Internet DRAFT - draft-huang-lable-collision
draft-huang-lable-collision
Network Working Group Zheng Huang
Internet Draft Yang Yang
Huawei
Expires: December 2006 June 16, 2006
The Solution of Label Collision Between Multicast and Unicast
draft-huang-lable-collision-00.txt
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Abstract
Upstream Label Suggestion and upstream label allocation schemes are
introduced and simply defined in [OVER][UPSTREAM]. But it is possible
that the same label will be allocated for unicast LSP and multicast
LSP on the same link. In upstream label allocation, it solves the
collision through layer 2 encapsulation and context-specific label
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space. But in the Upstream Label Suggestion allocation scheme, the
collision can not be solved if downstream LSRs do not support
context-specific label space. This document details the solution of
label collision between multicast and unicast for the Upstream Label
Suggestion allocation scheme.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119.
Table of Contents
1. Introduction................................................2
2. Collision Scene.............................................3
3. Label collision solution.....................................4
4. Upstream Label Suggestion for Multicast......................4
5. Some Advice.................................................5
6. IANA Considerations.........................................6
7. References..................................................6
7.1. Normative References....................................6
7.2. Informative References..................................6
8. Author's Addresses..........................................6
9. Intellectual Property Statement..............................6
Disclaimer of Validity.........................................7
Copyright Statement............................................7
Acknowledgment.................................................7
1. Introduction
On multi-access links for P2MP LSPs, a labeled packet is delivered to
multiple routers. All next-hop downstream routers will receive the
packet with the same label. In order to optimize packet transmission
and avoid branch LSR traffic replication for P2MP LSPs, all next-hop
downstream routers must process the same label. This will sit well
with upstream Label Suggestion and upstream label allocation schemes
[OVER] [UPSTREAM].
When unicast label is allocated before multicast label, it is
possible that the same label will be allocated on the same LSR for
unicast LSP and multicast LSP on the same link. The unicast LSP and
the multicast LSP will use the same incoming label value on the same
data link. This means one label collision between multicast and
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unicast. In upstream label allocation, it solves the collision
through layer 2 encapsulation and context-specific label space. But
in the Upstream Label Suggestion allocation scheme, the collision can
not be solved if downstream LSRs do not support context-specific
label space. This document details the solution of label collision
between multicast and unicast for the Upstream Label Suggestion
allocation scheme.
2. Collision Scene
By way of illustration, the figure below demonstrates the scene of
the collision between multicast label and unicast label.
D
/
/
A---B---C---E
\
\
F
There already exists one unicast LSP (A-B-C-E). And the incoming
label on LSR C is equal to K.
There also exist one multicast tree (A-B, B-D, B-F). And the incoming
label on LSR D and LSR F is equal to K. LSR D and LSR F are the next-
hop downstream router of LSR B, and their incoming labels must same.
Subsequently, LSR C wants to join the multicast tree. LSR C will also
become the next-hop downstream router of LSR B. The incoming
multicast label on LSR C must be equal to K, just like LSR D and LSR
F on the same multicast tree.
So there will appear the two same incoming labels on LSR C if we
don't distinguish between multicast and unicast.
The collision can be solved very well in [UPSTREAM]. But in the
Upstream Label Suggestion allocation scheme, these multicast labels
are assigned by downstream LSRs just like unicast. The collision can
not be solved if downstream LSRs do not support context-specific
label space. As a result, all next-hop downstream LSRs can not hold
the same label in multicast tree and the traffic can not be optimized.
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3. Label collision solution
This document proposes one solution for label collision. When
collision appears, unicast will drop the label to multicast and re-
assign one new different label for itself. Then multicast tree can be
built successfully.
The solution needs to extend label distribution instructions, for
example, LDP or RSVP-TE. We give one vivid description based on LDP
Mechanisms as follows.
Unicast labels are conventionally assigned by one downstream LSR. In
multicast, labels can be assigned by one upstream or downstream LSR.
