Network Working Group F. Templin, Ed.
Internet-Draft Boeing Phantom Works
Intended status: Informational May 30, 2008
Expires: December 1, 2008
The Subnetwork Encapsulation and Adaptation Layer (SEAL)
draft-templin-seal-15.txt
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Abstract
Subnetworks are connected network regions bounded by border nodes
that forward unicast and multicast packets over a virtual topology,
often manifested by encapsulation and/or tunneling. This virtual
topology may span multiple IP- and/or sub-IP layer forwarding hops,
and can introduce failure modes due to packet duplication and/or
links with diverse Maximum Transmission Units (MTUs). This document
specifies a Subnetwork Encapsulation and Adaptation Layer (SEAL) that
accommodates such virtual topologies over diverse underlying link
technologies.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Requirements . . . . . . . . . . . . . . . . . 4
3. Applicability Statement . . . . . . . . . . . . . . . . . . . 5
4. SEAL Protocol Specification - Tunnel Mode . . . . . . . . . . 6
4.1. Model of Operation . . . . . . . . . . . . . . . . . . . . 6
4.2. ITE Specification . . . . . . . . . . . . . . . . . . . . 7
4.2.1. Tunnel Interface MTU . . . . . . . . . . . . . . . . . 7
4.2.2. Accounting for Headers . . . . . . . . . . . . . . . . 8
4.2.3. Segmentation and Encapsulation . . . . . . . . . . . . 9
4.2.4. Packet Identification . . . . . . . . . . . . . . . . 11
4.2.5. Sending SEAL Protocol Packets . . . . . . . . . . . . 12
4.2.6. Sending Probes . . . . . . . . . . . . . . . . . . . . 12
4.2.7. Processing Raw ICMPv4 Messages . . . . . . . . . . . . 13
4.2.8. Processing SEAL-Encapsulated ICMPv4 Messages . . . . . 13
4.3. ETE Specification . . . . . . . . . . . . . . . . . . . . 14
4.3.1. Reassembly Buffer Requirements . . . . . . . . . . . . 14
4.3.2. IPv4-Layer Reassembly . . . . . . . . . . . . . . . . 14
4.3.3. Generating SEAL-Encapsulated ICMPv4 Fragmentation
Needed Messages . . . . . . . . . . . . . . . . . . . 14
4.3.4. SEAL-Layer Reassembly . . . . . . . . . . . . . . . . 16
4.3.5. Decapsulation and Generating Other ICMPv4 Errors . . . 16
5. SEAL Protocol Specification - Transport Mode . . . . . . . . . 17
6. Link Requirements . . . . . . . . . . . . . . . . . . . . . . 17
7. End System Requirements . . . . . . . . . . . . . . . . . . . 17
8. Router Requirements . . . . . . . . . . . . . . . . . . . . . 18
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 18
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
12.1. Normative References . . . . . . . . . . . . . . . . . . . 19
12.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Historic Evolution of PMTUD (written 10/30/2002) . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22
Intellectual Property and Copyright Statements . . . . . . . . . . 23
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1. Introduction
As Internet technology and communication has grown and matured, many
techniques have developed that use virtual topologies (frequently
tunnels of one form or another) over an actual IP network. Those
virtual topologies have elements which appear as one hop in the
virtual topology, but are actually multiple IP or sub-IP layer hops.
These multiple hops often have quite diverse properties which are
often not even visible to the end-points of the virtual hop. This
introduces many failure modes that are not dealt with well in current
approaches.
The use of IP encapsulation has long been considered as an
alternative for creating such virtual topologies. However, the
insertion of an outer IP header reduces the effective path MTU as-
seen by the IP layer. When IPv4 is used, this reduced MTU can be
accommodated through the use of IPv4 fragmentation, but unmitigated
in-the-network fragmentation has been shown to be harmful through
operational experience and studies conducted over the course of many
years [FRAG][FOLK][RFC4963]. Additionally, classical path MTU
discovery [RFC1191] has known operational issues that are exacerbated
by in-the-network tunnels [RFC2923][RFC4459].
For the purpose of this document, subnetworks are defined as virtual
topologies that span connected network regions bounded by
encapsulating border nodes. Examples include the global Internet
interdomain routing core, Mobile Ad hoc Networks (MANETs) and
enterprise networks. Subnetwork border nodes support the Internet
protocols [RFC0791][RFC2460] and forward unicast and multicast IP
packets over the virtual topology across multiple IP- and/or sub-IP
layer forwarding hops which may introduce packet duplication and/or
traverse links with diverse Maximum Transmission Units (MTUs).
This document proposes a Subnetwork Encapsulation and Adaptation
Layer (SEAL) for tunnel-mode operation of IP over subnetworks that
connect the Ingress- and Egress Tunnel Endpoints (ITEs/ETEs) of
border nodes. Operation in transport mode is also supported when
subnetwork border node upper-layer protocols negotiate the use of
SEAL during connection establishment. SEAL accommodates links with
diverse MTUs and supports efficient duplicate packet detection by
introducing a minimal mid-layer encapsulation. The SEAL
encapsulation introduces an extended Identification field for packet
identification and a mid-layer segmentation and reassembly capability
that allows simplified cutting and pasting of packets without
invoking in-the-network IPv4 fragmentation. The SEAL encapsulation
layer and protocol is specified in the following sections.
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2. Terminology and Requirements
The term "subnetwork" in this document refers to a virtual topology
that is configured over a connected network region bounded by border
nodes.
