larger than 1500 bytes, the ITE reduces its MINMTU value
for this ITE and discards the message. If the inner packet length is
larger than 1500 and the MTU value is not substantially less than
MINMTU 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 a small MTU
and inner packet length 4KB, it can rewrite the MTU to 2KB. If the
ITE subsequently receives an IPv4 SPTB message with a small MTU 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 SEAL path, it then writes the new MTU value
minus HLEN in MAXMTU.
The ITE then transcribes the message into a control message format
appropriate for the packet-in-error (i.e., the same as described for
transcribing ICMP messages in Section 5.4.7) and sends the resulting
transcribed message to the previous hop. When the previous hop is
reached via the same tunnel interface as the SCMP message arrived on,
the ITE performs a "reverse re-encapsulation" and replaces the outer
headers up to and including the SEAL header with headers that will be
recognized and accepted by the previous hop. This means that any
Identification and/or ICV values within the SEAL header within the
packet-in-error will be meaningless to the previous hop and should
therefore be ignored.
5.6.2.2. 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
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Destination Unreachable message. The ITE transcribes the message and
forwards it toward the source address of the inner packet within the
packet-in-error the same as specified for SPTB messages in Section
5.6.2.1.
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
different settings 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].
6. SEAL Transport Mode Specification
SEAL also provides a transport-mode of operation. Transport mode
refers to a SEAL encapsulation in which a layer-4 header appears
immediately following the SEAL header. The type of layer-4 header is
indicated in the "NEXTHDR" field the same as for tunnel mode. The
SEAL header is identical to the version used for tunnel mode, except
that the "LINK_ID" and "LEVEL" fields are omitted, and the transport
layer port numbers are included in each non-initial segment (see:
Figure 6).
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NEXTHDR |VER|C|P|I|V|R|M| Offset | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Port (when Offset!=0) | Dest Port (when Offset!=0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Integrity Check Vector (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ...
Figure 6: SEAL Header Format
The "Source Port" and "Dest Port" are taken from the corresponding
fields in the transport layer next header that appears immediately
following the SEAL header in the initial segment. For example, for
UDP [RFC0768] the transport layer source/dest ports are 16 bits in
length and are copied from the transport layer header. All
segmentation/reassembly is performed the same as specified for tunnel
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mode, and the SEAL Control Message Protocol (SCMP) operates the same
as for tunnel mode. For example, the source node can probe the path
MTU to the destination by setting the P bit in a probe packet and
wating for an SCMP acknowledgement message from the destination.
SEAL transport mode implementations SHOULD configure reassembly
buffers that are large enough to accommodate a maximum-sized
segmented SEAL packet, i.e., it is RECOMMENDED that they configure a
64KB reassembly buffer size.
SEAL transport mode is useful for transport layer protocols that have
no way to segment the large packets they send. It is a universal
format that can be applied to any such transport.
7. Link Requirements
Subnetwork designers are expected to follow the recommendations in
Section 2 of [RFC3819] when configuring link MTUs.
8. End System Requirements
End systems are encouraged to implement end-to-end MTU assurance
(e.g., using Packetization Layer Path MTU Discovery (PLPMTUD) per
[RFC4821]) even if the subnetwork is using SEAL.
When end systems use PLPMTUD, SEAL will ensure that the tunnel
behaves as a link in the path that assures an MTU of at least 1500
bytes while not precluding discovery of larger MTUs. The PLPMTUD
mechanism will therefore be able to function as designed in order to
discover and utilize larger MTUs.
9. Router Requirements
Routers within the subnetwork are expected to observe the standard IP
router requirements, including the implementation of IP fragmentation
and reassembly as well as the generation of ICMP messages
[RFC0792][RFC1122][RFC1812][RFC2460][RFC4443][RFC6434].
Note that, even when routers support existing requirements for the
generation of ICMP messages, these messages are often filtered and
discarded by middleboxes on the path to the original source of the
message that triggered the ICMP. It is therefore not possible to
assume delivery of ICMP messages even when routers are correctly
implemented.
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10. Nested Encapsulation Considerations
SEAL supports nested tunneling for up to 8 layers of encapsulation.
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.
In such nested arrangements, 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 TEs at other nesting
levels.Therefore, when an ITE 'A' within an outer nesting level needs
to return an error message to an ITE 'B' within an inner 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 5.4.7, but full message
origin authentication is not possible.
(Note that the SCMP protocol could instead be extended to allow an
outer nesting level ITE 'A' to return an SCMP message to an inner
nesting level ITE 'B' rather than return an ICMP message. This would
conceptually allow the control messages to 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.)
11. Reliability Considerations
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.
12. Integrity Considerations
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.
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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
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. It is therefore essential
that IPv4 fragmentation and reassembly be detected early and tuned
out through proper application of SEAL segmentation and reassembly.
