Re: [pim] Q on the congestion awareness of routing protocols

Hi Toerless,

with respect to routing in general, there is a trade-off between the
timeliness of the routing information and rate limiting. Let's assume
you use a TCP-like congestion control for routing protocols: in case
your sender is throttled heavily, the delay of routing information
propagation is probably too high and the routing information lacks
seriously behind, thus being outdated when received.

I think one has to distinguish where the bottleneck is:
1) link bandwidth (link congestion)
2) routing message processing (CPU congestion)

So in case of 1), routing messages will be dropped, which may
lead to retransmissions in case the routing protocol needs
reliable message delivery and also cause sending rate reduction
in case there is congestion control in place.
In case 2) queues build up inside the router causing also
serious delay and also the potential problem of obsolete
routing information. So the whole routing system could become
instable in extreme cases.

A number of typical pitfalls is summarized in this presentation here:
https://datatracker.ietf.org/meeting/102/materials/slides-102-rtgarea-those-who-do-not-learn-history-are-doomed-to-repeat-it-00

This presentation also mentions the widely known practice
to prioritize routing control messages over data plane traffic
so that the routing control traffic is not adversely affected
by congestion in the data plane. Moreover, it also mentions
the oscillation effects that happened with delay-based routing
as well as the OSPF Flooding issue in denser topologies.

I actually have not the operational experience as others
may have, but my guess is that practically CPU congestion
occurs more often than link congestion (solely caused by
control plane packets). While I believe that TCP congestion control
may potentially help to fix short-term congestion situations,
it is not a solution for persistent link congestion – I think that
such a system may not be able to function correctly.

So there are typically dampening mechanisms in place to aggregate 
routing information or to wait before announcing certain
route updates.
When using TCP, the CPU congestion problem would cause
flow control to kick in and automatically throttle
the sender to the receiver's processing speed.
However, if the generation rate of routing messages is
permanently too high, the system will not be stable.

Regards,
  Roland

On 02.12.22 at 18:03 Toerless Eckert wrote:
> Dear routing-discussion / TSV folks
> (sorry for escalating this, but it really bugs me - Cc'ing PIM/BIER)
> 
> What are these days the expectations against let's say a full Internet Standard
> for a routing protocol to support in terms of congestion safe behavior ? And
> what are congestion control expectation for new routing protocl RFCs even if
> just proposed standard ?
> 
> I am asking, because i think that our core IP multicast routing protocol
> fails miserably on this end, and quite frankly i do not understand how
> PIM-SM (RFC7761) could have become a full Internet standard given how it
> has zilch discussion about congestion or loss handling.
> 
> [ Especially, when in comparison a protocol like RFC7450 where TSV did raise concerns
>    about multicast data plane congestion awareness, and it  was held up for years, and
>    GregS as the WG-chair for the WG responsible for RFC7450 had to even help
>    co-author RFC8085 to cut through the congestion control concern-cord. But likely
>    all for the better!].
> 
> To quickly summarize the issue with PIM-SM to those who do not know it:
> 
>                   /- R2 -------- R6 -\
>       Rcvrs ... R1                    R7 ... Senders
>                   \- R3 -- R4 -- R5 -/
> 
>          CE ... PE .. P    P     P    PE  CE ...
> 
> R1 has let's say 100,000 ulticast/PIM (S,G) states with sources behind R7, so
> it has to maintain 1000,000 so-called PIM (S,G) joins across the path R2, R6, R7.
> Lets say roughly an (S,G) join for IPv6 is about 38 byte (IPv6), maybe 35 (S,G)
> per 1500 byte packet, so 2857 packets of 1500 byte to carry all 100,000 (S,G).
> 
> Assume link R6/R7 fails, IGP reconverges, R1 recognizes that it needs to
> change path, so it sends 2857 PIM-SM packets with prunes to R2 and 2857 PIM -SM
> packets with joins to R3.
> 
> Assume R1 is a PE, R2 and R3 are P routers in an SP, and actually R2/R3 connect
> to lets say 100 routers like R1. Now R2 and R3 get 100 x 2857 1500 byte packets.
> 
> And there is nothing in the PIM-SM spec that talks about how to throttle this
> heap of PIM-SM packets. Typically, routers would just send them back-to-back.
> And those packets repeat every 60 seconds given how PIM-SM is datagram / periodic
> soft-state.  In fact, if you try to scale this in production networks, you will
> most likely fail a lot more than IP multicast in those routers, because PIM not
> only will badly compete on control-plane CPU time, but even more so on control-plane
> to hardware-forwarding time when updating the 100,000 (S,G) hardware forwarding entries.
> 
> Correct me if i am wrong, but did the same type of issues in ISIS/OSPF in
> DC because of so many parallel paths and hence duplication of LSA recently
> lead to the creation of multiple IETF working groups in RTG to solve these
> issues ?
> 
> In IP multicast, we where well aware of these issues and they where a core
> reason to not build a PIM-based MPLS multicast protocol, but use the TCP based LDP
> to specify mLDP (RFC6388). Same thing, when various BGP multicast work was
> done as an alternative to PIM for SPs (BCP also being TCP based).
> 
> We did even fix this problem in PIM by specifying RFC6559 (PIM over TCP),
> but instead of making that mechanisms mandatory and become the only option
> for PIM when moving PIM up the IETF standards ladder to RFC7761, that
> RFC had seemingly fallen into ignorance in the IP Multicast community,
> because most IP multicast deployments are small enough that these issues
> do not occur.
> 
> So, why do i escalate this issue now ?
> 
> We have a great new multicast architecture called BIER that eliminates
> all this PIM multicast state issues from the P routers of such large
> service provider networks by being stateless. But it still leaves the
> need for overlay signaling, such as with PIM to operate between the
> PE, such as in above picture the hundreds if not thousands
> of receiver PE R1' and sender PE R7'. In which case you would have
> PIM directly between those R1'/R7' across multihop paths, leading
> to even more congestion considerations. And in support of such BIER networks,
> there is a draft draft-hb-pim-light proposed to PIM-WG to optimize PIM explicitly
> for this type of deployment. And when i said in PIM@IETF115, that such a draft IMHO
> should only allowed to proceed when it is written to say it MUST
> be based on PIM over TCP (RFC6388), all other people responding
> on the thread said at best it could be be a MAY. Aka: Congestion control optional.
> 
> Am i a congestion control extremist ? I really only want to have
> scaleable, reliably multicast RFCs, especially when they aspire and
> go to full IETF standard and are meant to support our next-gen IP Multicast
> architectures (BIER). I do fully understand how there is a lot
> of cost pressure on vendor development, and having procrastinated
> to implement, proliferate and deploy PIM over TCP so far (almost a decade!)
> does make this a less attractive choice short term. And the whole purpose
> of the PIM light draft of course is to reduce the amount of development needed
> by making PIM more "light" (which is a good think). But when it
> carries forward the problems of PIM to another generation of networks
> (using BIER) that was especially built to scale better, then one
> should IMHO really become worried. At least i do. But i also struggled to
> implement datagram PIM processing for 100,000 states in a prior life
> and then pushed for PIM over TCP...
> 
> Thanks!
>      Toerless
> 
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