Re: What's your favorite MTU?
Drew Daniel Perkins <ddp+@andrew.cmu.edu> Sat, 14 April 1990 03:47 UTC
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Date: Fri, 13 Apr 90 23:44:28 -0400 (EDT)
From: Drew Daniel Perkins <ddp+@andrew.cmu.edu>
To: mtudwg
Subject: Re: What's your favorite MTU?
Cc: postel@isi.edu
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Excerpts from mail: 13-Apr-90 Re: What's your favorite MTU? Jeffrey Mogul@decwrl.dec (2452) > Don't 16Mbit TRs support much larger MTUs? > RFC1042 mentions that 802.5 systems can potentially support these MTUs: > 8188, 4092, 2044, 1020, 508 > It isn't clear from the RFC just which of these actually get used. > I have plateaus at or just below all of these values, except for > 4092. Putting one at 4092 would probably cause some problems as > FDDI becomes ubiquitous; either there would be an extra RTT spent > getting from an FDDI MTU to an Ethernet MTU, or I would have to > remove the FDDI plateau (now changed to 4352, in the lastest > draft RFC) and potentially waste 6% of the FDDI MTU. If you > expect that 802.5 networks with MTU=4092 are going to be common, > then I will have to take that into account. > RFC 1042 mentions that 802.5 source routing BRIDGES support those numbers; it does NOT say that 802.5 TOKEN RINGS support those sizes. Understanding this requires an understanding of IBM source routing. Source routing utilizes an IBM (not IEEE) specified field in the packet called a Routing Information Field (RIF). The RIF includes a field called the Largest Frame (LF) field. It is used to find the maximum MTU supported along a source routed path (same thing we're trying to do actually). I guess they didn't want to use two (or more) full octets, so they encoded it, originally as a 3-bit field. That encoding is what is described in RFC 1042. I'm not certain of this, I would guess that IBM chose the values based on the formula 2^(9+x) + 4, giving them LLC data lengths corresponding to powers of two (512, 1024, 2048, 4096, and 8192; the LLC header is 4 octets). However, somebody (probably IEEE) later decided that the length field should be four bits and should indicate more useful values, where useful means that they correspond to other 802 networks. Below is a table containing all the values. The ones marked with * are the original IBM codes which are now "reserved". LF 802 MAC Info IP Code Network Length Length MTU ---------------------------------------- 0000* 552 516 508 0001 " " " 0010* 1064 1028 1020 0011 802.3 1536 1500 1492 0100* 2088 2052 2044 0101 " " " 0110* 4136 4100 4092 0111 802.5 4508 4472 4464 1000* 8232 8196 8188 1001 ~802.4 8227 8191 8183 Now to make the waters muddier... The lengths above do NOT correspond to the MTUs of IBM Token Rings. Theoretically, the maximum packet size on a token ring is based on the maximum time a node may hold a token. This varies with the number of nodes on the ring and the ring speed. Worse yet, various token ring implementations have all supported different maximum packet lengths. The original TI implementation will support an IP MTU of up to 4464 octets, which (I think) corresponds to a ring with the maximum number of nodes. However, IBM's original token ring implementation (the IBM PC Token Ring Card) will only support an IP MTU of 2002. Therefore, in RFC 1042 we suggested that all implementations support ATLEAST 2002. You CAN support more (like the full 4464), and I think some people like cisco actually do. In the meantime, IBM has made two more changes. Their latest 4 Mb token ring interface seems to support an IP MTU of 4408. The 16 Mb interface should support up to 17914 (increasing the data rate increases the MTU with the same token hold time). In conclusion, for the purposes of this WG, I would suggest that we use the values 2002, 4408, and 17914 for the token ring MTUs of interest. Drew
- What's your favorite MTU? Jeffrey Mogul
- Re: What's your favorite MTU? Art Berggreen
- Re: What's your favorite MTU? Jeffrey Mogul
- Re: What's your favorite MTU? Drew Daniel Perkins
- Re: What's your favorite MTU? Drew Daniel Perkins
- What's your favorite MTU? Fred Bohle acc_gnsc
- Re: What's your favorite MTU? Jeffrey Mogul
- Re: What's your favorite MTU? Drew Daniel Perkins