MPLS B. Black
Internet-Draft Layer8 Networks
Expires: December 30, 2002 K. Kompella
Juniper Networks
July 1, 2002
MTU Signalling Extensions for LDP
draft-ietf-mpls-ldp-mtu-extensions-00
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Proper functioning of RFC 1191 path MTU detection requires that IP
routers have knowledge of the MTU for each link to which they are
connected. As currently specified, LDP does not have the ability to
signal the MTU for an LSP to ingress LSRs. In the absence of this
functionality, the MTU for each LSP must be statically configured by
network operators or by equivalent, off-line mechanisms.
This document specifies extensions to the LDP label distribution
protocol in support of LSP MTU signalling.
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1. Introduction
As currently specified in [3], the LDP protocol for MPLS does not
support signalling of the MTU for LSPs to ingress LSRs. This
functionality is essential to the proper functioning of RFC 1191 path
MTU detection [1]. Without knowledge of the MTU for an LSP, edge
LSRs may transmit packets along that LSP which are, according to [4],
too big. Such packets may be silently discarded by LSRs along the
LSP, effectively preventing communication between certain end hosts.
The solution proposed in this document enables automatic
determination of the MTU for an LSP with the addition of a TLV to
carry MTU information for a FEC between adjacent LSRs in LDP Label
Mapping messages. This information is sufficient for a set of LSRs
along the path followed by an LSP to discover either the exact MTU
for that LSP, or an approximation which is no worse than could be
generated with local information on the ingress LSR.
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2. MTU Signalling
The signalling procedure described in this document employs the
addition of a single TLV to LDP Label Mapping messages and a simple
algorithm for LSP MTU calculation.
2.1 Signalling Procedure
The procedure for signalling the MTU is performed hop-by-hop by each
LSR L along an LSP. The steps are as follows:
1. First, L computes the MTU for each FEC:
1. If L is the egress LSR for the FEC, L set the MTU to the MTU
of the egress interface, unless local policy specifies
otherwise.
2. If L is not the egress LSR for the FEC, L SHOULD set the MTU
to 0xffff, indicating that it is not the egress LSR and has
not yet received an MTU other than 0xffff from downstream
LSRs. Local policy may dictate the selection of a value
other than 0xffff, but the default in the absence of such
policy should be 0xffff.
3. If L is not the egress LSR for a FEC, and L receives a
Mapping for a FEC which includes an MTU TLV with a value
other than 0xffff, L calculates the MTU according to the
rules in Section 2.2. If L receives multiple Mapping
messages for this FEC, it first chooses between them by some
policy, then applies the calculation for the chosen Mapping.
This is the "active Mapping" for this FEC.
4. If L receives a Mapping for a FEC without an MTU TLV from a
directly connected neighbor, L MAY act as if it received an
MTU TLV with MTU 0xffff, and follow the procedure in Step
1.2. Otherwise, L MUST send Mappings for this FEC without an
MTU TLV.
5. If L receives a Mapping for a FEC from a peer to which it is
not directly connected, it must first find an LSP by which L
can reach the peer. (Note that this procedure may be
recursively applied.) Once the appropriate LSP has been
determined, the MTU is calculated according to the rules in
Section 2.2, using the MTU of the selected LSP as the link
MTU.
2. For each direct LDP neighbor of L to which L decides to send a
Mapping for a FEC, L attaches an MTU TLV with the MTU that it
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computed for this FEC. Mapping messages sent to "remote" LDP
neighbors need not have an MTU TLV.
3. When a new MTU is received for a label mapping from a downstream
LSR, or the active Mapping for a FEC changes, L returns to Step
1. If the newly computed MTU is unchanged, L does not advertise
new information to its neighbors.
This behavior is standard for attributes such as path vector and
hop count, and the same rules apply, as specified in [3].
4. In some cases, a node may act as both an LER and an LSR for the
same LSP. In these situations, the node will calculate multiple
MTUs: the MTU advertised to upstream LSRs for labelled traffic
and the MTU used locally when processing unlabelled traffic. The
procedure for calculating each of these MTUs is unchanged from
the steps above, although the series of steps taken will differ
depending on which MTU is being calculated.
2.2 Calculating Local MTU
There is a wide variety of policies which may be used in determining
the MTU advertised by a node, however there are restrictions which
MUST be adhered to in order to ensure proper operation of MTU
signalling and minimization of signalling traffic during topology
changes.
If the local policy is based entirely on the egress interface for
the LSP, the calculated MTU must be less than or equal to the
egress interface MTU.
If the local policy is based on a group of egress interfaces, the
calculated MTU MUST be less than or equal to the MTU of the egress
interface with the largest MTU in the group minus any label
overhead, but SHOULD be less than or equal to the MTU of the
egress interface with the smallest MTU in the group minus any
label overhead.
