INTERNET-DRAFT T. Herbert
Intended Status: Proposed Standard Facebook
Expires: September 2015 F. Templin
Boeing Research & Technology
March 25, 2015
Fragmentation option for Generic UDP Encapsulation
draft-herbert-gue-fragmentation-00
Abstract
This specification describes a fragmentation and reassembly
capability with an associated header option for Generic UDP
Encapsulation.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Option format . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 Procedures . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1 Fragmentation . . . . . . . . . . . . . . . . . . . . . . . 6
3.2 Reassembly . . . . . . . . . . . . . . . . . . . . . . . . . 9
4 Security Considerations . . . . . . . . . . . . . . . . . . . . 11
5 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 11
6 References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.1 Normative References . . . . . . . . . . . . . . . . . . . 11
6.2 Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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1 Introduction
This specification describes a fragmentation and reassembly
capability in Generic UDP Encapsulation (GUE) [I.D.herbert-gue]. This
entails adding a GUE option and procedures for fragmentation and
reassembly in the encapsulation layer. This specification adapts the
procedures for IP fragmentation and reassembly described in [RFC0791]
and [RFC2460]. Fragmentation may be performed on both data and
control messages in GUE.
1.1 Motivation
This section describes the motivation for having a fragmentation
option in GUE.
MTU and fragmentation issues with In-the-Network Tunneling are
described in [RFC4459]. Considerations need to be made when a packet
is received at a tunnel ingress point which may be too large to
traverse the path between tunnel endpoints.
There are four suggested alternatives in [RFC4459] to deal with this:
1) Fragmentation and Reassembly by the Tunnel Endpoints
2) Signaling the Lower MTU to the Sources
3) Encapsulate Only When There is Free MTU
4) Fragmentation of the Inner Packet
Many tunneling protocol implementations have assumed that
fragmentation should be avoided, and in particular alternative #3
seems preferred for deployment. In this case, it is assumed that an
operator can configure the MTUs of links in the paths of tunnels to
ensure that they are large enough to accommodate any packets and
required encapsulation overhead. This method, however, may not be
feasible in certain deployments and may be prone to misconfiguration
in others.
Similarly, the other alternatives have drawbacks that are described
in [RFC4459]. Alternative #2 implies use of something like Path MTU
Discovery which is not known to be sufficiently reliable. Alternative
#4 is not permissible with IPv6 or when the DF bit is set for IPv4,
and it also introduces other known issues with IP fragmentation.
For alternative #1, fragmentation and reassembly at the tunnel
endpoints, there are two possibilities: encapsulate the large packet
and then perform IP fragmentation, or segment the packet and then
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encapsulate each segment (a non-IP fragmentation approach).
Performing IP fragmentation on an encapsulated packet has the same
issues as that of normal IP fragmentation. Most significant of these
is that the Identification field is only sixteen bits in IPv4 which
introduces problems with wraparound as described in [FRAGHRM].
The second possibility follows the suggestion expressed in [RFC2764]
and the fragmentation feature described in the AERO protocol
[I.D.templin-aerolink], that is for the tunneling protocol itself to
incorporate a segmentation and reassembly capability that operates at
the tunnel level. In this method fragmentation is part of the
encapsulation and an encapsulation header contains the information
for reassembly. This is different from IP fragmentation in that the
IP headers of the original packet are not replicated for each
fragment.
Incorporating fragmentation into the encapsulation protocol has some
advantages:
o A 32 bit identifier can be defined to avoid issues of the 16 bit
Identification in IPv4.
o Encapsulation mechanisms for security and identification such as
virtual network identifiers can be applied to each segment.
o This allows the possibility of using alternate fragmentation and
reassembly algorithms (e.g. fragmentation with Forward Error
Correction).
o Fragmentation is transparent to the underlying network so it is
unlikely that fragmented packet will be unconditionally dropped
as might happen with IP fragmentation.
1.2 Scope
This specification describes the mechanics of fragmentation in
Generic UDP Encapsulation. The operational aspects and details for
higher layer implementation must be considered for deployment, but
are considered out of scope for this document. The AERO protocol
[I.D.templin-aerolink] defines one use case of fragmentation with
encapsulation.
1.3 Terminology
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 RFC 2119 [RFC2119].
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2 Option format
Fragments in GUE are sent with a fragmentation option in the GUE
header. The format of this option is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment offset |Res|M| Orig-proto | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
o Fragment offset: This field indicates where in the datagram this
fragment belongs. The fragment offset is measured in units of 8
octets (64 bits). The first fragment has offset zero.
o Res: Two bit reserved field. Must be set to zero for
transmission. If set to non-zero in a received packet then the
packet MUST be dropped.
o M: More fragments bit. Set to 1 when there are more fragments
following in the datagram, set to 0 for the last fragment.
o Orig-proto: The control type (when C bit is set) or the IP
protocol (when C bit is not set) of the fragmented packet.
o Reserved: Must be set to 0 on transmission. If set to non-zero
in a received packet then the packet MUST be dropped.
o Identification: Identifies fragments of a fragmented packet.
