Network Working Group Toerless Eckert
Internet Draft Eric C. Rosen (editor)
Intended Status: Standards Track Cisco Systems, Inc.
Expires: May 1, 2008
Updates: RFCs 3032 and 4023 Rahul Aggarwal
Yakov Rekhter
Juniper Networks, Inc.
November 1, 2007
MPLS Multicast Encapsulations
draft-ietf-mpls-multicast-encaps-07.txt
Status of this Memo
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Abstract
RFC 3032 established two data link layer codepoints for MPLS: one to
indicate that the data link layer frame is carrying an MPLS unicast
packet, and the other to indicate that the data link layer frame is
carrying an MPLS multicast packet. This specification updates RFC
3032 by redefining the meaning of these two codepoints. The former
"multicast codepoint" is now to be used only on multiaccess media,
and it is to mean "the top label of the following label stack is an
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upstream-assigned label". The former "unicast codepoint" is to be
used in all other cases. Whether the data link layer payload is a
unicast MPLS packet or a multicast MPLS packet is now to be
determined by looking up the top label, rather than by the codepoint.
RFC 3032 does not specify the destination address to be placed in the
"MAC DA" field of an ethernet frame which carries an MPLS multicast
packet. This document provides that specification.
This document updates RFC 3032 and RFC 4023.
Contents
1 Specification of Requirements ........................... 3
2 Introduction ............................................ 3
3 Upstream-Assigned vs. Downstream-Assigned ............... 4
4 Ethernet Codepoints ..................................... 6
5 PPP Protocol Field ...................................... 6
6 GRE Protocol Type ....................................... 6
7 IP Protocol Number ...................................... 7
8 Ethernet MAC DA for Multicast MPLS ...................... 7
9 IANA Considerations ..................................... 8
10 Security Considerations ................................. 8
11 Normative References .................................... 9
12 Informative References .................................. 9
13 Authors' Addresses ...................................... 9
14 Full Copyright Statement ................................ 10
15 Intellectual Property ................................... 10
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1. Specification of Requirements
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 [RFC2119].
2. Introduction
RFC 3031 [RFC3031] defines the "Next Hop Label Forwarding Entry"
(NHLFE). The NHLFE for a particular label maps the label into a next
hop (among other things). When an MPLS packet is received, its top
label is mapped to an NHLFE, and the packet is sent to the next hop
specified by the NHLFE.
We define a particular MPLS label to be a "multicast label" in a
particular context if the NHLFE to which it is mapped in that context
specifies a set of next hops, with the semantics that the packet is
to be replicated, and a copy of the packet sent to each of the
specified next hops. Note that this definition accommodates the case
where the set of next hops contains a single member. What makes a
label a multicast label in a particular context is the semantics
attached to the set, i.e., the intention to replicate the packet and
transmit to all members of the set if the set has more than one
member.
RFC 3032 [RFC3032] established two data link layer codepoints for
MPLS: one to indicate that the data link layer frame is carrying an
MPLS unicast packet, and the other to indicate that the data link
layer frame is carrying an MPLS multicast packet. The term
"multicast packet" is not precisely defined in RFC 3032, though one
may presume that the "multicast" codepoint is intended to identify
the packet's top label as a multicast label. However, the multicast
codepoint has never been deployed, and further development of the
procedures for MPLS multicast have shown that, while there is a need
for two codepoints, the use of the two codepoints is not properly
captured by RFC 3032.
In particular, there is no need for the codepoint to indicate whether
the top MPLS label is a multicast label. When the receiver of an
MPLS packet looks up the top label, the NHLFE will specify whether
the label is a multicast label or not.
This document updates RFC 3032 and RFC 4023 by re-specifying the use
of the codepoints.
While RFC 3032 allows an MPLS packet to be carried in an ethernet
multicast frame, it fails to specify how the Medium Access Layer
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Destination Address (MAC DA) field is to be set in that case. This
document provides that specification.
3. Upstream-Assigned vs. Downstream-Assigned
According to RFC 3031 [RFC3031], if two MPLS Label Switching Routers
(LSRs) are adjacent in a label switched path (LSP), with respect to
that LSP, one of them may be called the "upstream" LSR and the other
the "downstream" LSR. Call these Ru and Rd respectively. Before Ru
can send an MPLS packet to Rd with label L at the top of the label
stack, Ru and Rd must agree on the Forwarding Equivalence Class (FEC)
which is bound to L. A particular binding of L to FEC F is called a
"downstream-assigned" binding if the binding is first made by Rd and
then advertised to Ru. If the binding is first made by Ru and then
advertised to Rd, it is called an "upstream-assigned" binding.