If the local LSR is the ingress LER for the FEC in question, any
label overhead introduced must be subtracted from the calculated
MTU to determine the actual path MTU. For example, if 2 labels
are pushed onto the stack, the LSR MUST subtract 8 bytes from the
MTU value it has calculated based on local link MTUs and MTU
values received from downstream LDP neighbors.
Under no circumstances must the advertised MTU exceed the received
MTU.
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2.3 MTU TLV
The MTU TLV encodes information on the maximum transmission unit for
an LSP, either for the entire path or only for a segment of the path.
The encoding for the MTU TLV is:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| MTU TLV (0x0XXX) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MTU
This is a 16-bit unsigned integer that represents the MTU in bytes
for an LSP or segment of an LSP.
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3. Example of Operation
The figure and below describes a simple LSR topology. Ri and Re are
the ingress and egress LSRs for LSP P1. Rx and Re are the ingress
and egress LSRs for LSP P2. From Rx to Re, LSP P1 is encapsulated in
LSP P2. Ry is an intermediate LSR which does not act as ingress or
egress for any LSPs. L1 through L3 are links connecting the LSRs.
Le is the egress link.
MTU
Media w/ P2
+--+ +--+ +--+ +--+ Link MTU overhead
--|Ri|--L1--|Rx|--L2--|Ry|--L3--|Re|--Le ---- ------ --------
+--+ +--+ +--+ +--+ L1 9216 9216
| | ^^ L2 4470 4466
| | || L3 9216 9212
| +---P2-------------+| Le 9216 9216
| |
+-------------P1--------------+
Figure 1. Sample LSR Topology
The following four time steps illustrate the calculation of the MTU
for P1. Let FEC F represent traffic mapped to LSP P1.
At t[0]:
1) Re sets the MTU for F to 9216 (the MTU of the egress interface)
and sends a Mapping message for F to Ry.
2) Ri, Rx, and Ry have not received Mappings for F.
At t[1]:
1) Ry receives a Mapping for F from Re with an MTU of 9216. Ry
compares 9216 to 9216 (Ry does not push a label onto the stack for
either P1 or P2), and sends a mapping message for F with an MTU of
9216 to Rx.
2) Ri and Rx have not received Mappings for F.
At t[2]:
1) Rx receives a Mapping for F from Ry with an MTU of 9216. Rx
compares 9212 (9216 - 4) to 4466, and sends a Mapping message for F
with an MTU of 4466 to Ri.
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2) Ri has not received Mappings for F.
At t[3]:
1) Ri receives a Mapping for F from Rx with an MTU of 4462. Ri
compares 4466 to 9216, and sets the MTU for P1 to 4462 (4466 minus
the overhead of 1 label pushed onto the stack).
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4. Protocol Interaction
4.1 Interaction With LSRs Which Do Not Support MTU Signalling
Changes in MTU for sections of an LSP may cause intermediate LSRs to
generate unsolicited label Mapping messages to advertise the new MTU.
LSRs which do not support MTU signalling MUST accept these messages,
but MAY ignore them (see Section 2.1).
4.2 Interaction with CR-LDP and RSVP-TE
The MTU TLV can be used to discover the Path MTU of both LDP LSPs and
CR-LDP LSPs. This proposal is not impacted in the presence of LSPs
created using CR-LDP, as specified in [2].
Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled
using LDP, CR-LDP or RSVP-TE [5]; the mechanism suggested here
applies in all these cases.
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5. Security Considerations
This mechanism does not introduce any new weaknesses in LDP. It is
possible to spoof TCP packets belonging to an LDP session to
manipulate the LSP MTU, but this sort of attack is not new to LDP.
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6. Acknowledgments
We would like to thank Andre Fredette for a number of detailed
comments on earlier versions of the signalling mechanism. Eric Gray
and Giles Heron have contributed numerous useful suggestions.
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References (Normative)
[1] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990.
[2] Jamoussi, J., "Constraint-Based LSP Setup Using LDP", July 2000.
[3] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
Thomas, "LDP Specification", RFC 3036, January 2001.
[4] Rosen, E., Tappan, D., Federkow, G., Rekhter, Y., Farinacci, D.,
Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
January 2001.
[5] Awduche, D., Berger, L. and D. Gan, "RSVP-TE: Extensions to RSVP
for LSP Tunnels", February 2001.
Authors' Addresses
Benjamin Black
Layer8 Networks
EMail: ben@layer8.net
Kireeti Kompella
Juniper Networks
1194 N. Mathilda Ave
Sunnyvale, CA 94089
US
EMail: kireeti@juniper.net
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