Network Working Group Eric C. Rosen
Internet Draft Cisco Systems, Inc.
Expiration Date: September 2004
Updates RFC 3032
March 2004
Removing a Restriction on the use of MPLS Explicit NULL
draft-ietf-mpls-explicit-null-00.txt
Status of this Memo
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Abstract
The label stack encoding for MPLS (Multi-protocol Label Switching)
defines a reserved label value known as "IPv4 Explicit NULL" and a
reserved label value known as "IPv6 Explicit NULL". Previously,
these labels were only legal when they occurred at the bottom of the
MPLS label stack. This restriction is now removed, so that these
label values may legally occur anywhere in the stack.
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Contents
1 Introduction ......................................... 2
2 Detail of Change ..................................... 2
3 Reasons for Change ................................... 4
4 Acknowledgments ...................................... 5
5 Normative References ................................. 5
6 Informative References ............................... 5
7 Author's Address ..................................... 6
8 Intellectual Property Notice ......................... 6
9 Copyright Notice ..................................... 6
1. Introduction
RFC 3032 defines a reserved label value known as "IPv4 Explicit NULL"
and a reserved label value known as "IPv6 Explicit NULL". It states
that these label values are only legal at the bottom of the MPLS
label stack. However, no reason is given for this restriction.
It has turned out that in practice there are some situations in which
it is useful to send MPLS packets which have Explicit NULL occur
other than at that bottom of the label stack. While the intended
semantics are obvious enough, the fact that such packets are
gratuitously declared by RFC 3032 to be illegal has made it difficult
to handle these situations in an interoperable manner.
This document updates RFC 3032 by removing the unnecessary
restriction, so that the two aforementioned label values are legal
anywhere in the label stack.
2. Detail of Change
RFC 3032 states on page 4:
There are several reserved label values:
i. A value of 0 represents the "IPv4 Explicit NULL Label". This
label value is only legal at the bottom of the label stack.
It indicates that the label stack must be popped, and the
forwarding of the packet must then be based on the IPv4
header.
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iii. A value of 2 represents the "IPv6 Explicit NULL Label". This
label value is only legal at the bottom of the label stack.
It indicates that the label stack must be popped, and the
forwarding of the packet must then be based on the IPv6
header.
Paragraph i is hereby changed to read:
i. A value of 0 represents the "IPv4 Explicit NULL Label".
An IPv4 Explicit NULL at the top of the label stack means that
the stack must be popped.
If the NULL was not the only label on the stack, this will
cause the label beneath it to rise to the top of the stack.
The disposition of the packet is based on the label that has
now risen to the top.
If, on the other hand, the NULL was the only label on the
stack, then the stack is now empty. The resulting packet is
treated as an IPv4 packet, and its disposition is based on the
IPv4 header.
Paragraph iii is hereby changed to read:
iii. A value of 2 represents the "IPv6 Explicit NULL Label".
An IPv6 Explicit NULL at the top of the label stack means that
the stack must be popped.
If the NULL was not the only label on the stack, this will
cause the label beneath it to rise to the top of the stack.
The disposition of the packet is based on the label that has
now risen to the top.
If, on the other hand, the NULL was the only label on the
stack, then the stack is now empty. The resulting packet is
treated as an IPv6 packet, and its disposition is based on the
IPv6 header.
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3. Reasons for Change
Restricting Explicit NULL to the bottom of the stack has caused some
problems in practice.
With this restriction in place, one should not distribute, to a
particular label distribution peer, a binding of Explicit NULL to a
particular FEC, unless the following condition (call it "Condition
L") holds: all MPLS packets received by that peer with an incoming
label corresponding to that FEC contain only a single label stack
entry. If Explicit NULL is bound to the FEC, but Condition L doesn't
hold, the peer is being requested to create illegal packets. None of
the MPLS specifications say what the peer is actually supposed to do
in this case. This situation is made more troublesome by the facts
that, in practice, Condition L rarely holds, and it is not possible
in general to determine whether it holds or not.
Further, if one is supporting the Pipe Model of RFC3270, there are
good reasons to create label stacks in which Explicit NULL is at the
top of the label stack, but a non-null label is at the bottom.
RFC3270 specifies the procedures for MPLS support of Differentiated
Services. In particular, it defines a "Pipe Model", in which
(quoting from RFC3270, section 2.6.2):
"tunneled packets must convey two meaningful pieces of Diff-Serv
information:
- the Diff-Serv information which is meaningful to intermediate
nodes along the LSP span including the LSP Egress (which we
refer to as the 'LSP Diff-Serv Information'). This LSP Diff-
Serv Information is not meaningful beyond the LSP Egress:
Whether Traffic Conditioning at intermediate nodes on the LSP
span affects the LSP Diff-Serv information or not, this updated
Diff-Serv information is not considered meaningful beyond the
LSP Egress and is ignored.
- the Diff-Serv information which is meaningful beyond the LSP
Egress (which we refer to as the 'Tunneled Diff-Serv
Information'). This information is to be conveyed by the LSP
Ingress to the LSP Egress. This Diff-Serv information is not
meaningful to the intermediate nodes on the LSP span."
When the Pipe Model is in use, it is common practice for the LSP
Egress to bind Explicit Null to the tunnel's FEC. The intention is
that the LSP Diff-Serv information will be carried in the EXP bits
of the Explicit Null label stack entry, and the tunneled Diff-Serv
information will be carried in whatever is "below" the Explicit Null
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label stack entry, i.e., in the IP header DS bits or in the EXP bits
of the next entry on the MPLS label stack.
Naturally, this practice causes a problem if the Pipe Model LSP is
being used to tunnel MPLS packets (i.e., if Condition L does not
hold). With strict adherence to RFCs 3031 and 3036, this practice
results in an MPLS packet where Explicit NULL is at the top of the
label stack, even though it is not the only entry in the label
stack. However, RFC 3032 makes this packet illegal. Some
implementations simply transmit the illegal packet. Others try to
convert it to a legal packet by stripping off the Explicit NULL
before transmitting it. However, that breaks the Pipe Model by
discarding the LSP Diff-Serv information.
Of course the LSP egress is not compelled to bind Explicit NULL to
the tunnel's FEC; an ordinary label could be used instead. However,
using Explicit NULL enables the egress to determine immediately
(i.e., without need for lookup in the Label Information Base) that
the further forwarding of the packet is to be determined by whatever
is below the label. Avoiding this lookup can have favorable
implications on forwarding performance.
Removing the restriction that Explicit Null only occur at the bottom
of the stack is the simplest way to facilitate the proper operation
of the Pipe Model.
4. Acknowledgments
Thanks to Rahul Aggarwal, Francois LeFaucheur, Yakov Rekhter, and Dan
Tappan for their helpful comments.
5. Normative References
[RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001
6. Informative References
[RFC3270] "Multi-Protocol Label Switching (MPLS) Support of
Differentiated Services", Le Faucheur, et. al., May 2002
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7. Author's Address
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
Email: erosen@cisco.com
8. Intellectual Property Notice
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