Internet DRAFT - draft-black-ldp-mtu-extensions
draft-black-ldp-mtu-extensions
Network Working Group B. Black
Internet-Draft Layer8 Networks
Expires: May 21, 2002 K. Kompella
Juniper Networks
November 20, 2001
MTU Signalling Extensions for LDP
draft-black-ldp-mtu-extensions-02
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This document is an Internet-Draft and is in full conformance with
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Copyright Notice
Copyright (C) The Internet Society (2001). 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 [1]. As currently specified in [3], LDP does not have the
ability to signal the MTU for an LSP to ingress LSRs.
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 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 State TLV
from a directly connected neighbor, L MAY act as if it
received an MTU State TLV with MTU 0xffff, and follow the
procedure in Step 1.2. Otherwise, L MUST send Mappings for
this FEC without an MTU State TLV.
5. If L receives a Mapping for a FEC from a neighbor to which
it is not directly connected, it must first find an LSP by
which L can reach the neighbor. (Note that this procedure
may be recursively applied.) Once the appropriate LSP has
been determined, the MTU is calculated as usual, 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 State TLV with the MTU that
it computed for this FEC. Mapping messages sent to "remote" LDP
neighbors need not have an MTU State TLV.
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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].
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 minus any label overhead.
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.
Under no circumstances must the advertised MTU exceed the
received MTU.
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 TLV
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.
MTU
Media w/ P2
+--+ +--+ +--+ +--+ Link MTU overhead
--|Ri|--L1--|Rx|--L2--|Ry|--L3--|Re|-- ---- ------ --------
+--+ +--+ +--+ +--+ L1 9216 9216
| | ^^ L2 4470 4466
| | || L3 9216 9212
| +---P2-------------+|
| |
+-------------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 (9212 - 4), and sends a Mapping message for F with
an MTU of 9208 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 9212. Rx
compares 9208 to (4466 - 4), and sends a Mapping message for F with
an MTU of 4462 to Ri.
2) Ri has not received Mappings for F.
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At t[3]:
1) Ri receives a Mapping for F from Rx with an MTU of 4462. Ri
compares 4462 to (9216 - 4), and sets the MTU for P1 to 4462.
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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. Danny
McPherson and Vijay Gill also gave useful feedback on earlier
versions of the draft. Eric Gray has contributed numerous useful
suggestions.
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References
[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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