If Ru and RD are LSP adjacencies, then they transmit a MPLS packet to
each other through one of the following mechanisms:
1. by putting the MPLS packet in a data link layer frame and
transmitting the frame
2. by transmitting the MPLS packet through an MPLS tunnel, i.e.,
by pushing an additional label (or labels) onto the label
stack, and then invoking mechanism 1,
3. by transmitting the MPLS packet through an IP-based tunnel
(e.g., via RFC 4023 [RFC4023]), and then invoking mechanisms 1
and/or 2.
In short, an MPLS packet is transmitted either through a data link or
through an MPLS tunnel or through an IP tunnel. In any of those
cases, when the packet emerges through the tunnel, the downstream LSR
must know whether the label that now appears at the top of the label
stack has an upstream-assigned label binding or a downstream-assigned
label binding. For convenience, we will speak of a label with an
upstream-assigned label binding as an "upstream-assigned label".
Unicast labels MUST be downstream-assigned.
Under certain conditions, specified below, multicast labels MAY be
upstream-assigned. The ability to use upstream-assigned labels is an
OPTIONAL feature. Upstream-assigned labels MUST NOT be used unless
it is known that the downstream LSR supports them. How this is known
is outside the scope of this document.
We discuss three different types of data link or tunnel:
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- Point-to-Point. A point-to-point data link or tunnel associates
two systems, such that transmissions on that link or tunnel made
by the one are received by the other, and only by the other.
For a given direction of a given point-to-point data link or
tunnel, the following MUST be the case: either every MPLS packet
will carry an upstream-assigned label, or else every MPLS packet
will carry a downstream-assigned label. The procedures for
determining whether upstream-assigned or downstream-assigned
labels are being used are outside the scope of this
specification. However, in the absence of any other information,
the use of downstream-assigned labels MUST be presumed by
default.
- Point-to-Multipoint. A point-to-multipoint link or tunnel
associates n systems, such that only one of them can transmit
onto the link or tunnel, and the transmissions may be received by
the other n-1 systems.
The top labels (before applying the data link or tunnel
encapsulation) of all MPLS packets which are transmitted on a
particular point-to-multipoint data link or tunnel MUST be of the
same type; either all upstream-assigned or all downstream-
assigned. This means that all the receivers on the MPLS or IP
tunnel must know a priori whether upstream-assigned or
downstream-assigned labels are being used in the tunnel. How
this is known is outside the scope of this document.
- Multipoint-to-Multipoint. A multipoint-to-multipoint link or
tunnel associates n systems, such that any of them can transmit
on the link or tunnel, and the transmissions may be received by
the other n-1 systems.
If MPLS packets are transmitted on a particular multipoint-to-
multipoint link or tunnel, one of the following scenarios
applies:
1. It is known (by methods outside the scope of this document)
that the top label of every MPLS packet on the link or
tunnel is downstream-assigned
2. It is known (by methods outside the scope of this document)
that the top label of every MPLS packet on the link or
tunnel is upstream-assigned
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3. Some MPLS packets on the link may have upstream-assigned
top labels while some may have downstream-assigned top
labels
If (and only if) the third scenario applies, the data link or
tunnel encapsulation MUST provide a codepoint which specifies
whether the top label of the encapsulated MPLS packet is
upstream-assigned or downstream-assigned. If a particular type
of data link or tunnel does not provide such a codepoint, then
the third scenario MUST NOT be used.
The remainder of this document specifies procedures for setting the
data link layer codepoints and address fields.
4. Ethernet Codepoints
Ethernet is an example of a multipoint-to-multipoint data link.
Ethertype 0x8847 is used whenever a unicast ethernet frame carries an
MPLS packet.
Ethertype 0x8847 is also used whenever a multicast ethernet frame
carries an MPLS packet, EXCEPT for the case where the top label of
the MPLS packet has been upstream-assigned.
Ethertype 0x8848, formerly known as the "MPLS multicast codepoint",
is to be used only when an MPLS packet whose top label is upstream-
assigned is carried in a multicast ethernet frame.
5. PPP Protocol Field
PPP is an example of a point-to-point data link. When a PPP frame is
carrying an MPLS packet, the PPP Protocol field is always set to
0x0281.
6. GRE Protocol Type
RFC 4023 is modified as described below.
If the IP destination address of the GRE encapsulation is a unicast
IP address, then the ethertype value 0x8847 MUST be used in all cases
for the MPLS-in-GRE encapsulation.
If the IP destination address of the GRE encapsulation is a multicast
IP address, then:
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- the ethertype value 0x8847 MUST be used when the top label of the
encapsulated MPLS packet is downstream-assigned,
- the ethertype value 0x8848 MUST be used when the top label of the
encapsulated MPLS packet is upstream-assigned.