The terms "inner", "mid-layer" and "outer" respectively refer to the
innermost IP {layer, protocol, header, packet, etc.} before any
encapsulation, the mid-layer IP {protocol, header, packet, etc.)
after any mid-layer '*' encapsulation and the outermost IP {layer,
protocol, header, packet etc.} after SEAL/*/IPv4 encapsulation.
The notation IPvX/*/SEAL/*IPvY refers to an inner IPvX packet
encapsulated in any mid-layer '*' encapsulations followed by the SEAL
header followed by any outer '*' encapsulations followed by an outer
IPvY header. The notation "IP" means either IP protocol version
(IPv4 or IPv6).
The following abbreviations correspond to terms used within this
document and elsewhere in common Internetworking nomenclature:
Subnetwork - a connected network region bounded by border nodes
SEAL - Subnetwork Encapsulation and Adaptation Layer
ITE - Ingress Tunnel Endpoint
ETE - Egress Tunnel Endpoint
MTU - Maximum Transmission Unit
ENCAPS - the length of any mid-layer '*' headers plus the length
of the outer encapsulating SEAL/*/IPv4 headers
S_MTU - the per-ETE SEAL Maximum Transmission Unit
S_MRU- the per-ETE SEAL Maximum Reassembly Unit
S_MSS - the SEAL Maximum Segment Size, derived from S_MTU
PTB - an ICMPv6 "Packet Too Big" or an ICMPv4 "Fragmentation
Needed" message
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FLEN - the MTU value included in an ICMPv4 "Fragmentation Needed"
message
DF - the IPv4 header "Don't Fragment" flag
SEAL-ID - a 32-bit Identification value; randomly initialized and
monotonically incremented for each SEAL protocol packet
SEAL_PROTO - an IPv4 protocol number used for SEAL
SEAL_PORT - a TCP/UDP service port number used for SEAL
SEAL_OPTION - a TCP option number used for (transport-mode) SEAL
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
document, are to be interpreted as described in [RFC2119].
3. Applicability Statement
SEAL was motivated by the specific use case of subnetwork abstraction
for Mobile Ad-hoc Networks (MANETs), however the domain of
applicability also extends to subnetwork abstractions of enterprise
networks, the interdomain routing core, etc. The domain of
application therefore also includes the map-and-encaps architecture
proposals in the IRTF Routing Research Group (RRG) (see: http://
www3.tools.ietf.org/group/irtf/trac/wiki/RoutingResearchGroup).
SEAL introduces a minimal new sublayer for IPvX in IPvY encapsulation
(e.g., as IPv6/SEAL/IPv4), and appears as a subnetwork encapsulation
as seen by the inner IP layer. SEAL can also be used as a sublayer
for encapsulating inner IP packets within outer UDP/IPv4 header
(e.g., as IP/SEAL/UDP/IPv4) such as for the Teredo domain of
applicability [RFC4380]. When it appears immediately after the outer
IPv4 header, the SEAL header is processed exactly as for IPv6
extension headers.
SEAL can also be used in "transport-mode", e.g., when the inner layer
includes upper layer protocol data rather than an encapsulated IP
packet. For instance, TCP peers can negotiate the use of SEAL for
the carriage of protocol data encapsulated as TCP/SEAL/IPv4. In this
sense, the "subnetwork" becomes the entire end-to-end path between
the TCP peers and may potentially span the entire Internet.
The current document version is specific to the use of IPv4 as the
outer encapsulation layer, however the same principles apply when
IPv6 is used as the outer layer.
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4. SEAL Protocol Specification - Tunnel Mode
4.1. Model of Operation
SEAL supports the encapsulation of inner IP packets in mid-layer and
outer encapsulating headers/trailers. For example, an inner IP
packet would appear as IP/*/SEAL/*/IPv4 after mid-layer and outer
encapsulations, where '*' denotes zero or more additional
encapsulation sublayers. Ingres Tunnel Endpoints (ITEs) add mid-
layer '*' and outer SEAL/*/IPv4 encapsulations to the inner packets
they inject into a subnetwork, where the outermost IPv4 header
contains the source and destination addresses of the subnetwork
entry/exit points (i.e., the ITE/ETE), respectively. SEAL defines a
new IP protocol type and a new encapsulation sublayer for both
unicast and multicast. The ITE encapsulates an inner IP packet in
mid-layer and outer encapsulations as shown in Figure 1:
+-------------------------+
| |
~ Outer */IPv4 headers ~
| |
I +-------------------------+
n | SEAL Header |
n +-------------------------+ +-------------------------+
e ~ Any mid-layer * headers ~ ~ Any mid-layer * headers ~
r +-------------------------+ +-------------------------+
| | | |
I --> ~ Inner IP ~ --> ~ Inner IP ~
P --> ~ Packet ~ --> ~ Packet ~
| | | |
P +-------------------------+ +-------------------------+
a ~ Any mid-layer trailers ~ ~ Any mid-layer trailers ~
c +-------------------------+ +-------------------------+
k ~ Any outer trailers ~
e +-------------------------+
t
(After mid-layer encaps.) (After SEAL/*/IPv4 encaps.)
Figure 1: SEAL Encapsulation
where the SEAL header is inserted as follows:
o For simple IP/IPv4 encapsulations (e.g.,
[RFC2003][RFC2004][RFC4213]), the SEAL header is inserted between
the inner IP and outer IPv4 headers as: IP/SEAL/IPv4.
o For tunnel-mode IPsec encapsulations over IPv4, [RFC4301], the
SEAL header is inserted between the {AH,ESP} header and outer IPv4
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headers as: IP/*/{AH,ESP}/SEAL/IPv4.
o For IP encapsulations over transports such as UDP, the SEAL header
is inserted immediately after the outer transport layer header,
e.g., as IP/*/SEAL/UDP/IPv4.
