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Network Working Group F. Templin, Ed.
Internet-Draft Boeing Research & Technology
Updates: RFC1701, RFC2784, RFC2890 (if July 29, 2016
approved)
Intended status: Informational
Expires: January 30, 2017
GRE Tunnel Level Fragmentation
draft-templin-intarea-grefrag-04.txt
Abstract
GRE tunnels use IP fragmentation for delivery packets that exceed the
path MTU. However, IP fragmentation has been shown to be susceptible
to reassembly errors at high data rates, and IP fragments may be
unconditionally dropped by some middleboxes. This document therefore
introduces GRE tunnel level fragmentation, which overcomes these
issues.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 30, 2017.
Copyright Notice
Copyright (c) 2016 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
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publication of this document. Please review these documents
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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 . . . . . . . . . . . . . . . . . . . . . . . . 2
2. GRE Fragmentation Header . . . . . . . . . . . . . . . . . . 3
3. GRE Tunnel Level Fragmentation and Reassembly . . . . . . . . 4
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 5
6. Implementation Status . . . . . . . . . . . . . . . . . . . . 5
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction
GRE is specified in the following RFCs:
[RFC1701][RFC2784][RFC2890][RFC7676]. GRE fragmentation
considerations are further discussed in [RFC7588]. In its current
manifestation, GRE allows for fragmentation of the payload packet
only if it is an IPv4 packet with the Don't Fragment (DF) bit set to
0. GRE also allows for IP fragmentation of the delivery packet, but
IP fragmentation has been shown to be susceptible to reassembly
errors at high data rates [RFC4963] and IP fragments may be
unconditionally dropped by some middleboxes [I-D.taylor-v6ops-
fragdrop].
A third option (introduced here) is for the GRE tunnel to perform
"tunnel level" fragmentation and reassembly on the payload packet at
the GRE layer. In this way, the ingress can fragment the payload
packet (while treating the payload packet's headers as ordinary data)
and encapsulate each fragment in a separate delivery header. The GRE
header requires a new fragment header field to support this.
This tunnel level fragmentation method was first suggested in
Section 3.1.7 of [RFC2764], and also appears in more recent works
[I-D.templin-aerolink] [I-D.herbert-gue-fragmentation].
[I-D.ietf-intarea-tunnels] provides the architectural background for
tunnel fragemntation and reassembly.
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2. GRE Fragmentation Header
Figure 1 shows the GRE header as specified in [RFC1701] but with a
new optional "Fragment Header" and a new control bit "F":
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|C|R|K|S|s|Recur|F| Flags | Ver | Protocol Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Checksum (optional) | Offset (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number (optional) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment Header (Optional) |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Routing (optional)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: GRE Header with Fragment Header
In this format, when the "F" bit (i.e., bit 8) is set to 1 the GRE
header includes a Fragment 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Fragment offset |Res|M| Reserved(2) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| Identification |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: GRE Fragemnt Header Format
The fields of the option are:
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.
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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 Reserved(2): Eight 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 Identification: 40 bits. Identifies fragments of a fragmented
packet.
Note that these formats are the same as specified in
[I-D.herbert-gue-fragmentation] with the exception that the
Reserved(2) field replaces the "Original Type" field since the GRE
header already includes a Protocl Type.
3. GRE Tunnel Level Fragmentation and Reassembly
GRE tunnel level fragmentation treats the entire GRE payload packet
(including the payload headers) as opaque data. The GRE tunnel
ingress breaks the payload packet into N fragments and encapsulates
each fragment in a separate GRE header and GRE delivery header. For
the first fragment, the ingress writes the IEEE802 protocol number in
the Protocol Type field the same as for any GRE packet. For other
fragments, the ingress instead writes the length of the fragment in
the Protocol Type field. This value MUST be no larger than 1500,
which the egress will interpret as a length instead of a protocol
type. (This implies that the maximum size for a non-initial fragment
is 1500 bytes.) The GRE tunnel ingress then sends each fragment to
the GRE tunnel egress.
When the GRE tunnel egress receives the fragments, it reassembles the
GRE payload packet by concatenating the data portions of each
fragment according to their offsets. In order to support this tunnel
level fragmentation and reassembly procedure, the GRE tunnel ingress
must know the maximum sized packet the GRE tunnel egress is capable
of reassembling, i.e., the Maximum Reassembly Unit (MRU). In order
to avoid interactions with Path MTU Discovery, the GRE tunnel egress
MUST configure a minimum MRU of 1500 bytes plus the GRE delivery
encapsulation overhead, and MAY configure a larger MRU.
4. IANA Considerations
This document introduces no IANA considerations.
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5. Security Considerations
Security considerations for GRE apply also to this document.
6. Implementation Status
The SEAL proposal uses tunnel level fragmentation in the same way as
proposed here for GRE. Both SEAL and GRE fragmentation can be
implemented through simple modifications of the widely-avaialble,
well understood and well-tested IP fragmentation code bases.
An implementation of SEAL fragmentation and reassembly has been
published and is available at the following URL:
http://linkupnetworks.org/seal/sealv2-1.0.tgz
7. Acknowledgements
The following are acknowledged for their helpful comments: Tom
Herbert, Carlos Pignataro, Joe Touch.
8. References
8.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<http://www.rfc-editor.org/info/rfc791>.
[RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, "Generic
Routing Encapsulation (GRE)", RFC 1701,
DOI 10.17487/RFC1701, October 1994,
<http://www.rfc-editor.org/info/rfc1701>.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460,
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", RFC 2764, DOI 10.17487/RFC2764, February 2000,
<http://www.rfc-editor.org/info/rfc2764>.
[RFC2784] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
DOI 10.17487/RFC2784, March 2000,
<http://www.rfc-editor.org/info/rfc2784>.
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[RFC2890] Dommety, G., "Key and Sequence Number Extensions to GRE",
RFC 2890, DOI 10.17487/RFC2890, September 2000,
<http://www.rfc-editor.org/info/rfc2890>.
[RFC7588] Bonica, R., Pignataro, C., and J. Touch, "A Widely
Deployed Solution to the Generic Routing Encapsulation
(GRE) Fragmentation Problem", RFC 7588,
DOI 10.17487/RFC7588, July 2015,
<http://www.rfc-editor.org/info/rfc7588>.
[RFC7676] Pignataro, C., Bonica, R., and S. Krishnan, "IPv6 Support
for Generic Routing Encapsulation (GRE)", RFC 7676,
DOI 10.17487/RFC7676, October 2015,
<http://www.rfc-editor.org/info/rfc7676>.
8.2. Informative References
[I-D.herbert-gue-fragmentation]
Herbert, T. and F. Templin, "Fragmentation option for
Generic UDP Encapsulation", draft-herbert-gue-
fragmentation-02 (work in progress), October 2015.
[I-D.ietf-intarea-tunnels]
Touch, D. and W. Townsley, "IP Tunnels in the Internet
Architecture", draft-ietf-intarea-tunnels-03 (work in
progress), July 2016.
[I-D.templin-aerolink]
Templin, F., "Asymmetric Extended Route Optimization
(AERO)", draft-templin-aerolink-70 (work in progress),
July 2016.
[RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly
Errors at High Data Rates", RFC 4963,
DOI 10.17487/RFC4963, July 2007,
<http://www.rfc-editor.org/info/rfc4963>.
Author's Address
Fred L. Templin (editor)
Boeing Research & Technology
P.O. Box 3707
Seattle, WA 98124
USA
Email: fltemplin@acm.org
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