Hi all,
I have read the Segment Routing Problem Statement and Requirements draft and I
have a couple of comments on it.
Editorial:
The Abstract states that "Multicast use-cases and requirements are out of scope
of this document", but this (or equivalent) statement does not appear anywhere
in the body of the document. IMHO and FWIW this contradicts the last para in
Section 4.3 of RFC 7322 that states that "the RFC should be self-contained as
if there were no Abstract".
Technical:
The draft requires, in Section 2, that "The SPRING architecture SHOULD leverage
the existing MPLS dataplane without any modification...".
In addition, in Section 3.3 it requires that "The SPRING architecture SHOULD
allow incremental and selective deployment without any requirement of flag day
or massive upgrade of all network elements" .
My reading (FWIW) of these two requirements is that SPRING with MPLS dataplane
should work on existing MPLS forwarding HW.
If this understanding is correct, it is in potential conflict with another
requirement formulated in the Section1 of the draft: "The SPRING architecture
SHOULD allow optimal virtualization: put policy state in the packet header and
not in the intermediate nodes along the path".
This conflict stems from the following (admittedly, naïve) observation:
1. The policy state representing the desired source route must be pushed
in its entirety onto the packet by the source (here source is interpreted in
the same way as in the draft itself) and must be parsed by all the transit
nodes.
2. The amount of the policy state grows (linearly?) as the number of
elements in the source route selected by the packet. In particular, the policy
representing a strict route could be potentially quite large.
3. In the nodes that use hardware-based forwarding, the size of the
policy state that can be pushed and parsed with the expected throughput is
inherently limited. These limits differ for different implementations but they
usually cannot be exceeded without replacing the forwarding hardware.
4. Passing "offending" packets for software handling could result in
dramatic decrease of throughput. S
In the case of the MPLS dataplane, the policy state is expressed as the MPLS
label stack where each segment is represented by a label stack entry. AFAIK,
existing (and probably future) forwarding HW that supports MPLS is inherently
limited (the limits differ for different implementations) both regarding the
number of labels that could be pushed on the packet, and regarding the total
depth of the label stack that it can parse.
Note: The limit on the number of labels that can be pushed on a packet by
forwarding HW is obvious. The limit on that can be parsed becomes essential in
the scenarios when ECMP is used, because:
* As per RFC 7325, Section 2.4.5.1., "The most entropy is
generally found in the label stack entries near the bottom of the label stack
(innermost label, closest to S=1 bit)"
* As per Section 2.4.5.2 of the same RFC, "Inspecting the IP
payload provides the most entropy in provider networks. The practice of
looking past the bottom of stack label for an IP payload is well accepted and
documented in [RFC4928] and in other RFCs".
* Both these methods (hashing the label stack and hashing IP
header) obviously require parsing the entire label stack.
The limits of forwarding HW could be considered an implementation problem, were
it not for the draft requiring (and I fully agree with validity of this
requirement) that SPRING could be used on existing MPLS-capable HW.
From my POV the document should at least explicitly acknowledge this conflict
as part of the SPRING problem statement. Preferably it should also include
some guidelines for its resolution:
* One possibility that comes to mind could be a requirement
to provide the information about hardware-specific limitations to
traffic-engineering entities in order to avoid computation of paths that do not
meet HW-imposed constraints.
* Another possibility is to clearly indicate that loose
route options are preferable for using with SPRING. To the best of my
understanding this could be translated into a requirement for a new type of
constrained path computation algorithms that yield loose (rather than strict)
routes
Of course there may be other (and, possibly, better) ways to resolve this
conflict. But, from my POV, if it is not acknowledged explicitly, its
resolution becomes much more problematic.
Hopefully, these LC comments would be useful.
Regards,
Sasha
Office: +972-39266302
Cell: +972-549266302
Email: Alexander(_dot_)Vainshtein(_at_)ecitele(_dot_)com
-----Original Message-----
From: spring [mailto:spring-bounces(_at_)ietf(_dot_)org] On Behalf Of The IESG
Sent: Tuesday, December 15, 2015 8:11 PM
To: IETF-Announce
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Subject: [spring] Last Call: <draft-ietf-spring-problem-statement-06.txt>
(SPRING Problem Statement and Requirements) to Informational RFC
The IESG has received a request from the Source Packet Routing in Networking WG
(spring) to consider the following document:
- 'SPRING Problem Statement and Requirements'
<draft-ietf-spring-problem-statement-06.txt> as Informational RFC
The IESG plans to make a decision in the next few weeks, and solicits final
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Abstract
The ability for a node to specify a forwarding path, other than the
normal shortest path, that a particular packet will traverse,
benefits a number of network functions. Source-based routing
mechanisms have previously been specified for network protocols, but
have not seen widespread adoption. In this context, the term
'source' means 'the point at which the explicit route is imposed' and
therefore it is not limited to the originator of the packet (i.e.:
the node imposing the explicit route may be the ingress node of an
operator's network).
This document outlines various use cases, with their requirements,
that need to be taken into account by the Source Packet Routing in
Networking (SPRING) architecture for unicast traffic. Multicast use-
cases and requirements are out of scope of this document.
The file can be obtained via
https://datatracker.ietf.org/doc/draft-ietf-spring-problem-statement/
IESG discussion can be tracked via
https://datatracker.ietf.org/doc/draft-ietf-spring-problem-statement/ballot/
No IPR declarations have been submitted directly on this I-D.
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