SEAL-encapsulated packets include a 32-bit SEAL-ID formed from the
concatenation of the 16-bit ID Extension field in the SEAL header as
the most-significant bits, and with the 16-bit ID value in the outer
IPv4 header as the least-significant bits. (For tunnels that
traverse IPv4 Network Address Translators, the SEAL-ID is instead
maintained only within the 16-bit ID Extension field in the SEAL
header.) Routers within the subnetwork use the SEAL-ID for duplicate
packet detection, and ITEs/ETEs use the SEAL-ID for SEAL segmentation
and reassembly.
SEAL enables a multi-level segmentation and reassembly capability.
First, the ITE can use IPv4 fragmentation to fragment inner IPv4
packets with DF=0 before SEAL encapsulation to avoid lower-level
segmentation and reassembly. Secondly, the SEAL layer itself
provides a simple mid-layer cutting-and-pasting of mid-layer packets
to avoid IPv4 fragmentation on the outer packet. Finally, ordinary
IPv4 fragmentation is permitted on the outer packet after SEAL
encapsulation and used to detect and dampen any in-the-network
fragmentation as quickly as possible.
The following sections specifiy the SEAL-related operations of the
ITE and ETE, respectively:
4.2. ITE Specification
4.2.1. Tunnel Interface MTU
The ITE configures a tunnel virtual interface over one or more
underlying links that connect the border node to the subnetwork. The
tunnel interface must present a fixed MTU to the inner IP layer
(i.e., Layer 3) as the size for admission of inner IP packets into
the tunnel. Since the tunnel interface may support a potentially
large set of ETEs, however, care must be taken in setting a greatest-
common-denominator MTU for all ETEs while still upholding end system
expectations.
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 PTB message returned. However,
the network may not always deliver the necessary PTBs, leading to
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MTU-related black holes [RFC2923]. The ITE therefore requires a
means for conveying 1500 byte (or smaller) packets to the ETE without
loss due to MTU restrictions and without dependence on PTB messages
from within the subnetwork.
In common deployments, there may be many forwarding hops between the
original source and the ITE. Within those hops, there may be
additional encapsulations (IPSec, L2TP, etc.) such that a 1500 byte
packet sent by the original source might grow to a larger size by the
time it reaches the ITE for encapsulation as an inner IP packet.
Similarly, additional encapsulations on the path from the ITE to the
ETE could cause the encapsulated packet to become larger still and
trigger in-the-network fragmentation. In order to preserve the end
system expectations, the ITE therefore requires a means for conveying
these larger packets to the ETE even though there may be links within
the subnetwork that configure a smaller MTU.
The ITE should therefore set a tunnel virtual interface MTU of 1500
bytes plus extra room to accommodate any additional encapsulations
that may occur on the path from the original source (i.e., even if
the underlying links do not support an MTU of this size). The ITE
can set larger MTU values still (up to the maximum MTU size of the
underlying links), but should select a value that is not so large as
to cause excessive PTBs coming from within the tunnel interface (see:
Sections 4.2.2 and 4.2.6). The ITE can also set smaller MTU values,
however care must be taken not to set so small a value that original
sources would experience an MTU underflow. In particular, IPv6
sources must see a minimum path MTU of 1280 bytes, and IPv4 sources
should see a minimum path MTU of 576 bytes.
The inner IP layer consults the tunnel interface MTU when admitting a
packet into the interface. For inner IPv4 packets larger than the
tunnel interface MTU and with the IPv4 Don't Fragment (DF) bit set to
0, the inner IP layer uses IPv4 fragmentation to break the packet
into IPv4 fragments no larger than the tunnel interface MTU then
admits each fragment into the tunnel as an independent packet. For
all other inner packets (IPv4 or IPv6), the ITE admits the packet if
it is no larger than the tunnel interface MTU; otherwise, it drops
the packet and sends an PTB message with an MTU value of the tunnel
interface MTU to the source.
4.2.2. Accounting for Headers
As for any upper-layer protocol, ITEs use the MTU of the underlying
IPv4 interface and the length of the encapsulating SEAL/*/IPv4
headers to determine the maximum-sized upper layer payload. For
example, when the underlying IPv4 interface advertises an MTU of 1500
bytes and the ITE inserts a minimum-length (i.e., 20 byte) IPv4
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header, the ITE sees an upper layer payload of 1480 bytes. When the
ITE inserts IPv4 header options, the size is further reduced by as
many as 40 additional bytes (the maximum length for IPv4 options)
such that as few as 1440 bytes may be available for the upper layer
payload. When other outer '*' encapsulations are inserted before the
SEAL header, the available MTU for the upper layer payload is reduced
further still.
The ITE must additionally account for the length of the SEAL header
as an extra encapsulation that further reduces the size available for
the upper layer payload. The length of the SEAL header is not
incorporated in the IPv4 header length, therefore the network does
not observe the SEAL header as an IPv4 option. In this way, the SEAL
header is inserted after the IPv4 options but before the upper layer
payload in exactly the same manner as for IPv6 extension headers.
