Path: utzoo!attcan!uunet!ncrlnk!ncr-sd!hp-sdd!hplabs!ucbvax!PARK-STREET.BBN.COM!brescia From: brescia@PARK-STREET.BBN.COM (Mike Brescia) Newsgroups: comp.protocols.tcp-ip Subject: Re: UDP [IP] max datagram size ? Message-ID: <8809221448.AA03993@ucbvax.Berkeley.EDU> Date: 22 Sep 88 12:55:11 GMT References: <170@hsi86.hsi.UUCP> Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 23 > Is there an agreed upon max datagram size for UDP packets ?? There is an assumption at the IP level that says an IP implementation does not have to be prepared to reassemble and buffer individual packets of up to 576 octets. This is listed in RFC791 (IP protocol spec) in the description of the IP header Total Length field. "It is recommended that hosts only send datagrams larger than 576 octets if they have assurance that the destination is prepared to accept the larger datagrams." This assurance is only provided by specifications in higher layer protocols. Some IP implementations are capable of reassembling up to the full 64K datagram size without prior agreement. For TCP, there is an exchange at the time of opening the connection where the 2 parties can agree on datagram sizes larger (or smaller) than 576, using the TCP MSS option. Two hosts on an ethernet may well negotiate for 1024 data octets. In answer to your question about UDP, I would have to say that, yes, there is an agreement on datagram size, but it is specific to the higher layer protocol. TFTP uses 576. Sun NFS uses 8K. I think you would have to implement the UDP part of a system so that it would be able to handle whatever a higher layer would want.