In upstream label assignment, the collision can be resolve when all
LSRs support Upstream Label Assignment and Context Specific Label
Space. It is possible that the capability of Context Specific Label
Space is not supported in Upstream Label Suggestion allocation scheme.
So the collision question still exists.
We assume the collided label is equal to K. When Downstream LSR
detects a collision, unicast LSP will give up the collided label and
reassign one new unicast label. So the multicast can use the label K
to build one entire tree.
4. Upstream Label Suggestion for Multicast
In Upstream Label Suggestion for Multicast scheme, all of multicast
labels are assigned by all downstream LSRs. The message exchange for
Multicast Upstream Label Suggestion is shown in the figure below to
resolve the label collision between two adjacent LSRs. The figure
shows one detailed process of downstream unsolicited label with
Suggestion label distribution.
There include there stages in the message exchanges between two
adjacent LSRs:
a) Downstream LSR assigns label to upstream LSR based on Upstream
Label Suggestion for Multicast. When downstream LSR sends Label
Mapping message with special label to it, upstream LSR will realize
that downstream wants one suggested label according to the special
label. Upstream LSR returns Label Request with one suggested label to
downstream LSR.
b) Downstream LSR defect the collision, and unicast will assign one
new different label after abandoning the collided label. The
suggested label value is equal to the value of unicast label in the
same downstream LSR. So the downstream LSR will defect a label
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collision and can not assign the label for multicast. Consequently,
downstream LSR will give up the collided label during sending Label
Withdraw message and accepting Label Release message from the
upstream LSR in unicast LSP. Then downstream LSR assigns one new
label for unicast again.
c) Downstream LSR re-assigns the label to upstream LSR for Multicast.
After unicast gives up the label, downstream LSR will find that the
label is available for multicast and the collision disappears. So it
will re-assign the label to upstream LSR for multicast by using Label
Mapping message again.
It must be noticed that the first message is not used in Downstream
On-Demand label distribution.
Upstream LSR Downstream LSR
| Label Mapping |
| (Special Label) | (Multicast)
|<------------------------------|
| Label Request |
| (Suggested Label = K) | (Multicast)
|------------------------------>|
| | (Detect a collision)
| Label Withdraw | (Unicast give up label)
| (Label = K) | (Unicast)
|<------------------------------|
| |
| Label Release | (Unicast)
|------------------------------>|
| Label Mapping |
| (new label) | (Unicast)
|<------------------------------|
| Label Mapping |
| (Label = K) | (Multicast)
|<------------------------------|
5. Some Advice
It is possible to interrupt unicast traffic when unicast assigns one
new label after giving up the collided label. In order to avoid the
traffic interrupt, unicast should assign one new label before giving
up the old label. Maybe there need some other extensions for current
mechanism, we maybe discuss more details in the latter version.
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6. IANA Considerations
This document will propose some minor extensions to LDP that may
require the allocation of new code points under the care of IANA. A
future version of this document will include the relevant information.
7. References
7.1. Normative References
[RFC3036] L. Andersson, P. Doolan, N. Feldman, A. Fredette, B. Thomas,
"LDP Specification", RFC3036, January 2001.
7.2. Informative References
[OVER] Seisho Yasukawa, Adrian Farrel, "Support of LDP Multicast
Label Switched Paths over Point-to-Multipoint Label
Switched Path Tunnels and on Multi-Access Links", draft-
yasukawa-mpls-ldp-mcast-over-p2mp-lsps-01.txt, Work in
progress.
[UPSTREAM] R. Aggarwal, Y. Rekhter, E. Rosen, "MPLS Upstream Label
Assignment and Context Specific Label Space", draft-ietf-
mpls-upstream-label-00.txt, Work In Progress.
8. Author's Addresses
Zheng Huang
Huawei Technologies Co., Ltd.
Bantian industry base, Longgang district
Shenzhen, China
Email: szhuangzheng@huawei.com
Yang Yang
Huawei Technologies Co., Ltd.
Bantian industry base, Longgang district
Shenzhen, China
Email: healthinghearts@huawei.com
9. Intellectual Property Statement
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
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