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Re: [lwip-users] Pbuf pool size and network interface
From: |
Enrico Murador - Research & Development - CET |
Subject: |
Re: [lwip-users] Pbuf pool size and network interface |
Date: |
Mon, 04 Jul 2011 16:07:25 +0200 |
User-agent: |
Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.2.18) Gecko/20110616 Thunderbird/3.1.11 |
Dear Kieran,
Is such a small MSS valid? All IP networks are supposed to guarantee
that they can receive a segment of 576 bytes without fragmenting, so
assuming the default header sizes that would make a TCP segment of 536
bytes. While setting it smaller might be OK in most situations, you
might find that this causes interoperability problems with other stacks,
as they could ignore your MSS request and just use 536 instead.
thank you for the tip, I was not aware of this kind of situation.
By "googling out" a while, I found this discussion:
http://www.ietf.org/mail-archive/web/tsvwg/current/msg04483.html
In particular, this explanation seems useful:
For IPv4, RFC 791 defines the minimum IP packet size as 68 bytes. On
page 25:
Every internet module must be able to forward a datagram of 68
octets without further fragmentation. This is because an internet
header may be up to 60 octets, and the minimum fragment is 8 octets.
Every internet destination must be able to receive a datagram of 576
octets either in one piece or in fragments to be reassembled.
From this description, it seems to me that a MSS equal or more than 68
bytes "should" be OK, as long as there are sufficient resources to
reassemble a 576 bytes long (minimum) fragmented packet...
Enrico
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