4.2.3. Segmentation and Encapsulation
The ITE maintains a SEAL Maximum Transmission Unit (S_MTU) value for
each ETE as soft state within the tunnel interface (e.g., in the IPv4
destination cache). The ITE initializes S_MTU to the MTU of the
underlying IPv4 interface, and decreases or increases S_MTU based on
any ICMPv4 Fragmentation Needed messages received (see: Section
4.2.6). The ITE additionally maintains a SEAL Maximum Reassembly
Unit (S_MRU) value for each ETE. The ITE initializes S_MRU to a
value no larger than 2KB (2048 bytes), and uses this value to
determine when to set the "Dont Reassemble" bit (see below).
The ITE performs segmentation and encapsulation on inner packets that
have been admitted into the tunnel interface. For each inner packet,
the ITE stores the length of any mid-layer '*' encapsulation headers
and trailers (e.g., for '*' = AH, ESP, NULL, etc.) plus the length of
the outer SEAL/*/IPv4 encapsulation headers in a per-packet variable
'ENCAPS' and sets a per-packet variable 'S_MSS" to (S_MTU-ENCAPS).
Next, for inner IPv4 packets with the DF bit set to 0, if the length
of the inner packet is larger than MIN(S_MSS, S_MRU) the ITE uses
IPv4 fragmentation to break the packet into IPv4 fragments no larger
than MIN(S_MSS, S_MRU). For unfragmentable inner packets (e.g., IPv6
packets, IPv4 packets with DF=1, etc.), if the length of the inner
packet is larger than MAX(S_MSS, S_MRU) the ITE drops the packet and
sends an PTB message with an MTU value of MAX(S_MSS, S_MRU) back to
the original source.
The ITE then encapsulates each inner packet/fragment in any mid-layer
'*' headers and trailers. For each such resulting mid-layer packet,
if the packet is no larger than S_MRU but is larger than S_MSS, the
ITE breaks it into N segments (N <= 16) that are no larger than S_MSS
bytes each. Each segment except the final one MUST be of equal
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length, while the final segment MUST be no larger than the initial
segment. The first byte of each segment MUST begin immediately after
the final byte of the previous segment, i.e., the segments MUST NOT
overlap.
Note that this SEAL segmentation is used only for packets that are no
larger than S_MRU; packets that are larger than S_MRU (and also no
larger than S_MSS) are instead encapsulated as a single segment.
Note also that this SEAL segmentation ignores the fact that the mid-
layer packet may be unfragmentable. This segmentation process is a
mid-layer (not an IP layer) operation employed by the ITE to adapt
the mid-layer packet to the subnetwork path characteristics, and the
ETE will restore the packet to its original form during
decapsulation. Therefore, the fact that the packet may have been
segmented within the subnetwork is not observable after
decapsulation.
The ITE next encapsulates each segment in a SEAL header 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ID Extension |P|R|D|M|Segment| Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: SEAL Header Format
where the header fields are defined as follows:
ID Extension (16)
a 16-bit extension of the ID field in the outer IPv4 header;
encodes the most-significant 16 bits of a 32 bit SEAL-ID value.
P (1)
the "Probe" bit. Set to 1 if the ITE wishes to receive an
explicit acknowledgement from the ETE.
R (1)
the "Report Fragmentation" bit. Set to 1 if the ITE wishes to
receive a report from the ETE if any IPv4 fragmentation occurs.
D (1)
the "Dont Reassemble" bit. Set to 1 if the reassembled SEAL
protocol packet is to be discarded by the ETE if any IPv4
reassemly is required.
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M (1)
the "More Segments" bit. Set to 1 if this SEAL protocol packet
contains a non-final segment of a multi-segment mid-layer packet.
Segment (4)
a 4-bit Segment number. Encodes a segment number between 0 - 15.
Next Header (8) an 8-bit field that encodes an IP protocol number
the same as for the IPv4 protocol and IPv6 next header fields.
For single-segment mid-layer packets, the ITE encapsulates the
segment in a SEAL header with (M=0; Segment=0). For N-segment mid-
layer packets (N <= 16), the ITE encapsulates each segment in a SEAL
header with (M=1; Segment=0) for the first segment, (M=1; Segment=1)
for the second segment, etc., with the final segment setting (M=0;
Segment=N-1). For each SEAL-encapsulated packet with Segment=0, the
ITE sets D=0 in the SEAL header if the ETE is permitted to reassemble
the packet if it arrives as multiple IPv4 fragments and/or SEAL
segments; in particular, the ITE sets D=0 in the SEAL header of each
segment for all mid-layer packets no larger than S_MRU. The ITE
instead sets D=1 in the SEAL header if the ETE is to discard the
packet if it arrives as multiple IPv4 fragments and/or SEAL segments;
in particular, the ITE sets D=1 in the SEAL header of each segment
for all mid-layer packets larger than S_MRU.
The ITE next sets the P and R bits in the SEAL header of each segment
as specified in Section 4.2.5, then writes the IP protocol number
corresponding to the mid-layer packet in the SEAL 'Next Header'
field. Next, the ITE encapsulates the segment in the requisite
*/IPv4 outer headers according to the specific encapsulation format
(e.g., [RFC2003], [RFC4213], [RFC4380], etc.), except that it writes
'SEAL_PROTO' in the protocol field of the outer IPv4 header (when
simple IPv4 encapsualtion is used) or writes 'SEAL_PORT' in the outer
destination service port field (e.g., when UDP/IPv4 encapsulation is
used). The ITE finally sets packet identification values and sends
the packets as described in the following sections.
4.2.4. Packet Identification
For the purpose of packet identification, the ITE maintains a 32-bit
SEAL-ID value as per-ETE soft state, e.g. in the IPv4 destination
cache. The ITE randomly-initializes SEAL-ID when the soft state is
created and monotonically increments it (modulo 2^32) for each
successive SEAL protocol packet it sends to the ETE. For each
packet, the ITE writes the least-significant 16 bits of the SEAL-ID
value in the ID field in the outer IPv4 header, and writes the most-
significant 16 bits in the ID Extension field in the SEAL header.
