Documents uploaded
Callaghan, Robert
Robert.Callaghan at ICN.SIEMENS.COM
Thu Oct 26 14:14:28 EDT 2000
As Brian says, there are several different answers.
In reality, the max length IP packet supported is constrained by the MTU
(max transmission unit) supported by the host at each end (usually a
function of receive buffers) and of the subnets in the path.
Since IP is an internetworking protocol, it runs over a number of
networking technologies and protocols, each with its own frame or packet
size limitation. For example, running IP over an X.25 network (e.g., ISDN
D-channel X.25) might limit the MTU size to 128 octets. Ethernet allows
1500 bytes.
RFC1191 contains a table of MTU sizes for various subnet technologies and
the RFC numbers in which they are defined. I copy the table below (one
advantage of having ASCII specs :-).
Plateau MTU Comments Reference
------ --- -------- ---------
65535 Official maximum MTU RFC 791
65535 Hyperchannel RFC 1044
65535
32000 Just in case
17914 16Mb IBM Token Ring ref. [6]
17914
8166 IEEE 802.4 RFC 1042
8166
4464 IEEE 802.5 (4Mb max) RFC 1042
4352 FDDI (Revised) RFC 1188
4352 (1%)
2048 Wideband Network RFC 907
2002 IEEE 802.5 (4Mb recommended) RFC 1042
2002 (2%)
1536 Exp. Ethernet Nets RFC 895
1500 Ethernet Networks RFC 894
1500 Point-to-Point (default) RFC 1134
1492 IEEE 802.3 RFC 1042
1492 (3%)
1006 SLIP RFC 1055
1006 ARPANET BBN 1822
1006
576 X.25 Networks RFC 877
544 DEC IP Portal ref. [10]
512 NETBIOS RFC 1088
508 IEEE 802/Source-Rt Bridge RFC 1042
508 ARCNET RFC 1051
508 (13%)
296 Point-to-Point (low delay) RFC 1144
296
68 Official minimum MTU RFC 791
Table 7-1: Common MTUs in the Internet
In a general case implementation, I would support at least 1500 bytes MTU
sizes. As for your specific application (video), it might not require 1500
byte packets. Supporting some type of Path MTU Discovery (RFC1191 or
RFC1981) might be useful to avoid fragmentation since some paths don't
support 1500 byte MTUs. For example, many tunneling technologies (GRE,
L2TP, IPSEC) reduce the effective packet size.
Chip
At 10:09 PM 10/25/00 -0400, Rosen, Brian wrote:
>There is the "real" answer, and the practical answer.
>
>The real answer is that depending on what kind of network you are on,
>you could see, in theory, packets of up to 64K bytes. 4K bytes can actually
>be
>seen in some Gig-E systems. Whether you will find a video codec using such
>large packets sizes or not is uncertain; I've never seen one.
>
>It used to be that we thought 1510-odd bytes was the biggest practical
>packet you
>see on a LAN, but that's no longer the case. There is a lot of code around
>that
>fails above that 1500 ish number.
>
>Brian
>
> > -----Original Message-----
> > From: Chris Farmer [mailto:cfarmer at smithmicro.com]
> > Sent: Wednesday, October 25, 2000 9:05 PM
> > To: ITU-SG16 at mailbag.cps.intel.com
> > Subject: Sanity check on Packet Sizes....
> >
> >
> > Okay,
> >
> > I was researching an arcane fact on the theoretical and
> > practical sizes for
> > RTP packet sizes for
> > video conferencing.
> >
> > What I have come up with is:
> > There is specific RFC's about implimenting H.263, H.261,
> > G.723, but I'm
> > looking for just an overall
> > practical size to check input buffer sizes.
> >
> > RFC 1889 RTP packet structure referes to 768, RFC 791
> > RFC 768 UDP refers to obsolete RFC 760.
> > RFC 791 states in the Glossory,
> > ARPANET messages max size is about 1012 octets (8096)bits.
> > ARPANET packet max size is about 126 octets (1008) bits.
> >
> > As a client side EP, should I be concerned about the ARPANET
> > message size or
> > the ARPANET packet size?
> >
> > RFC 760 states:
> > 65,536 octets theoretically, put not practical
> > Page 22, every destination should be able to
> > receive a datagram
> > of 576 octets in one piece or as fragments to be reassembled
> >
> > so my guess here is 4608 bits max? Or is this too low level
> > and there is
> > another RFC that I should reference?
> >
> > Any help in this direction?
> >
> > Thanks,
> >
> >
> > Christopher B. Farmer
> > Software Engineer
> > Smith Micro Software, Inc.
> > (503) 641-1221 ext 28
> > ======================================================================
> > Daily C\C++ Tidbits at www.EngageUs.com\c
> > "Challenges in life are inevitable. Failure is optional" -
> > Roger Crawford
> > ======================================================================
> >
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > For help on this mail list, send "HELP ITU-SG16" in a message to
> > listserv at mailbag.intel.com
> >
>
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Chip Sharp Consulting Engineering
Cisco Systems
"The best way to pick a lock is with a key."
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