RE: multiple TCP connections

["Merhar, Milan" <mmerhar@pirus.com>]

Steve,

The problem with relying on Layer 2 link aggregation 
such as 802.1ad, is that it protects against packet reordering 
by keeping all traffic between a given source/destination
pair on a single pipe of the multi-link.  So, unless you 
have multiple attachments at each end (with multiple MAC 
address and multiple IP addresses,) you're limited to a
maximum of (in the case of TCP/IP and Gigabit Ethernet) 
around 920 Mbps of aggregate traffic between two end points. 

And, although 900+ Mbps seems to be plenty of bandwidth for 
any single storage device, concerns have been raised that 
it may not be sufficient for the case, for example, of a 
storage array controller talking to a mainframe.

This, in fact, had been presented as one rationale for 
spawning multiple TCP sessions within a single iSCSI session.
Each TCP could potentially have separate IP and MAC end point 
addresses, allowing them to be carried in parallel through 
802.1ad inter-switch links.  (Although, I agree with you that
being able to take advantage of multiple protocol accelerators
is in itself a pretty good justification.)

On the other hand, It may be a close race between introduction 
of Storage-over-IP devices that can take advantage of multiple
parallel Gigabit Ethernet links, and initial availablity of 
10 Gigabit Ethernet LANs.  We might find ourselves having 
designed a clever, but more complex, way of accessing 
greater bandwidth, which is superceded by the brute-force
solution of faster transport technology.

- milan


> -----Original Message-----
> From: Steve Byan [mailto:steve_byan@hotmail.com]
> Sent: Wednesday, August 02, 2000 3:14 PM
> To: ips@ece.cmu.edu
> Subject: Re: multiple TCP connections
> 
> 
> Sorry, I didn't understand TCP multihoming. I've been reading 
> RFC 1122 and 
> RFC 1123 and discovered that TCP binds to exactly one of the local 
> multihomed addresses, and that TCP applications must choose 
> exactly one of 
> the multihomed addresses for the remote host. Multiple 
> sessions starts to 
> make much more sense.
> 
> I noticed that there is a standard for ethernet link 
> aggregation (IEEE  
> 802.3ad), and that routers are available that will 
> load-balance over a set 
> of links. Given these two, it seems to me that multiple TCP 
> connections 
> aren't needed for speed or reliability, assuming ethernet 
> links at the 
> end-points. Am I still missing something?
> 
> The argument that makes sense to me is that of parallelizing 
> the TCP load 
> across multiple hardware accelerators.
> 
> Thanks.
> 
> Regards,
> -Steve
> 
> Steve Byan <stephen.byan@quantum.com>
> 
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