Through procedures which are outside the scope of this specification,
it may be known that if the destination address of a GRE packet is a
multicast IP address, then the top label of the GRE payload is
upstream-assigned. In such a case, the occurrence of the 8847
codepoint in a GRE packet with a multicast destination IP address
MUST be considered an error, and the packet MUST be discarded.
7. IP Protocol Number
RFC 4023 is modified as follows: the IPv4 Protocol Number field or
the IPv6 Next Header field is always set to 137, whether or not the
encapsulated MPLS packet is an MPLS multicast packet.
If the IP destination address of the IP encapsulation is an IP
multicast address, the IP tunnel may be considered to be a point-to-
multipoint tunnel or a multipoint-to-multipoint tunnel. In either
case, either all encapsulated MPLS packets in the particular tunnel
have a downstream-assigned label at the top of the stack, or all
encapsulated MPLS packets in that tunnel have an upstream-assigned
label at the top of the stack. The means by which this is determined
for a particular tunnel is outside the scope of this specification.
8. Ethernet MAC DA for Multicast MPLS
When an LSR transmits a multicast MPLS packet in a multicast ethernet
frame, it MUST set the Destination MAC Address to the value
01-00-5e-8v-wx-yz, where vwxyz is a 20-bit (five-nibble) value set as
follows:
1. vwxyz MAY be set to 0
2. vwxyz MAY be set to the value of one of the MPLS labels on the
packet's label stack.
Which of these procedures is the default procedure in any particular
LSR is implementation-dependent. However, LSRs using the two
different procedures MUST interoperate. That is, an LSR MUST NOT
filter packets for which vwxyz has been set to zero, and it MUST NOT
indiscriminately filter all packets for which vwxyz has not been set
to zero.
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If an LSR follows the procedure of setting vwxyz to the value of one
of the MPLS labels on the packet's label stack, and if that label
stack contains two or more labels, then by default, vwxyz MUST be set
to the value of the second MPLS label on the packet's label stack.
By "the second label", we mean the label that is in the label stack
entry that immediately follows the topmost label stack entry. The
LSR MAY, if configured to do so, allow a a label other than the
second to be used for this purpose. If the MPLS packet has only one
label, the value of that label will be used instead of the value of
the (non-existent) second label.
It is expected that the LSR will follow the procedures of [UPSTREAM],
pushing on two labels, with the topmost label being a "context label"
that is the same for all MPLS packets being transmitted by the LSR
onto the ethernet, but with the second label being different for
different LSPs. Thus if the MAC DA value is a function of the second
label, more of the LSP-specific information about the packet appears
in the MAC DA field. This can be used to filter multicast packets
with "unexpected" non-zero values of vwxyz. Further discussion of
such filtering or its uses is outside the scope of this document.
9. IANA Considerations
IANA already owns the set of ethernet multicast addresses in the
range 01-00-5e-00-00-00 to 01-00-5e-ff-ff-ff. Addresses in the range
01-00-5e-00-00-00 to 01-00-5e-7f-ff-ff are reserved for use when an
ethernet multicast frame carries an IP multicast packet. IANA shall
reserve ethernet addresses in the range 01-00-5e-80-00-00 to
01-00-5e-8f-ff-ff for use when an ethernet multicast frame carries an
MPLS multicast packet.
10. Security Considerations
The security considerations of RFC 3032 and RFC 4023 apply.
Malicious changing of the codepoint may result in loss or misrouting
of packets. However, altering the codepoint without also altering the
label does not result in a predictable effect.
Malicious alteration of the MAC DA on an ethernet can result in
packets being received by a third party, rather than by the intended
recipient.
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11. Normative References
[RFC2119] "Key words for use in RFCs to Indicate Requirement
Levels.", Bradner, March 1997
[RFC3031] "Multiprotocol Label Switching Architecture", Rosen,
Viswanathan, Callon, January 2001
[RFC3032] "MPLS Label Stack Encoding", Rosen, et. al., January 2001
[RFC4023] "Encapsulating MPLS in IP or GRE", Worster, Rekhter, Rosen,
March 2005
12. Informative References
[UPSTREAM] "MPLS Upstream Label Assignment and Context Specific Label
Space", Aggarwal, Rekhter, Rosen, draft-ietf-mpls-upstream-
label-03.txt, November 2007.
13. Authors' Addresses
Toerless Eckert
Cisco Systems, Inc.
170 Tasman Drive
San Jose, CA, 95134
Email: eckert@cisco.com
Eric C. Rosen
Cisco Systems, Inc.
1414 Massachusetts Avenue
Boxborough, MA 01719
Email: erosen@cisco.com
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: rahul@juniper.net
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Yakov Rekhter
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: yakov@juniper.net
14. Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
15. Intellectual Property
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