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The format of the fragmentation option within the GUE header is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0x0|C| Hlen | Proto/ctype |V|SEC|K|F| Flags |E|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ VNID, security fields, checksum (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment offset |Res|M| Orig-proto | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Extension flags(optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Extension fields (optional) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Pertinent fields to fragmentation are:
o C: This bit is set for each fragment based on the whether the
original packet being fragmented is a control or data message.
o Proto/ctype - For the first fragment (fragment offset is zero)
this is set to that of the original packet being fragmented
(either will be a control type or IP protocol). For other
fragments, this is set to zero for a control message being
fragmented, or to "No next header" (protocol number 59) for a
data message being fragmented.
o F bit - Set to indicate presence of the fragmentation option
field.
3 Procedures
3.1 Fragmentation
If an encapsulator determines that a packet must be fragmented (e.g.
the packets size exceed the Path MTU of the tunnel) it may divide the
packet into fragments and send each fragment as a separate GUE
packet, to be reassembled at the decapsulator (tunnel egress).
For every packet that is to be fragmented, the source node generates
an Identification value. The Identification must be different than
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that of any other fragmented packet sent within the past 60 seconds
(Maximum Segment Lifetime) with the same tunnel identification-- that
is the same outer source and destination addresses, same UDP ports,
same orig-proto, and same virtual network identifier if present.
The initial, unfragmented, and unencapsulated packet is referred to
as the "original packet". This will be a layer 2 packet, layer 3
packet, or the payload of a GUE control message:
+-------------------------------//------------------------------+
| Original packet |
| (e.g. an IPv4, IPv6, Ethernet packet) |
+------------------------------//-------------------------------+
Fragmentation and encapsulation are performed on the original packet
in sequence. First the packet is divided up in to fragments, and then
each fragment is encapsulated. Each fragment, except possibly the
last ("rightmost") one, is an integer multiple of 8 octets long.
Fragments MUST be non-overlapping. The number of fragments should be
minimized, and all but the last fragment should be approximately
equal in length.
The fragments are transmitted in separate "fragment packets" as
illustrated:
+--------------+--------------+---------------+--//--+----------+
| first | second | third | | last |
| fragment | fragment | fragment | .... | fragment |
+--------------+--------------+---------------+--//--+----------+
Each fragment is encapsulated as the payload of a GUE packet. This is
illustrated as:
+------------------+----------------+-----------------------+
| IP/UDP header | GUE header | first |
| header | w/ frag option | fragment |
+------------------+----------------+-----------------------+
+------------------+----------------+-----------------------+
| IP/UDP header | GUE header | second |
| header | w/ frag option | fragment |
+------------------+----------------+-----------------------+
o
o
+------------------+----------------+-----------------------+
| IP/UDP header | GUE header | last |
| header | w/ frag option | fragment |
+------------------+----------------+-----------------------+
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Each fragment packet is composed of:
(1) Outer IP and UDP headers as defined for GUE encapsulation.
o The IP addresses and UDP destination port must be the same
for all fragments of a fragmented packet.
o The source port selected for the inner flow identifier must
be the same value for all fragments of a fragmented packet.
(2) A GUE header that contains:
o The C bit which is set to the same value for all the
fragments of a fragmented packet based on whether a control
message or data message was fragmented.
o A proto/ctype. In the first fragment this is set to the
value corresponding to the next header of the original
packet and will be either an IP protocol or a control type.
For subsequent fragments, this field is set to 0 for a
fragmented control message or 59 (no next header) for a
fragmented data messages.
o The F bit is set and fragment option is present. See below.
o Other GUE options. Note that options apply to the individual
GUE packet. For instance, the security option would be
validated before reassembly.
(2) The GUE fragmentation option. The option contents include:
o Orig-proto that identifies the first header of the original
packet.
o A Fragment Offset containing the offset of the fragment, in
8-octet units, relative to the start of the of the original
packet. The Fragment Offset of the first ("leftmost")
fragment is 0.
o An M flag value of 0 if the fragment is the last
("rightmost") one, else an M flag value of 1.
o The Identification value generated for the original packet.
(3) The fragment itself.
