Subject: [manet] Thoughts on draft-ietf-manet-jitter-01.txt

From: Fred Baker <[email-masked]>
Date: Wed, 25 Jul 2007 11:08:48 -0500

I read this document, and it seems generally reasonable to me. Some  
of its specific recommendations seem odd though, such as suggesting  
that the jitter imposed on a message might correctly make the inter- 
transmission interval less than the minimum inter-transmission  
interval. In general, I would suggest that the actual inter- 
transmission interval for some message should never be less than the  
minimum inter-transmission interval (or zero if that is not  
specified), and never be greater than the maximum inter-transmission  
interval.

In general, though, the minimum inter-transmission interval is far  
smaller than the maximum, on the order of tens of milliseconds vs  
some number of seconds. Specifying the jitter in terms of the minimum  
inter-transmission interval is therefore a good way to get far too  
much traffic. Generally, the jitter specifies an interval on the  
order of 10-25% of the maximum inter-transmission interval for the  
message in question. For example, if an OSPF Hello is being sent  
nominally every ten seconds, the jitter is uniformly distributed over  
one second, so that actual inter-transmission intervals are somewhere  
between nine and ten seconds.

A review of the following documents will let you know how jitter is  
used in common routing protocols along the lines you have suggested.  
I would suggest that you adjust your text to agree with their  
algorithms, which are proven in the field. As I read the text, I  
wasn't at all sure that it would have considered them to be correct.

http://www.ietf.org/rfc/rfc1142.txt
1142 OSI IS-IS Intra-domain Routing Protocol. D. Oran, Ed.. February
      1990. (Format: TXT=425379, PS=1440951, PDF=490300 bytes) (Status:
      INFORMATIONAL)

http://www.ietf.org/rfc/rfc1765.txt
1765 OSPF Database Overflow. J. Moy. March 1995. (Format: TXT=21613
      bytes) (Status: EXPERIMENTAL)

http://www.ietf.org/rfc/rfc1768.txt
1768 Host Group Extensions for CLNP Multicasting. D. Marlow. March
      1995. (Format: TXT=111499 bytes) (Status: EXPERIMENTAL)

http://www.ietf.org/rfc/rfc4271.txt
4271 A Border Gateway Protocol 4 (BGP-4). Y. Rekhter, Ed., T. Li, Ed.,
      S. Hares, Ed.. January 2006. (Format: TXT=222702 bytes) (Obsoletes
      RFC1771) (Status: DRAFT STANDARD)

For the record, OSPF, RIP, and other routing implementations are  
generally jittered as well, although there isn't an obvious  
specification for that. Timer jitter is generally on the order of 10%.

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