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Conference npss::gigaswitch

Title:	GIGAswitch
Notice:	GIGAswitch/FDDI Jan 97 BL3.1 914.0 documentation 412.1ion 412.1
Moderator:	NPSS::MDLYONS

Created:	Wed Jul 29 1992
Last Modified:	Fri Jun 06 1997
Last Successful Update:	Fri Jun 06 1997
Number of topics:	995
Total number of notes:	4519

945.0. "HOL blocking in Giga/FDDI" by STKHLM::WEBJORN (Gullik Webj�rn Network Advisory) Mon Mar 10 1997 10:40

    
    I have been approaced by the people building the nordic university
    backbone, and who are in charge of the GIX, internet interconnection
    point in Sweden.
    
    The GIX is based on a GigaSwitch FDDI with 20 ports (to be 24, and in
    the short term 2 switches and 44 ports) where all operators plug in
    their routers.
    
    An issue has come up when a destination port is temporarily overloaded.
    
    The claim is that when the Giga is unable to transmit 'cut-through' to 
    a port, the blocked and queued packet causes head-of-line blocking
    in the switch preventing other packets arriving on that input port
    to be switched to other (free) output ports. This affects throughput
    to other connected routers/networks.
    
    Could someone with internal knowledge please comment on this, or send
    me a mail for discussions about this offline.
    
    Gullik

T.R	Title	User	Personal Name	Date	Lines
945.1		NPSS::MDLYONS	Michael D. Lyons DTN 226-6943	`Mon Mar 10 1997 11:03`	7
	...they are correct - although it has nothing to do with cut through. See the keyword head-of-line-blocking for previous discussion on the topic. MDL
945.2	technical customers need the GIGAswitch System Overview paper	NPSS::WATERS	I need an egg-laying woolmilkpig.	`Thu Mar 13 1997 10:51`	10
	You should supply the GIGAswitch(/FDDI) System Description paper to all technical customers. There is some discussion of HOLB effect on switch efficiency, and I thought of the increased efficiency afforded by hunt groups. (Use a hunt group between the two Gsw/FDDI systems, and HOLB will be greatly reduced for traffic flowing between switches.) Since this key paper disappeared from //www.networks.digital.com/ a while ago, you have to get it directly from the FTP archive of the Digital Technical Journal at gatekeeper.dec.com/pub/DECinfo/DTJ/ or something like that. The pointer will of course be in this conference.
945.3	make sure cust isn't distracted by cut-through	NPSS::BENZ	I'm an idiot, and I vote	`Thu Mar 13 1997 12:27`	8
	And be sure that the customer isn't focusing on the cut-through concept, which could lead him to believe that waiting for the token on a ring could lead to HOLB. That isn't so - the packet goes directly into the output linecard's buffers. Cut-through is just that it may start onto the link before the packet finishes getting through the crossbar. \chuck
945.4		AMCFAC::RABAHY	dtn 471-5160, outside 1-810-347-5160	`Fri Mar 14 1997 09:41`	2
	ftp://gatekeeper.dec.com/pub/DEC/DECinfo/DTJ/v6n1/GIGAswitch_System_A_Highperf_0 1jul1994DTJD01SC.txt