13. IANA Considerations
The IANA is requested to allocate a User Port number for "SEAL" in
the 'port-numbers' registry. The Service Name is "SEAL", and the
Transport Protocols are TCP and UDP. The Assignee is the IESG
(iesg@ietf.org) and the Contact is the IETF Chair (chair@ietf.org).
The Description is "Subnetwork Encapsulation and Adaptation Layer
(SEAL)", and the Reference is the RFC-to-be currently known as
'draft-templin-intarea-seal'.
14. Security Considerations
SEAL provides a segment-by-segment message origin authentication,
integrity and anti-replay service. 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.
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.
Security issues that apply to tunneling in general are discussed in
[RFC6169].
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15. 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.
SEAL, along with the Virtual Enterprise Traversal (VET)
[I-D.templin-intarea-vet] tunnel virtual interface abstraction, are
the functional building blocks for the Interior Routing Overlay
Network (IRON) [I-D.templin-ironbis] and Routing and Addressing in
Networks with Global Enterprise Recursion (RANGER) [RFC5720][RFC6139]
architectures.
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 [RFC5320].
16. Implementation Status
An early implementation of the first revision of SEAL [RFC5320] is
available at: http://isatap.com/seal.
17. 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
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Droms, Aurnaud Ebalard, Gorry Fairhurst, Washam Fan, Dino Farinacci,
Joel Halpern, Sam Hartman, John Heffner, Thomas Henderson, Bob
Hinden, Christian Huitema, Eliot Lear, Darrel Lewis, Joe Macker, Matt
Mathis, Erik Nordmark, Dan Romascanu, Dave Thaler, Joe Touch, Mark
Townsley, Ole Troan, Margaret Wasserman, Magnus Westerlund, Robin
Whittle, James Woodyatt, and members of the Boeing Research &
Technology NST DC&NT group.
Discussions with colleagues following the publication of [RFC5320]
have provided useful insights that have resulted in significant
improvements to this, the Second Edition of SEAL.
This document received substantial review input from the IESG and
IETF area directorates in the February 2013 timeframe. IESG members
and IETF area directorate representatives who contributed helpful
comments and suggestions are gratefully acknowledged. Discussions on
the IETF IPv6 and Intarea mailing lists in the summer 2013 timeframe
also stimulated several useful ideas.
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.
18. References
18.1. 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
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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.
18.2. 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.massar-v6ops-ayiya]
Massar, J., "AYIYA: Anything In Anything",
draft-massar-v6ops-ayiya-02 (work in progress), July 2004.
[I-D.taylor-v6ops-fragdrop]
Jaeggli, J., Colitti, L., Kumari, W., Vyncke, E., Kaeo,
M., and T. Taylor, "Why Operators Filter Fragments and
What It Implies", draft-taylor-v6ops-fragdrop-01 (work in
progress), June 2013.
[I-D.templin-intarea-vet]
Templin, F., "Virtual Enterprise Traversal (VET)",
draft-templin-intarea-vet-40 (work in progress), May 2013.
[I-D.templin-ironbis]
Templin, F., "The Interior Routing Overlay Network
(IRON)", draft-templin-ironbis-15 (work in progress),
May 2013.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[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.
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[RFC1146] Zweig, J. and C. Partridge, "TCP alternate checksum
options", RFC 1146, March 1990.
[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.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[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,
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August 2002.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4213] Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
for IPv6 Hosts and Routers", RFC 4213, October 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[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.
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[RFC5927] Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.
[RFC6139] Russert, S., Fleischman, E., and F. Templin, "Routing and
Addressing in Networks with Global Enterprise Recursion
(RANGER) Scenarios", RFC 6139, February 2011.
[RFC6169] Krishnan, S., Thaler, D., and J. Hoagland, "Security
Concerns with IP Tunneling", RFC 6169, April 2011.
[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.
[RFC6434] Jankiewicz, E., Loughney, J., and T. Narten, "IPv6 Node
Requirements", RFC 6434, December 2011.
[RFC6438] Carpenter, B. and S. Amante, "Using the IPv6 Flow Label
for Equal Cost Multipath Routing and Link Aggregation in
Tunnels", RFC 6438, November 2011.
[RFC6864] Touch, J., "Updated Specification of the IPv4 ID Field",
RFC 6864, February 2013.
[RFC6935] Eubanks, M., Chimento, P., and M. Westerlund, "IPv6 and
UDP Checksums for Tunneled Packets", RFC 6935, April 2013.
[RFC6936] Fairhurst, G. and M. Westerlund, "Applicability Statement
for the Use of IPv6 UDP Datagrams with Zero Checksums",
RFC 6936, April 2013.
[RIPE] De Boer, M. and J. Bosma, "Discovering Path MTU Black
Holes on the Internet using RIPE Atlas", July 2012.
[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.
[WAND] Luckie, M., Cho, K., and B. Owens, "Inferring and
Debugging Path MTU Discovery Failures", October 2005.
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Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
Templin Expires February 3, 2014 [Page 42]
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