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IETF RFC 3988
Maximum Transmission Unit Signalling Extensions for the Label Distribution Protocol
Last modified on Monday, January 31st, 2005
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Network Working Group B. Black
Request for Comments: 3988 Layer8 Networks
Category: Experimental K. Kompella
Juniper Networks
January 2005
Maximum Transmission Unit Signalling Extensions
for the Label Distribution Protocol
 Status of This Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2005).
Abstract
Proper functioning of RFC 1191 path Maximum Transmission Unit (MTU)
discovery requires that IP routers have knowledge of the MTU for each
link to which they are connected. As currently specified, the Label
Distribution Protocol (LDP) does not have the ability to signal the
MTU for a Label Switched Path (LSP) to the ingress Label Switching
Router (LSR). 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 experimental extensions to LDP in support of
LSP MTU discovery.
1. Introduction
As currently specified in [2], 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 [3]. Without knowledge of the MTU for an LSP, edge
LSRs may transmit packets along that LSP which are, according to [4],
too big. These packets may be silently discarded by LSRs along the
LSP, effectively preventing communication between certain end hosts.
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RFC 3988 MTU Signalling Extensions for LDP January 2005
The solution proposed in this document enables automatic
determination of the MTU for an LSP by adding a Type-Length-Value
triplet (TLV) to carry MTU information for a Forwarding Equivalence
Class (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 that is no worse than could be generated with local
information on the ingress LSR.
1.1. 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 BCP 14, RFC 2119 [1].
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. Definitions
Link MTU: The MTU of a given link. This size includes the IP header
and data (or other payload) and the label stack but does not include
any lower-layer headers. A link may be an interface (such as
Ethernet or Packet-over-SONET), a tunnel (such as GRE or IPsec), or
an LSP.
Peer LSRs: For LSR A and FEC F, this is the set of LSRs that sent a
Label Mapping for FEC F to A.
Downstream LSRs: For LSR A and FEC F, this is the subset of A's peer
LSRs for FEC F to which A will forward packets for the FEC.
Typically, this subset is determined via the routing table.
Hop MTU: The MTU of an LSP hop between an upstream LSR, A, and a
downstream LSR, B. This size includes the IP header and data (or
other payload) and the part of the label stack that is considered
payload as far as this LSP goes. It does not include any lower-level
headers. (Note: If there are multiple links between A and B, the Hop
MTU is the minimum of the Hop MTU of those links used for
forwarding.)
LSP MTU: The MTU of an LSP from a given LSR to the egress(es), over
each valid (forwarding) path. This size includes the IP header and
data (or other payload) and any part of the label stack that was
received by the ingress LSR before it placed the packet into the LSP
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RFC 3988 MTU Signalling Extensions for LDP January 2005
(this part of the label stack is considered part of the payload for
this LSP). The size does not include any lower-level headers.
2.2. Example
Consider LSRs A - F, interconnected as follows:
M P
_____ C =====
/ | \
A ~~~~~ B ===== D ----- E ----- F
L N Q R
Say that the link MTU for link L is 9216; for links M, Q, and R,
4470; and for N and P, is 1500.
Consider an FEC X for which F is the egress, and say that all LSRs
advertise X to their neighbors.
Note that although LDP may be running on the C - D link, it is not
used for forwarding (e.g., because it has a high metric). In
particular, D is an LDP neighbor of C, but D is not one of C's
downstream LSRs for FEC X.
E's peers for FEC X are C, D, and F. Say that E chooses F as its
downstream LSR for X. E's Hop MTU for link R is 4466. If F
advertised an implicit null label to E, then E MAY set the Hop MTU
for R to 4470.
C's peers for FEC X are B, D, and E. Say that C chooses E as its
downstream LSR for X. Similarly, A chooses B, B chooses C and D
(equal cost multi-path), D chooses E, and E chooses F (respectively)
as downstream LSRs.
C's Hop MTU to E for FEC X is 1496. B's Hop MTU to C is 4466 and to
D is 1496. A's LSP MTU for FEC X is 1496. If A has another LSP for
FEC Y to F (learned via targeted LDP) that rides over the LSP for FEC
X, the MTU for that LSP would be 1492.