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For tunnels that may traverse IPv4 Network Address Translators
(NATs), the ITE instead maintains SEAL-ID as a 16-bit value that it
randomly-initializes when the soft state is created and monotonically
increments (modulo 2^16) for each successive SEAL protocol packet.
For each packet, the ITE writes SEAL-ID in the ID extension field of
the SEAL header and writes a random 16-bit value in the ID field in
the outer IPv4 header. This requires that both the ITE and ETE
participate in this alternate scheme.
4.2.5. Sending SEAL Protocol Packets
Following SEAL segmentation and encapsulation, the ITE sets DF=0 in
the outer IPv4 header of every outer packet it sends. For
"expendable" packets (e.g., for NULL packets used as probes - see:
Section 4.2.6), the ITE may instead set DF=1.
The ITE then sends each outer packet that encapsulates a segment of
the same mid-layer packet into the tunnel in canonical order, i.e.,
Segment 0 first, followed by Segment 1, etc. and finally Segment N-1.
4.2.6. Sending Probes
When S_MSS is larger than 128, the ITE sends ordinary data packets as
implicit probes to detect in-the-network IPv4 fragmentation and to
determine new values for S_MTU. The ITE sets R=1 in the SEAL header
and DF=0 in the outer IPv4 header of each segment of a SEAL-segmented
packet to be used as an implicit probe, and will receive ICMPv4
Fragmentation Needed messages from the ETE if any IPv4 fragmentation
occurs. When S_MSS=128, the ITE instead sets R=0 in the SEAL header
to avoid generating fragmentation reports for unavoidable in-the-
network fragmentation.
The ITE may additionally send explicit probes periodically to manage
a window of SEAL-IDs of outstanding probes as a means to validate any
ICMPv4 messages it receives. The ITE sets P=1 in the SEAL header of
each segment of a SEAL-segmented packet to be used as an explicit
probe, where the probe can be either an ordinary data packet or a
NULL packet created by setting the 'Next Header' field in the SEAL
header to a value of "No Next Header".
The ITE should periodically probe to detect increases in S_MTU. The
ITE can 1) reset S_MTU to the MTU of the underlying IPv4 interface,
and/or 2) send probes that are larger than the current S_MTU using
either a NULL packet or an ordinary data packet that is padded at the
end by setting the outer IPv4 length field to a larger value than the
packet's true length.
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4.2.7. Processing Raw ICMPv4 Messages
The ITE may receive "raw" ICMPv4 error messages from routers within
the subnetwork that comprise an outer IPv4 header followed by an
ICMPv4 header followed by a portion of the SEAL packet that generated
the error (also known as the "packet-in-error"). For such messages,
the ITE can use the 32-bit SEAL ID encoded in the packet-in-error as
a nonce to confirm that the ICMP message came from an on-path router
within the subnetwork. The ITE MAY process raw ICMPv4 messages as
soft errors indicating that the path to the ETE may be failing, but
it discards any raw ICMPv4 Fragmentation Needed messages for which
the IPv4 header of the packet-in-error has the DF=0.
4.2.8. Processing SEAL-Encapsulated ICMPv4 Messages
In addition to any raw ICMPv4 messages, the ITE may receive SEAL-
encapsulated ICMPv4 messages from subnetwork border nodes that
comprise outer ICMPv4/*/SEAL/*/IPv4 headers followed by a portion of
the packet-in-error. The ITE can use the 32-bit SEAL ID encoded in
the packet-in-error as well as the outer IPv4 and SEAL headers as
nonces to confirm that the ICMP message came from a legitimate ETE.
The ITE then verifies that the SEAL-ID encoded in the packet-in-error
is within the current window of outstanding SEAL-IDs for this ETE.
If the SEAL-ID is outside of the window, the ITE discards the
message; otherwise, it advances the window and processes the message.
The ITE processes SEAL-encapsulated ICMPv4 messages other than ICMPv4
Fragmentation Needed exactly as specified in [RFC0792]. For SEAL-
encapsulated ICMPv4 Fragmentation Needed messages, if the MTU value
reported in the message is zero the ITE discards the message.
Otherwise, if the MTU value is 576 or larger, the ITE sets S_MTU to
this new value. If the MTU value is smaller than 576, the ITE sets
S_MTU to MAX(S_MTU/2, 128).
Note that 576 is assumed as the nominal minimum MTU for common IPv4
links, and accounts for normal-case IPv4 first fragments. When an
ETE reports an MTU smaller than 576, the ITE performs a "limited
halving" of S_MTU that accounts for IPv4 links with unusually small
MTUs or cases in which the ETE otherwise receives an undersized IPv4
first-fragment [RFC1812]. This limited halving may require multiple
iterations of sending probes and receiving ICMPv4 Fragmentation
Needed messages, but will soon converge to a stable S_MTU value.
After deterimining a new value for S_MTU, if the IPv4 header of the
packet-in-error has M=1 and its SEAL header has D=1, the ITE discards
the SEAL/*/IPv4 headers plus any mid-layer '*' headers/trailers (with
a combined length of ENCAPS bytes). The ITE then encapsulates the
remaining inner IP packet portion in a PTB messsage to send back to
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the original source, with the MTU field set to MAX((S_MTU-ENCAPS),
S_MRU).
