Subject: [P2PSIP] HoL blocking and performance issues, UDP and TCP both

From: "Bruce Lowekamp" <[email-masked]>
Date: Fri, 19 Dec 2008 12:33:55 -0500

Two of the topics that came up several times in the recent discussion
about fragmentation were HoL blocking and the performance impact of
reassembling messages other than at the destination peer.  These
questions are significant well beyond the question of UDP
fragmentation.

I've been giving these concerns a lot of thought, and (regardless of
the other issues related to fragmentation) I think there's an
underlying large message/performance question here that we need to
address.  I thought that this issue had been resolved in MSP, but want
to confirm consensus and the scope of that consensus.

At MSP, we agreed not to support "large" messages, leaving it up to
the overlay configuration to specify the notion of large.  For most
deployments that essentially store and retrieve pointers to resources
and set up connections, my current belief is that messages wind up
being in the 2000-4000 byte range, so 2-3 fragments on a 1500 MTU
network.

HoL blocking and latency penalties were raised as objections to my
proposal that the simplest way to solve issues with UDP fragmentation
was to reassemble the message at every hop and to not interleave
fragments from different messages on transmission.

However, I think it's important to note that what I proposed we should
do with UDP is exactly what the TCP link protocol does.  So if those
are performance concerns, we need to redesign the TCP link protocol,
at the least.

My belief from MSP is that we want to ignore all of these issues
currently, and we believe that the current message size is "small
enough" that we don't need to worry about the performance issues.
There are solutions we can apply for dealing with larger messages and
maximizing performance, but that's not the goal of the current
protocol.  (though could be added as new overlay link protocols)

OTOH, if there is consensus in the group that we need to solve these,
then we should do it, but we need to address it as a problem with all
of our link protocols, not just UDP.

Bruce
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From: Cullen Jennings <[email-masked]>
Date: Fri, 26 Dec 2008 12:03:47 -0700

> of our link protocols, not just UDP.

I don't see the HoL blocking as a problem we should solve. Bulk  
transfer is best solved by forming a connection explictily for the  
bulk transfer. I think the current TCP solution is fine for TCP and  
there is no need to make the UDP solution better than the TCP solution  
in way other than NAT traversal.

Cullen <with my individual contributor hat on>