Subject: RE: [Tsvwg] Re: I-D ACTION:draft-ietf-sigtran-usctp-01.txt

From: [email-masked]
Date: Tue, 27 Feb 2001 14:55:08 +0200

Hi Jon,

If I could jump in here.

> [Jon] It is not immediately obvious to me why it is OK for IP 
> fragmentation to occur on unreliable messages but not on reliable 
> messages. If I better understood the reasoning for the "MUST" in 
> the RFC, this might help.

Case 1: IP level frag, reliable transport

SCTP packet A is larger than MTU, so IP fragments it into IP packet 
A1 & IP packet A2.  IP packet2 gets lost, so SCTP packet A is never
delivered, so both IP packet A1 & IP packet A2 both need to be
retransmitted.

Case 2: SCTP level frag, reliable transport

SCTP packet A is larger than MTU, so SCTP fragments it into SCTP packet 
A1 & SCTP packet A2.  SCTP packet2 gets lost, so SCTP retransmits
only packet A2.

Therefore Case 2 is more efficient.

Case 3: IP and/or SCTP level frag, unreliable transport

If a packet is fragmented at SCTP or IP, it doesn't matter much,
because it won't be retransmitted.

br,
John

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