4.3. ETE Specification
4.3.1. Reassembly Buffer Requirements
ETEs MUST be capable of using IPv4-layer reassembly to reassemble
SEAL protocol outer IPv4 packets of at least 2KB plus the size of the
maximum-length outer SEAL/*/IPv4 plus mid-layer '*' headers and
trailers. For example, for simple IP/SEAL/IPv4 encapsulation, the
ETE must be capable of reassembling an outer IPv4 packet of 2KB + 4 +
60 bytes
The ETE MUST also be capable of using SEAL-layer reassembly to
reassemble mid-layer packets of 2KB.
4.3.2. IPv4-Layer Reassembly
The ETE performs IPv4 reassembly as-normal, and should maintain a
conservative high- and low-water mark for the number of outstanding
reassemblies pending for each ITE. When the size of the reassembly
buffer exceeds this high-water mark, the ETE actively discards
incomplete reassemblies (e.g., using an Active Queue Management (AQM)
strategy) until the size falls below the low-water mark. The ETE
should also use a reduced IPv4 maximum segment lifetime value (e.g.,
15 seconds), i.e., the time after which it will discard an incomplete
IPv4 reassembly for a SEAL protocol packet.
After reassembly, the ETE either accepts or discards the reassembled
packet based on the current status of the IPv4 reassembly cache
(congested vs uncongested). The SEAL-ID included in the IPv4 first-
fragment provides an additional level of reassembly assurance, since
it can record a distinct arrival timestamp useful for associating the
first-fragment with its corresponding non-initial fragments. The
choice of accepting/discarding a reassembly may also depend on the
strength of the upper-layer integrity check if known (e.g., IPSec/ESP
provides a strong upper-layer integrity check) and/or the corruption
tolerance of the data (e.g., multicast streaming audio/video may be
more corruption-tolerant than file transfer, etc.). In the limiting
case, the ETE may choose to discard all IPv4 reassemblies and process
only the IPv4 first-fragment for SEAL-encapsulated error generation
purposes (see the following sections).
4.3.3. Generating SEAL-Encapsulated ICMPv4 Fragmentation Needed
Messages
During IPv4-layer reassembly, the ETE determines whether the packet
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belongs to the SEAL protocol by checking for SEAL_PROTO in the outer
IPv4 header (i.e., for simple IPv4 encapsulation) or for SEAL_PORT in
the outer */IPv4 header (e.g., for '*'=UDP).
During reassembly, when the ETE processes the IPv4 first-fragment
(i.e, one with DF=1 and Offset =0 in the IPv4 header) of a SEAL
protocol packet with (R=1; Segment=0) in the SEAL header, it sends a
SEAL-encapsulated ICMPv4 Fragmentation Needed message back to the ITE
with the MTU value set to the length of the IPv4 first-fragment.
Following reassembly, when the ETE processes a SEAL protocol packet
with (P=1; Segment=0), it sends a SEAL-encapsulated ICMPv4
Fragmentation Needed message back to the ITE with the MTU value set
to 0 (i.e., while treating the reassembled SEAL protocol packet as an
IPv4 first-fragment).
The ETE prepares the ICMPv4 Fragmentation Needed message by
encapsulating as much of the first fragment as possible in outer
*/SEAL/*/IPv4 headers without the length of the message exceeding 576
bytes as shown in Figure 3:
+-------------------------+ -
| | \
~ Outer */SEAL/*/IPv4 hdrs~ |
| | |
+-------------------------+ |
| ICMPv4 Header | |
|(Dest Unreach; Frag Need)| |
+-------------------------+ |
| | > Up to 576 bytes
~ IP/*/SEAL/*/IPv4 ~ |
~ hdrs of first-fragment ~ |
| | |
+-------------------------+ |
| | |
~ Data of first-fragment ~ |
| | /
+-------------------------+ -
Figure 3: SEAL-encapsulated ICMPv4 Fragmentation Needed Message
The ETE next sets D=0, P=0, R=0, M=0 and Segment=0 in the outer SEAL
header, sets the SEAL-ID the same as for any SEAL packet, then sets
the SEAL Next Header field and the fields of the outer */IPv4 headers
the same as for ordinay SEAL encapsulation (see: Sections 4.2.3 and
4.2.4). The ETE then sets outer IPv4 destination address to the
source address of the first-fragment and sets the outer IPv4 source
address to the destination address of the first-fragment. If the
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destination address in the first-fragment was multicast, the ETE
instead sets the outer IPv4 source address to an address assigned to
the underlying IPv4 interface. The ETE finally sends the SEAL-
encapsulated ICMPv4 message to the ITE the same as specified in
Section 4.2.5.
4.3.4. SEAL-Layer Reassembly
Following IPv4 reassembly of a SEAL protocol packet, the ETE adds the
SEAL packet to a SEAL-Layer pending-reassembly queue (if necessary).
If the packet arrived as multiple IPv4 fragments and with D=1 in the
SEAL header, the ETE marks the packet and/or pending reassembly queue
as "discard following reassembly". The ETE also marks the packet as
"discard following reassembly" if the (Next Header, P, R, D) fields
of the packet's SEAL header differ from their respective values in
other SEAL segments already in the queue, i.e., the (Next Header, P,
R, D)-tuple serves as a reassembly nonce.
