RFC 1626 (rfc1626) - Page 1 of 5

Default IP MTU for use over ATM AAL5

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Network Working Group                                        R. Atkinson
Request for Comments: 1626                     Naval Research Laboratory
Category: Standards Track                                       May 1994


                  Default IP MTU for use over ATM AAL5

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Default Value for IP MTU over ATM AAL5

   Protocols in wide use throughout the Internet, such as the Network
   File System (NFS), currently use large frame sizes (e.g. 8 KB).
   Empirical evidence with various applications over the Transmission
   Control Protocol (TCP) indicates that larger Maximum Transmission
   Unit (MTU) sizes for the Internet Protocol (IP) tend to give better
   performance.  Fragmentation of IP datagrams is known to be highly
   undesirable. [KM87] It is desirable to reduce fragmentation in the
   network and thereby enhance performance by having the IP Maximum
   Transmission Unit (MTU) for AAL5 be reasonably large.  NFS defaults
   to an 8192 byte frame size.  Allowing for RPC/XDR, UDP, IP, and LLC
   headers, NFS would prefer a default MTU of at least 8300 octets.
   Routers can sometimes perform better with larger packet sizes because
   most of the performance costs in routers relate to "packets handled"
   rather than "bytes transferred".  So there are a number of good
   reasons to have a reasonably large default MTU value for IP over ATM
   AAL5.

   RFC 1209 specifies the IP MTU over SMDS to be 9180 octets, which is
   larger than 8300 octets but still in the same range. [RFC-1209] There
   is no good reason for the default MTU of IP over ATM AAL5 to be
   different from IP over SMDS, given that they will be the same
   magnitude.  Having the two be the same size will be helpful in
   interoperability and will also help reduce incidence of IP
   fragmentation.

   Therefore, the default IP MTU for use with ATM AAL5 shall be 9180
   octets.  All implementations compliant and conformant with this
   specification shall support at least the default IP MTU value for use
   over ATM AAL5.





Atkinson
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