Subject: [Tsvwg] IP fragmentation in relation with TCP on Host

From: fabien passilly <[email-masked]>
Date: Fri, 1 Mar 2002 10:33:11 +0100 (CET)

Hello,

Sorry for this second mail, but I forgot to mention in
my previous mail, when asking this question, I
consider a host.

I have one question concerning IP fragmentation in
relation with the upper protocols UDP and TCP on a
host.

I know that IP fragmentation should be classically
implemented "under" the IP layer (without any
distinction between the upper protocols) but I would
like to get a clarification on a particular point:

- for UDP, I can understand the utility of IP
fragmentation; 

- however it is not clear for TCP. Indeed in this
protocol, the packet size is already managed in a way
to avoid sending packets bigger than the MTU size
(through the MSS value). What can an IP fragmentation
mechanism bring as advantages?? Especially if we
consider that, if the IP fragmentation mechanism is
based on the MTU size (as the biggest size that could
be sent), the IP (under TCP) packets may never be
fragmented.

I thank you in advance for your answer,

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From: "Brian F. G. Bidulock" <[email-masked]>
Date: Fri, 1 Mar 2002 15:56:02 -0600

fabien,

> be sent), the IP (under TCP) packets may never be

The sending host does not have precise information about the
path MTU size.  This is only determined using path MTU
discovery (setting the DF bit in the IP header).  When an MTU
falls, the sending host gets an ICMP error indicating that the
IP packet could not be transmitted.  Both TCP and SCTP must
perform a retransmission in this event.  It is possible to
retransmit immediately or to wait until the ack timer expires.
Only when retransmitting immediately does is it really
expedient for the sender to refragment on retransmission.
Almost no implementations do this.  Many TCP implementations
just retransmit with the DF bit cleared.

In SCTP the IPv4 ICMP message does not contain enough
information to indicate the TSNs which were rejected, so it is
not possible for SCTP to perform this simple retransmission.
All that it can do is to adjust the path MTU and fall back on
retransmission timers.  When SCTP goes to retransmit and a
DATA chunk now exceeds the MTU, it is possible to refrag
the DATA chunk, but I think most imps just sent the packet
with the DF bit clear and let IP fragmentation do the job.

In SCTP there is also the possibility that an INIT ACK or
COOKIE ECHO chunk could exceed the MTU (really big cookie).
There is no mechanism in SCTP to fragment these chunks and
IP fragmentation would have to be used.  Also, a SACK chunk
could exceed the path MTU (many gap and dup reports) but it
is less critical if these get lost.

--brian

-- 
Brian F. G. Bidulock    ¦ The reasonable man adapts himself to the ¦
[email-masked]    ¦ world; the unreasonable one persists in  ¦
http://www.openss7.org/ ¦ trying  to adapt the  world  to himself. ¦
                        ¦ Therefore  all  progress  depends on the ¦
                        ¦ unreasonable man. -- George Bernard Shaw ¦