The ETE performs SEAL-layer reassembly for multi-segment mid-layer
packets through simple in-order concatenation of the encapsulated
segments from N consecutive SEAL protocol packets from the same mid-
layer packet. SEAL-layer reassembly requires the ETE to maintain a
cache of recently received SEAL packet segments for a hold time that
would allow for reasonable inter-segment delays. The ETE uses a SEAL
maximum segment lifetime of 15 seconds for this purpose, i.e., the
time after which it will discard an incomplete reassembly. However,
the ETE should also actively discard any pending reassemblies that
clearly have no opportunity for completion, e.g., when a considerable
number of new SEAL packets have been received before a packet that
completes a pending reassembly has arrived.
The ETE reassembles the mid-layer packet segments in SEAL protocol
packets that contain Segment numbers 0 through N-1, with M=1/0 in
non-final/final segments, respectively, and with consecutive SEAL-ID
values. That is, for an N-segment mid-layer packet, reassembly
entails the concatenation of the SEAL-encapsulated segments with
(Segment 0, SEAL-ID i), followed by (Segment 1, SEAL-ID ((i + 1) mod
2^32)), etc. up to (Segment N-1, SEAL-ID ((i + N-1) mod 2^32)). (For
tunnels that may traverse IPv4 NATs, the ETE instead uses only a 16-
bit SEAL-ID value, and uses mod 2^16 arithmetic to associate the
segments of the same packet.)
4.3.5. Decapsulation and Generating Other ICMPv4 Errors
Following SEAL-layer reassembly, if the packet had the value "No Next
Header" in the SEAL header's Next Header field, or if the packet was
marked "discard following reassembly" the ETE silently discards the
reassembled mid-layer packet; otherwise, the ETE decapsulates the
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inner packet and processes it as normal. If the ETE determines that
the decapsulated inner packet cannot be processed further, it drops
the packet and prepares an appropriate SEAL-encapsulated ICMPv4 error
message and sends it back to the ITE exactly as for ICMPv4
Fragmentation Needed messages (See: Section 4.3.3).
5. SEAL Protocol Specification - Transport Mode
Section 4 specifies the operation of SEAL in "tunnel mode", i.e.,
when there is both an inner and outer IP layer and with a SEAL
encapsulation layer between. However, the SEAL protocol can also be
used in a "transport mode" of operation in which the inner layer
corresponds to an upper layer protocol (e.g., UDP, TCP, etc.) instead
of an additional IP layer.
For example, two TCP endpoints connected to the same subnetwork
region can negotiate the use of transport-mode SEAL for a connection
by inserting a 'SEAL_OPTION' TCP option during the connection
establishment phase. If both TCPs agree on the use of SEAL, their
protocol messages will be carriaged as TCP/SEAL/IPv4 and the
connection will be serviced by the SEAL protocol using TCP (nstead of
an encapsulating tunnel endpoint) as the upper layer protocol. The
SEAL protocol for transport mode otherwise observes the same
specifications as for Section 4.
6. Link Requirements
Subnetwork designers are strongly encouraged to follow the
recommendations in [RFC3819] when configuring link MTUs, where all
IPv4 links SHOULD configure a minimum MTU of 576 bytes. Links that
cannot configure an MTU of at least 576 bytes (e.g., due to
performance characteristics) SHOULD implement transparent link-layer
segmentation and reassembly such that an MTU of at least 576 can
still be presented to the IP layer.
7. End System Requirements
SEAL provides robust mechanisms for returning PTB messages to the
original source, however end systems that send unfragmentable IP
packets larger than 1500 bytes are strongly encouraged to use
Packetization Layer Path MTU Discovery per [RFC4821].
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8. Router Requirements
IPv4 routers within the subnetwork observe the requirements in
[RFC1812], and are strongly encouraged to implement IPv4
fragmentation such that the first fragment is the largest and
approximately the size of the underlying link MTU.
9. IANA Considerations
SEAL_PROTO, SEAL_PORT and SEAL_OPTION are taken from their respective
range of experimental values documented in [RFC3692][RFC4727]. These
values are for experimentation purposes only, and not to be used for
any kind of deployments (i.e., they are not to be shipped in any
products). This document therefore has no actions for IANA.
10. Security Considerations
Unlike IPv4 fragmentation, overlapping fragment attacks are not
possible due to the requirement that SEAL segments be non-
overlapping.
An amplification/reflection attack is possible when an attacker sends
IPv4 first-fragments with spoofed source addresses to an ETE,
resulting in a stream of ICMPv4 Fragmentation Needed messages
returned to a victim ITE. The encapsulated segment of the spoofed
IPv4 first-fragment provides mitigation for the ITE to detect and
discard spurious ICMPv4 Fragmentation Needed messages.
The SEAL header is sent in-the-clear (outside of any IPsec/ESP
encapsulations) the same as for the IPv4 header. As for IPv6
extension headers, the SEAL header is protected only by L2 integrity
checks and is not covered under any L3 integrity checks.
11. Acknowledgments
Path MTU determination through the report of fragmentation
experienced by the final destination was first proposed by Charles
Lynn of BBN on the TCP-IP mailing list in May 1987. An historical
analysis of the evolution of path MTU discovery appears in
http://www.tools.ietf.org/html/draft-templin-v6v4-ndisc-01 and is
reproduced in Appendix A of this document.
The following individuals are acknowledged for helpful comments and
suggestions: Jari Arkko, Fred Baker, Teco Boot, Iljitsch van Beijnum,
Brian Carpenter, Steve Casner, Ian Chakeres, Remi Denis-Courmont,
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Aurnaud Ebalard, Gorry Fairhurst, Joel Halpern, John Heffner, Bob
Hinden, Christian Huitema, Joe Macker, Matt Mathis, Dan Romascanu,
Dave Thaler, Joe Touch, Magnus Westerlund, Robin Whittle, James
Woodyatt and members of the Boeing PhantomWorks DC&NT group.
