[e2e] Reordering in routers
cannara at attglobal.net
Sun Aug 10 19:35:57 PDT 2003
Abhijit, check out the IBM and Motorola network processors, as well as Vitesse
(IQ2200), TI and Intel (IXP2800). Cisco uses the IQ, the IXP and, I believe
some IBM ones. The designers of most NPUs considered reordering a no-no, so
provided built-in hardware to track flows, as identified in simplest form by
tuples, like: input-interface, port/channel, IP details, etc. However, the
output process, no matter how parallel the input, can be controlled more
finely in most NPUs by the firmware designer, so he/she can extend the
ordering checks at pkt-output time to see if any pkt on a particular output
queue shouold be released form transmission, regardless of which input path it
arrived on. The IXP2400, in fact, doesn't support anything but programmer
Abhijit Bare wrote:
> I understand that multiple input interfaces at a given NPU may process packets
> of a single stream in different orders. I was wondering if anybody can direct
> me to some detailed description of such architectures in use.
> Also, as we move on to high speed routers, the router architectures are
> becoming more parallel in their design. For example, VOQ structures or
> multi-plane switching fabcrics etc. Does this mean that we should expect more
> packet reordering as the network speeds increase ? Certainly, it depends upon
> the specific router architectures. But still internally they may end up in
> more reordering, putting higher burden on the packet re-sequencing units.
> >===== Original Message From cannara at attglobal.net =====
> >Exactly right, in fact single-chip network processors from most all the
> >vendors have internal hardware (e.g., CAMs) simply to prevent pkts from being
> >released before all prior ones that arrived via the same input path.
> >since an input path is typically a channel on an interface, such as SPIx and
> >multiple interfaces exist on NPUs, there may be instances of pkts that
> >traversed different upstream paths getting reordered at a given processor
> >can't know better.
> >"Ghanwani, Anoop" wrote:
> >> As far as I know, under normal circumstances most routers do not
> >> reorder packets. I think Juniper routers had a problem with reordering,
> >> but the general design practice (as far as I know) is that reordering
> >> is a no-no.
> >> See
> >> http://www.lightreading.com/document.asp?doc_id=4009&page_number=8
> >> for an interesting discussion on this subject.
> >> -Anoop
> >> > -----Original Message-----
> >> > From: abhijit [mailto:abhijit at engr.colostate.edu]
> >> > Sent: Friday, August 08, 2003 2:45 PM
> >> > To: end2end-interest at postel.org
> >> > Subject: [e2e] Reordering in routers
> >> >
> >> >
> >> > Hi all,
> >> > I am studying the packet-reordering problem over the Internet
> >> > from the point
> >> > of view of the routers being a reason of reordering.
> >> >
> >> > In the paper by "Packet Reordering is Not Pathological
> >> > Network Behavior" by
> >> > Bennet, Partridge, and Shectman, the router queuing structure
> >> > was said to be
> >> > the reason behind the reordering observed at MAE-east. The
> >> > DEC gigaswitch used
> >> > at MAE-east point used hunt groups i.e. collection of ports
> >> > together acting as
> >> > input ports that caused the packets getting distributed and
> >> > hence reordered.
> >> >
> >> > I am studying queuing architectures and queuing disciplines
> >> > of present
> >> > routers. But I do not find any routers with parallel input
> >> > queuing sothat
> >> > incoming packets are distributed over a number of queues
> >> > before getting
> >> > processed. It will be great if anybody can give me
> >> > information or pointers
> >> > over this problem, especially:
> >> > 1. Are the internal structure issues i.e. queuing
> >> > architectures (input, output
> >> > queues, VOQ, VIQ etc), disciplines (packet scheduling or
> >> > buffer management
> >> > schemes) responsible or can be responsible for reordering?
> >> > 2. Or the external reasons such as route flapping, multi-path
> >> > routing,
> >> > parallel links between two routers, etc. are more responsible
> >> > for reordering
> >> > happening in the present Internet?
> >> >
> >> > I appreciate any information shared on this problem.
> >> > Regards,
> >> > Abhijit
> >> >
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