Christian Hammers
2002-Jul-28 04:02 UTC
"For ethernet, no packet uses less than 64 bytes" - why?
Hi Well, subject says all. In Chapter 9.2.2.1, TBF, the parameter mpu or "minimum packet size" is explained as: > A zero-sized packet does not use zero bandwidth. For ethernet, no packet > uses less than 64 bytes. The Minimum Packet Unit determines the minimal > token usage for a packet. In my understanding an ethernet packet needs at least 14 (2*6+2) bytes or 54 bytes if there's an ip header present. If this is a kernel issue it should be noted. To which layer does the term "zero-sized" apply anyway? I mean, how can I tell C to send "nothing"? bye, -christian- -- Christian Hammers WESTEND GmbH - Aachen und Dueren Tel 0241/701333-0 ch@westend.com Internet & Security for Professionals Fax 0241/911879 WESTEND ist CISCO Systems Partner - Authorized Reseller
Hi Please consider applying this patch: - Clearifies the relation between 'rate' and 'burst' - Corrects statement about non-existing input qdisc (-> IMQ) bye, -christian- Index: lartc.db =================================================================== RCS file: /var/cvsroot/2.4routing/lartc.db,v retrieving revision 1.12 diff -u -B -b -r1.12 lartc.db --- lartc.db 20 Jul 2002 14:44:26 -0000 1.12 +++ lartc.db 28 Jul 2002 11:25:06 -0000 @@ -2102,18 +2102,27 @@ </Para> </VarListEntry> <VarListEntry> -<Term>burst/buffer/maxburst</Term> +<Term>rate and burst/buffer/maxburst</Term> <ListItem> <Para> -Size of the bucket, in bytes. This is the maximum amount of bytes that -tokens can be available for instantaneously. In general, larger shaping -rates require a larger buffer. For 10mbit/s on Intel, you need at least -10kbyte buffer if you want to reach your configured rate! +Rate is the amount of tokens that are tried to put into the bucket every +second specified in mbit/s. +</Para> +<Para> +Burst is the size of the bucket, in bytes. This is the maximum amount of bytes +that tokens can be available for instantaneously. </Para> - <Para> -If your buffer is too small, packets may be dropped because more tokens -arrive per timer tick than fit in your bucket. +In general, larger shaping rates require a larger buffer. Remember that if +your buffer is too small, packets may be dropped because regardless how +much tokens the 'rate' parameter tries to put in the bucket, the ones that +actually fit into it are not sufficient for all waiting data. +</Para> +<Para>To archive a throughput of 10mbit/s on Intel you have to set the buffer +to 12.8kbyte. Calculation: On Intel the kernel HZ variable is 100, which means +that the token filler function is called 100 times per second. 100 times +12.8kbyte are 1310720byte or 10485760bit that are pushed into the bucket - +assuming a high enough value for the rate variable. </Para> </VarListEntry> <VarListEntry> @@ -2125,13 +2134,6 @@ token usage for a packet. </Para> </VarListEntry> -<VarListEntry> -<Term>rate</Term> -<ListItem> -<Para> -The speedknob. See remarks above about limits! -</Para> -</VarListEntry> </VariableList> </Para> @@ -2502,8 +2504,10 @@ <ListItem> <Para> Delaying or dropping packets in order to make traffic stay below a -configured bandwidth. In Linux, policing can only drop a packet and not -delay it - there is no 'ingress queue'. +configured bandwidth. In Linux, policing used to be only able to drop a packet +and not delay it - there was no 'ingress queue'. A currently developed solution +is the 'intermediate queueing device' (IMQ). It is discussed in a seperate +chapter. </Para> </VarListEntry> <VarListEntry> -- Christian Hammers WESTEND GmbH - Aachen und Dueren Tel 0241/701333-0 ch@westend.com Internet & Security for Professionals Fax 0241/911879 WESTEND ist CISCO Systems Partner - Authorized Reseller