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3.2 Reassembly
At the destination, fragment packets are decapsulated and reassembled
into their original, unfragmented form, as illustrated:
+-------------------------------//------------------------------+
| Original packet |
| (e.g. an IPv4, IPv6, Ethernet packet) |
+------------------------------//-------------------------------+
The following rules govern reassembly:
The IP/UDP/GUE headers of each packet are retained until all
fragments have arrived. The reassembled packet is then composed
of the decapsulated payloads in the GUE fragments, and the
IP/UDP/GUE headers are discarded
When a GUE packet is received with the fragment option, the
proto/ctype in the GUE header must be validated. In the case
that the packet is a first fragment (fragment offset is zero),
the proto/ctype in the GUE header must equal the orig-proto
value in the fragmentation option. For subsequent fragments
(fragment offset is non-zero) the proto/ctype in the GUE header
must be 0 for a control message or 59 (no-next-hdr) for a data
message. If the proto/ctype value is invalid then the packet
MUST be dropped.
An original packet is reassembled only from GUE fragment packets
that have the same outer Source Address, Destination Address,
UDP source port, UDP destination port, GUE header C bit, virtual
network identifier if present, orig-proto value in the
fragmentation option, and Fragment Identification. The protocol
type or control message type (depending on the C bit) for the
reassembled packet is the value of the GUE header proto/ctype
field in the first fragment.
The following error conditions may arise when reassembling fragmented
packets with GUE encapsulation:
If insufficient fragments are received to complete reassembly of
a packet within 60 seconds (or a configurable period) of the
reception of the first-arriving fragment of that packet,
reassembly of that packet must be abandoned and all the
fragments that have been received for that packet must be
discarded.
If the length of a fragment, as derived from the GUE fragment
packet's Payload Length field, is not a multiple of 8 octets and
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the M flag of that fragment is 1, then that fragment must be
discarded.
If the length and offset of a fragment are such that the Payload
Length of the packet reassembled from that fragment would exceed
65,535 octets, then that fragment must be discarded.
If a fragment overlaps another fragment already saved for
reassembly then the portion of data in the new fragment that
overlaps the existing fragment must be ignored.
If the first fragment is too small then it is possible that it
does not contain the necessary headers for a stateful firewall.
Sending small fragments like this has been used as an attack on
IP fragmentation. To mitigate this problem, an implementation
should ensure that the first fragment contains the headers of
the encapsulated packet at least through the transport header.
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4 Security Considerations
Exploits that have been identified with IP fragmentation are
conceptually applicable to GUE fragmentation.
Attacks on GUE fragmentation can be mitigated by:
o Hardened implementation that applies applicable techniques from
implementation of IP fragmentation.
o Application of GUE security [I.D.hy-gue-4-secure-transport] or
IPsec [RFC4301]. Security mechanisms can prevent spoofing of
fragments from unauthorized sources.
o Implement fragment filter techniques for GUE encapsulation as
described in [RFC1858] and [RFC3128].
o Do not accepted data in overlapping segments.
o Enforce a minimum size for the first fragment.
5 IANA Considerations
GUE fragmentation defines one flag bit in the GUE header and a
corresponding 64-bit field.
6 References
6.1 Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[I.D.herbert-gue] T. Herbert, L. Yong, and O. Zia, "Generic UDP
Encapsulation" draft-herbert-gue-03
6.2 Informative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
1981, <http://www.rfc-editor.org/info/rfc791>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998,
<http://www.rfc-editor.org/info/rfc2460>.
[RFC2764] Gleeson, B., Lin, A., Heinanen, J., Armitage, G., and A.
Malis, "A Framework for IP Based Virtual Private Networks",
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RFC 2764, February 2000, <http://www.rfc-
editor.org/info/rfc2764>.
[RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security
Considerations for IP Fragment Filtering", RFC 1858,
October 1995, <http://www.rfc-editor.org/info/rfc1858>.
[RFC3128] Miller, I., "Protection Against a Variant of the Tiny
Fragment Attack (RFC 1858)", RFC 3128, June 2001,
<http://www.rfc-editor.org/info/rfc3128>.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005,
<http://www.rfc-editor.org/info/rfc4301>.
[I.D.templin-aerolink] F. Templin, "Transmission of IP Packets over
AERO Links" draft-templin-aerolink-52.txt
[FRAGHRM] M. Mathis, J, Heffner, and B. Chandler, "Fragmentation
Considered Very Harmful", draft-mathis-frag-harmful-00
[I.D.hy-gue-4-secure-transport] L. Yong and T. Herbert, "Generic UDP
Encapsulation (GUE) for Secure Transport" draft-hy-gue-4-
secure-transport-01
Authors' Addresses
Tom Herbert
Facebook
Menlo Park, CA
USA
Email: tom@herbertland.com
Fred L. Templin
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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