12. References
12.1. 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.
[RFC1812] Baker, F., "Requirements for IP Version 4 Routers",
RFC 1812, June 1995.
[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.
12.2. Informative References
[FOLK] C, C., D, D., and k. k, "Beyond Folklore: Observations on
Fragmented Traffic", December 2002.
[FRAG] Kent, C. and J. Mogul, "Fragmentation Considered Harmful",
October 1987.
[I-D.ietf-manet-smf]
Macker, J. and S. Team, "Simplified Multicast Forwarding
for MANET", draft-ietf-manet-smf-07 (work in progress),
February 2008.
[I-D.templin-autoconf-dhcp]
Templin, F., Russert, S., and S. Yi, "The MANET Virtual
Ethernet (VET) Abstraction",
draft-templin-autoconf-dhcp-14 (work in progress),
April 2008.
[MTUDWG] "IETF MTU Discovery Working Group mailing list,
gatekeeper.dec.com/pub/DEC/WRL/mogul/mtudwg-log, November
1989 - February 1995.".
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[RFC1063] Mogul, J., Kent, C., Partridge, C., and K. McCloghrie, "IP
MTU discovery options", RFC 1063, July 1988.
[RFC1191] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, August 1996.
[RFC2003] Perkins, C., "IP Encapsulation within IP", RFC 2003,
October 1996.
[RFC2004] Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
October 1996.
[RFC2923] Lahey, K., "TCP Problems with Path MTU Discovery",
RFC 2923, September 2000.
[RFC3692] Narten, T., "Assigning Experimental and Testing Numbers
Considered Useful", BCP 82, RFC 3692, January 2004.
[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.
[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.
[RFC4380] Huitema, C., "Teredo: Tunneling IPv6 over UDP through
Network Address Translations (NATs)", RFC 4380,
February 2006.
[RFC4459] Savola, P., "MTU and Fragmentation Issues with In-the-
Network Tunneling", RFC 4459, April 2006.
[RFC4727] Fenner, B., "Experimental Values In IPv4, IPv6, ICMPv4,
ICMPv6, UDP, and TCP Headers", RFC 4727, November 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.
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[TCP-IP] "TCP-IP mailing list archives,
http://www-mice.cs.ucl.ac.uk/multimedia/mist/tcpip, May
1987 - May 1990.".
Appendix A. Historic Evolution of PMTUD (written 10/30/2002)
The topic of Path MTU discovery (PMTUD) saw a flurry of discussion
and numerous proposals in the late 1980's through early 1990. The
initial problem was posed by Art Berggreen on May 22, 1987 in a
message to the TCP-IP discussion group [TCP-IP]. The discussion that
followed provided significant reference material for [FRAG]. An IETF
Path MTU Discovery Working Group [MTUDWG] was formed in late 1989
with charter to produce an RFC. Several variations on a very few
basic proposals were entertained, including:
1. Routers record the PMTUD estimate in ICMP-like path probe
messages (proposed in [FRAG] and later [RFC1063])
2. The destination reports any fragmentation that occurs for packets
received with the "RF" (Report Fragmentation) bit set (Steve
Deering's 1989 adaptation of Charles Lynn's Nov. 1987 proposal)
3. A hybrid combination of 1) and Charles Lynn's Nov. 1987 proposal
(straw RFC draft by McCloughrie, Fox and Mogul on Jan 12, 1990)
4. Combination of the Lynn proposal with TCP (Fred Bohle, Jan 30,
1990)
5. Fragmentation avoidance by setting "IP_DF" flag on all packets
and retransmitting if ICMPv4 "fragmentation needed" messages
occur (Geof Cooper's 1987 proposal; later adapted into [RFC1191]
by Mogul and Deering).
Option 1) seemed attractive to the group at the time, since it was
believed that routers would migrate more quickly than hosts. Option
2) was a strong contender, but repeated attempts to secure an "RF"
bit in the IPv4 header from the IESG failed and the proponents became
discouraged. 3) was abandoned because it was perceived as too
complicated, and 4) never received any apparent serious
consideration. Proposal 5) was a late entry into the discussion from
Steve Deering on Feb. 24th, 1990. The discussion group soon
thereafter seemingly lost track of all other proposals and adopted
5), which eventually evolved into [RFC1191] and later [RFC1981].
In retrospect, the "RF" bit postulated in 2) is not needed if a
"contract" is first established between the peers, as in proposal 4)
and a message to the MTUDWG mailing list from jrd@PTT.LCS.MIT.EDU on
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Feb 19. 1990. These proposals saw little discussion or rebuttal, and
were dismissed based on the following the assertions:
o routers upgrade their software faster than hosts
o PCs could not reassemble fragmented packets
o Proteon and Wellfleet routers did not reproduce the "RF" bit
properly in fragmented packets
o Ethernet-FDDI bridges would need to perform fragmentation (i.e.,
"translucent" not "transparent" bridging)
o the 16-bit IP_ID field could wrap around and disrupt reassembly at
high packet arrival rates
The first four assertions, although perhaps valid at the time, have
been overcome by historical events leaving only the final to
consider. But, [FOLK] has shown that IP_ID wraparound simply does
not occur within several orders of magnitude the reassembly timeout
window on high-bandwidth networks.
(Authors 2/11/08 note: this final point was based on a loose
interpretation of [FOLK], and is more accurately addressed in
[RFC4963].)
Author's Address
Fred L. Templin (editor)
Boeing Phantom Works
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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