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Re: Path MTU discovery - Matt Mathis
this was never published as rja wanted to add 100kg of fluff and
confusion.
randy
---
INTERNET-DRAFT Tony Li, Procket
draft-ymbk-mtu-00.txt Ran Atkinson, Extreme
2001.12.23 Randy Bush, AT&T
Recommendations for MTU when Beyond Ethernet
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
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0. Abstract
While the Ethernet standard [ETHERNET] is limited to MTUs in the
range of 48 to 1500 bytes of payload, it is often advantageous to
configure the MTU to suit the internet WAN or LAN environment. This
document suggests the most common ways this is done.
1. Fragmentation Considered Harmful
When a packet is larger than the Maximum Transmission Unit (MTU) of
the interface to which is is being forwarded, the packet is
'fragmented' into two or more packets [RFC791]. Fragmentation is a
work-around, not a feature, and is undesirable [KENTMOGUL]. Path MTU
discovery [RFC1191] dynamically measures the MTU on an end-to-end
path so that the sender can adjust the originating MTU, but it is far
from a panacea [RFC2923].
Therefore, to minimize fragmentation on Ethernet-like links, we
recommend configuring them to the MTU of their larger neighbors.
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2. Wide Area Network (WAN)
Most IP backbones are running at rates of DS3 and above. These WAN
links have a default MTU of 4470 bytes, aka '4k'. Hence, to prevent
backbone packet fragmentation, it is recommended that operators
configure inter-router Ethernet-like links and switches to an MTU of
4470 bytes.
3. Local Area Network (LAN)
LANs with high-performance servers are often configured so that
Ethernet-like links and switches between the servers can accommodate
the common transfers of the servers, often 8192 bytes of data, with
an MTU of 9180, aka '9k'.
4. Mixed Environment
If a server farm LAN (2) is connected to a WAN (1), traffic from the
LAN to the WAN will be fragmented to match the WAN MTU. Hence it is
more sensible to run the LAN with the WAN's 4k MTU.
5. Warnings
The Ethernet CRC was designed for the Ethernet MTU range, and
provides weaker checking when MTUs are configured larger than that
range. IP packet fragmentation increases vulnerability and makes
filtering more complex [RFC1858].
It is often very hard to debug when all ends of an Ethernet-like link
are not properly configured to the same MTU.
6. Security Considerations
Non-standard MTUs are not known to open new security issues.
Fragmentation is known to complicate security tools [RFC1858]
[RFC3128].
7. Acknowledgments
I dunno
8. References
[KENTMOGUL]
Kent, C. and J. Mogul, "Fragmentation Considered Harmful", Proc.
ACM SIGCOMM 1987, August 1987.
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[ETHERNET]
IEEE Std. 802.3 [2000 Edition], ISO/IEC 8802-3:2000.
[RFC791]
Postel, J., Editor, "Internet Protocol - DARPA Internet Program
Protocol Specification", STD 5, RFC 791, September 1981.
[RFC1191]
J.C. Mogul, S.E. Deering,"Path MTU discovery", RFC 1191, November
1990.
[RFC1858]
G. Ziemba, D. Reed, P. Traina, "Security Considerations for IP
Fragment Filtering", RFC 1858, October 1995.
[RFC2923]
K. Lahey, "TCP Problems with Path MTU Discovery", RFC 2923, Septem-
ber 2000.
[RFC3128]
I. Miller, "Protection Against a Variant of the Tiny Fragment
Attack (RFC 1858)", RFC 3128, June 2001.
9. Authors' Addresses
Tony Li
1100 Cadillac Ct.
Milpitas, CA 95035
tony.li@tony.li
Ran Atkinson
ADDRESS INFO PLEASE
Randy Bush
5147 Crystal Springs
Bainbridge Island, WA US-98110
+1 206 780 0431
randy@psg.com
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10. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to others,
and derivative works that comment on or otherwise explain it or assist in
its implementation may be prepared, copied, published and distributed, in
whole or in part, without restriction of any kind, provided that the above
copyright notice and this paragraph are included on all such copies and
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way, such as by removing the copyright notice or references to the Internet
Society or other Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for copyrights
defined in the Internet Standards process must be followed, or as required
to translate it into languages other than English.
The limited permissions granted above are perpetual and will not be revoked
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DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
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PARTICULAR PURPOSE.
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