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List: mpls
Subject: Re: [mpls] [PWE3] Pseudowire Sequence Number in draft-ietf-mpls-forwarding: a minor comment
From: Alexander Vainshtein <Alexander.Vainshtein () ecitele ! com>
Date: 2013-11-26 16:26:43
Message-ID: F9336571731ADE42A5397FC831CEAA025AC56C9D () ILPTWPVEXMB02 ! ecitele ! com
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Curtis,
Lots of thanks for a prompt and very detailed response.
May I suggest an alternative version of the following text fragment?
<Curtis>
> Identifying the position of any lost packets is important
> for PW services which are attempting to reconstruct a bit stream
> which maintains bit timing, such as time division multiplexing (TDM)
> services. TDM and other PW services which require strict ordering
> also require that misordered packets be either dropped or reordered.
<Sasha>
Identifying lost PW packets and exact amount of lost payload is critical for PW \
services which maintain bit timing, such as Time Division Multiplexing (TDM) services \
since these services MUST compensate lost payload on a bit-for-bit basis.
With these services PW packets that have been received out of order also MUST also be \
identified and may be either re-ordered or dropped. Reordering requires, in addition \
to sequence numbering, a "de-jitter buffer" in the egress PE, and ability to reorder \
is limited by the depth of this buffer. The down side of maintaining a de-jitter \
buffer is added end-to-end service delay.
</Sasha>
Hopefully this will be useful.
Regards,
Sasha
> -----Original Message-----
> From: Curtis Villamizar [mailto:curtis@ipv6.occnc.com]
> Sent: Monday, November 25, 2013 8:30 PM
> To: Alexander Vainshtein
> Cc: curtis@occnc.com; kireeti@juniper.net; samante@apple.com;
> agmalis@gmail.com; cpignata@cisco.com; mpls@ietf.org; Yaakov Stein
> (yaakov_s@rad.com); pwe3 (pwe3@ietf.org)
> Subject: Re: [PWE3] Pseudowire Sequence Number in draft-ietf-mpls-
> forwarding: a minor comment
>
>
> In message
> <F9336571731ADE42A5397FC831CEAA025392F92F@ILPTWPVEXMB01.ecitele.c
> om>
> Alexander Vainshtein writes:
> >
> > Hi all,
> >
> > I would like to comment on the text in Section 2.1.8.1 "Pseudowire
> > Sequence Number" in draft-ietf-mpls-forwarding-03 - MPLS Forwarding
> > Compliance and Performance Requirements
> > <http://tools.ietf.org/html/draft-ietf-mpls-forwarding-03>.
> >
> > This section states, in short, that the main drive for using the
> > sequence number in the PW Control Word is handling of packet
> > reordering events. TDM PWs are presented as a major example, with CBR
> > ATM services as another example.
> >
> > In fact, this statement is not accurate.
>
> Yes. You are correct in pointing this out as inaccurate by omitting
> the other important function of identifying drops for the purpose of
> recunstructing TDM bit streams.
>
> Andy brought up resequencing being a strong provider request.
> Reordering is far more common than loss particularly for high priority
> services on provider networks and without resequencing reorder results
> in PW loss in what could otherwise be lossless service.
>
> So we should get the base requirements right but still reflect this,
> but strictly as advice with no normative wording. We had used the
> phrase "is beneficial" and will retain that.
>
> Andy brought up the topic. The incorrect wording in the existing
> draft is my fault. This text went in fairly early with a lot of other
> changes and it appears that no one had since given it a careful enough
> read and review until you came along.
>
> > The main drive for mandating the use of sequence number in TM PWs is
> > the need to detect and count lost packets because the egress PE MUST
> > compensate the lost payload bit for bit.
> >
> > Ability to compensate reordering of PW packets at egress is a side
> > effect of (a) sequence number usage and (b) usage of the de-jitter
> > buffer in the egress PW. It is not mandatory and in any case is
> > limited by the depth of the de-jitter buffer: re-ordered packets that
> > cannot be accommodated within this buffer are treated as lost.
> >
> > Additional details can be found, e.g., in section 6.2.2 of RFC
> > 4553<http://tools.ietf.org/html/rfc4553>. Ability to re-order
> > mis-ordered PW packets is defined there as OPTIONAL, while replacement
> > of the payload of lost PW packets is defined as MANDATORY.
>
> The change below at the top of the section more accurately reflects
> requirements in the RFCs but still states that resequencing can be
> beneficial. The comments about EPD and PPD are dropped and therefore
> also the informative reference.
>
> Context:
>
> 2.1.8.1. Pseudowire Sequence Number
>
> OLD
>
> Pseudowire (PW) sequence number support is most important for PW
> payload types with a high expectation of in-order delivery.
> Resequencing support, rather than dropping at egress on out of order
> arrival, is most important for PW payload types with a high
> expectation of lossless delivery. For example, TDM payloads require
> sequence number support and require resequencing support. The same
> is true of ATM CBR service. ATM VBR or ABR may have somewhat relaxed
> requirements, but generally require ATM Early Packet Discard (EPD) or
> ATM Partial Packet Discard (PPD) [ATM-EPD-and-PPD]. Though sequence
> number support and resequencing support are beneficial to PW packet
> oriented payloads such as FR and Ethernet, they are highly desirable
> but not as strongly required.
>
> NEW
>
> Pseudowire (PW) sequence number support is most important for PW
> payload types with a high expectation of lossless and/or in-order
> delivery. Identifying the position of any lost packets is important
> for PW services which are attempting to reconstruct a bit stream
> which maintains bit timing, such as time division multiplexing (TDM)
> services. TDM and other PW services which require strict ordering
> also require that misordered packets be either dropped or reordered.
>
> PW services which are not timing critical bit streams in nature are
> cell oriented or frame oriented. Though resequencing support is
> beneficial to PW cell and frame oriented payloads such as ATM, FR and
> Ethernet, they are highly desirable but not required.
>
> NEW (end of subsection after list of possible reording causes)
>
> In provider networks which use multipath techniques and which may
> occassionally rebalance traffic or which may change PW paths
> occasionally for other reasons, reordering may be far more common
> than loss. Where reordering is more common than loss, resequencing
> packets is beneficial, rather than dropping packets at egress when
> out of order arrival occus. Resequencing is most important for PW
> payload types with a high expectation of lossless delivery since in
> such cases out of order delivery within the network results in PW
> loss.
>
> The final paragraph sums up the motivation for highlighting
> resequencing as beneficial and desirable. It does so without any
> normative wording.
>
> > Hopefully these notes will be useful.
> >
> > Regards,
> > Sasha
>
> These notes are very useful. Thank you.
>
> Please let us know if you (and the WG) are OK with the rewording
> proposed above.
>
> Curtis
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