jst@cca.ucsf.edu (Joe Stong) (01/04/90)
Someone I know claims that most TCP/IP packets transmitted on T1 lines are
imbedded in X.25 protocol packets. Is this true, or what is actually
used? Or it TCP/IP alone with maybe an alternate checksum (CRC) sufficient?
Joe Stong jst@cca.ucsf.edudyer@spdcc.COM (Steve Dyer) (01/04/90)
In article <2695@ucsfcca.ucsf.edu> jst@cca.ucsf.edu (Joe Stong) writes: >Someone I know claims that most TCP/IP packets transmitted on T1 lines are >imbedded in X.25 protocol packets. Is this true, or what is actually >used? Or it TCP/IP alone with maybe an alternate checksum (CRC) sufficient? T1 is just a bit pipe. You usually have some sort of synchronous hardware/firmware on each end which also provides some form of point-to- point protocol. This could be byte-oriented DDCMP, as with DEC DMR-11 sync interfaces, or the bit-oriented LAPB/HDLC, sometimes referred to as X.25 level II, or even a minimal encapsulation with checksum, as with the DEC DMR-11 in "maintenance mode". What's important is that the two ends agree. Proprietary routers have already made their choices. I guess in the future, some form of the upcoming Point-to-Point Protocol could be used. I don't have any data on the reliability of data streams over T1 lines, which might drive a selection of protocol. We routinely ran in maintenance mode with DMR-11's between Harvard and MIT in the early days of the JVNCnet. The hardware discarded packets with bad link-level checksums, but otherwise did not perform any retransmissions. The sentiment was that it was undesirable to have lossiness be transformed into delay in the calculation of TCP round trip time and retransmission timeouts. -- Steve Dyer dyer@ursa-major.spdcc.com aka {ima,harvard,rayssd,linus,m2c}!spdcc!dyer dyer@arktouros.mit.edu, dyer@hstbme.mit.edu
Dave_Katz@UM.CC.UMICH.EDU (01/04/90)
>Someone I know claims that most TCP/IP packets transmitted on T1 lines are >imbedded in X.25 protocol packets. Is this true, or what is actually >used? Or it TCP/IP alone with maybe an alternate checksum (CRC) sufficient? It depends on which manufacturer makes the routers. A number of proprietary schemes are in use today; I know of at least one vendor that uses pieces of X.25. Running IP directly over the data link is insufficient if you ever intend to run multiple protocol families over the same link, since you need a method of demultiplexing the different protocols. For example, some routers use a proprietary header containing the ethertype of the protocol. The Point-to-Point protocol addresses these issues and is available as an Internet Draft; vendors will be quick to implement it once it becomes an Internet standard, after which routers from different vendors will actually interwork on point-to-point links.
krol@ux1.cso.uiuc.edu (Ed Krol) (01/04/90)
jst@cca.ucsf.edu (Joe Stong) writes: >Someone I know claims that most TCP/IP packets transmitted on T1 lines are >imbedded in X.25 protocol packets. Is this true, or what is actually >used? Or it TCP/IP alone with maybe an alternate checksum (CRC) sufficient? > Joe Stong jst@cca.ucsf.edu Not having counted packets much lately, but I would bet that most IP packets on T1 lines are imbedded in HDLC packets. Since that is what a lot of commercial routers use on their serial lines.