Subject: [Tsvwg] SCTP and IP Frag aliasing on LFNs ?

From: "WENDT,JIM (HP-Roseville,ex1)" <[email-masked]>
Date: Mon, 25 Feb 2002 18:18:23 -0500

Based upon following in RFC-2960:

> 6.9 Fragmentation and Reassembly
>   Note: Once a message is fragmented it cannot be re-fragmented.
>   Instead if the PMTU has been reduced, then IP fragmentation must be
>   used.  Please see Section 7.3 for details of PMTU discovery.

> 7.3 Path MTU Discovery
>   4) Since data transmission in SCTP is naturally structured in terms
>      of TSNs rather than bytes (as is the case for TCP), the discussion
>      in Section 6.5 of RFC 1191 applies: When retransmitting an IP
>      datagram to a remote address for which the IP datagram appears too
>      large for the path MTU to that address, the IP datagram SHOULD be
>      retransmitted without the DF bit set, allowing it to possibly be
>      fragmented.  Transmissions of new IP datagrams MUST have DF set.

Am I correct in understanding that IP fragmentation can occur
on SCTP packets across a IPv4 PMTU degrade, and that worst case,
IP fragmentation can occur for all packets in flight up to the
roundtrip pipe size between the sending endpoint and the last
hop router?

If so, and the pipe is large, has there been any discussion or
concern regarding aliasing or wrap-around of the 16-bit
Identification (ID) field in the IP header?  Is this an
issue, and if so, how is it being addressed?

Thx,
Jim

_______________________________________________
tsvwg mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/tsvwg

---

From: Randall Stewart <[email-masked]>
Date: Tue, 26 Feb 2002 06:05:23 -0600

Jim:

Some thoughts below...

> roundtrip pipe size between the sending endpoint and the last
I would think this can occur for any transport doing  PMTU discovery.
I do have  a hard time envisioning a very very highspeed inter-net
link that has a PMTU event without some sort of accompanying routing
re-convergence. And it seems to me if you hit a routing reconvergence
you are in for at LEAST 1 timeout putting you way back in slow start.

But lets do some rough calculations on a napkin anyway and see what
happens if a PMTU change occurs on say a 1 Gig-a-bit link with
lets say a 300ms round trip time. And just for fun we will assume
you are the ONLY user on this 1 Gig-a-bit link and you have fast
machines on both sides that can use it all.

Now lets say a 1500 byte MTU before the change and a 500 byte
MTU after the change. Now on a napkin, I come up with about 6.7
packets per ms... so we show up with 2,010 packets in flight.

Note I am rounding here a 1,000,000,000 bit -> 100,000,000 Mega Byte.
This may not be exact but it is quite close and allows for the
overhead in the e-net and anthing else :>

Now if I have only 2,010 packets all that need to be resent and with an
IP id of 16 bits or 65,535 I should be ok.. 

In fact I should be ok up to a RTT time of about 4.8 seconds.
-----

Now lets look at the same case with a 10 Gig a bit link.
We jump by a factor of 10 so... 

20,100 packets in flight .. no problem... 

Good up to 480ms round trip time..

So I guess there may be an issue if one has greater than a 480ms
round trip time and a 10 gigabit connection where you are using
the whole link all by yourself.

> issue, and if so, how is it being addressed?

Considering that the scenario is quite far fetched in todays
technology over an inter-net. Maybe in a Intra-net.. but then
one would hope you would not get more than a 400ms RTT...

And besides all that .. in the future if this really becomes
a problem I think SCTP is exandable enough that we can figure
out a new chunk or some such to help with this type of situation.

I think we can worry about this problem when we get a bit closer
to having a realistic scenario where a problem would exist...

I just don't see it yet... 

Maybe I have missed something in my back-of-the napkin calculations
or I am mis-interpreting what circumstances you are envisioning. If
so please correct me..

R

-- 
Randall R. Stewart
[email-masked] 815-342-5222 (cell phone)