Talk:ATM Adaptation Layer 5

IP standard
I'd like to know what IP standard is being referred to in the statement below:

''Although AAL5 can accept and transfer packets that contain up to 64K octets, the IP standards specify a default MTU of 9180 octets or less. IP must fragment any datagram larger than 9180 octets before passing it to AAL5.''

--Burt Harris 17:33, 16 September 2006 (UTC)


 * Possibly "R. Atkinson, "Default IP MTU for use over ATM Adaptation Layer 5 (AAL5)", Internet Draft, draft-ietf-atm-mtu-00.txt, available by anonymous FTP from ftp.nisc.sri.com, June, 1993.". A quick Google doesn't give me anything more for now.



Here's an abstract: A New Internet Draft is available from the on-line Internet-Drafts directories. This draft is a work item of the IP over Asynchronous Transfer Mode Working Group of the IETF.

Title    : Default IP MTU for use over ATM Adaptation Layer 5 (AAL 5) Author(s) : R. Atkinson Filename : draft-ietf-atm-mtu-00.txt Pages    : 4

This draft defines the default IP MTU for use over ATM Adaptation Layer 5 to be 9180 octets and describes an optional procedure whereby the two ATM endpoints may negotiate a different MTU value for use over an ATM virtual channel. A minimum MTU of 576 octets and a maximum MTU of 65535 octets are also defined.

I guess somebody has interpreted the proposed default MTU as a maximum somewhere. If negotiation is not possible, then it would be the maximum.



Actually, reading the following link makes it clearer - it is documented in a couple of RFCs. http://www.cell-relay.com/mhonarc/cell-relay/2000-Feb/msg00055.html



And here's the text of an extract from Section 6.1 of RFC 1755: Although the default IP over AAL 5/ATM is 9188 bytes, endstations are encouraged to support MTU sizes up to and including 64k.

And Section 7.1 of RFC 2225 7. 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 [16]. 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 [1]. 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.

So, depending on which RFC you want to use, the default is either 9188 or 9180 bytes, with the possibility of negotiation. If you can't or won't guaranteed negotiation, the maximum you can assume that will be available is, therefore, 9180 bytes. WLD 18:14, 16 September 2006 (UTC)

Wikification Project
Bensci54 19:29, 26 September 2007 (UTC)