If B had a targeted LDP session to E (e.g., over an RSVP-TE tunnel T)
and B received a Mapping for FEC X over the targeted LDP session,
then E would also be B's peer, and E may be chosen as a downstream
LSR for B. In that case, B's LSP MTU for FEC X would then be the
smaller of {(T's MTU - 4), E's LSP MTU for X}.
This memo describes how A determines its LSP MTU for FECs X and Y.
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RFC 3988 MTU Signalling Extensions for LDP January 2005
2.3. Signalling Procedure
The procedure for signalling the MTU is performed hop-by-hop by each
LSR L along an LSP for a given FEC, F. The steps are as follows:
1. First, L computes its LSP MTU for FEC F:
A. If L is the egress for F, L sets the LSP MTU for F to 65535.
B. [OPTIONAL] If L's only downstream LSR is the egress for F
(i.e., L is a penultimate hop for F) and L receives an implicit
null label as its Mapping for F, then L can set the Hop MTU for
its downstream link to the link MTU instead of (link MTU - 4
octets). L's LSP MTU for F is the Hop MTU.
C. Otherwise (L is not the egress LSR), L computes the LSP MTU for
F as follows:
a) L determines its downstream LSRs for FEC F.
b) For each downstream LSR Z, L computes the minimum of the Hop
MTU to Z and the LSP MTU in the MTU TLV that Z advertised to
L. If Z did not include the MTU TLV in its Label Mapping,
then Z's LSP MTU is set to 65535.
c) L sets its LSP MTU to the minimum of the MTUs it computed
for its downstream LSRs.
2. For each LDP neighbor (direct or targeted) of L to which L decides
to send a Mapping for FEC F, L attaches an MTU TLV with the LSP
MTU that it computed for this FEC. L MAY (because of policy or
for other reasons) advertise a smaller MTU than it has computed,
but L MUST NOT advertise a larger MTU.
3. When a new MTU is received for FEC F from a downstream LSR or the
set of downstream LSRs for F changes, L returns to step 1. If the
newly computed LSP MTU is unchanged, L SHOULD NOT advertise new
information to its neighbors. Otherwise, L readvertises its
Mappings for F to all its peers with an updated MTU TLV.
This behavior is standard for attributes such as path vector and
hop count, and the same rules apply, as specified in [2].
If the LSP MTU decreases, L SHOULD readvertise the new MTU
immediately; if the LSP MTU increases, L MAY hold down the
readvertisement.
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RFC 3988 MTU Signalling Extensions for LDP January 2005
2.4. MTU TLV
The MTU TLV encodes information on the maximum transmission unit for
an LSP, from the advertising LSR to the egress(es) over all valid
paths.
The encoding for the MTU TLV is as follows:
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|1| MTU TLV (0x0601) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MTU
This is a 16-bit unsigned integer that represents the MTU in octets
for an LSP or a segment of an LSP.
Note that the U and F bits are set. An LSR that doesn't recognize
the MTU TLV MUST ignore it when it processes the Label Mapping
message and forward the TLV to its peers. This may result in the
incorrect computation of the LSP MTU; however, silently forwarding
the MTU TLV preserves the maximal amount of information about the LSP
MTU.
3. Example of Operation
Consider the network example in Section 2.2. For each LSR, Table 1
describes the links to its downstream LSRs, the Hop MTU for the peer,
the LSP MTU received from the peer, and the LSR's computed LSP MTU.
Now consider the same network with the following changes: There is an
LSP T from B to E, and a targeted LDP session from B to E. B's peer
LSRs are A, C, D, and E; B's downstream LSRs are D and E; to reach E,
B chooses to go over T. The LSP MTU for LSP T is 1496. This
information is depicted in Table 2.
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RFC 3988 MTU Signalling Extensions for LDP January 2005
LSR | Link | Hop MTU | Recvd MTU | LSP MTU
--------------------------------------------------
F | - | 65535 | - | 65535
--------------------------------------------------
E | R | 4466 | F: 65535 | 4466
--------------------------------------------------
D | Q | 4466 | E: 4466 | 4466
--------------------------------------------------
C | P | 1496 | E: 4466 | 1496
--------------------------------------------------
B | M | 4466 | C: 1496 |
| N | 1496 | D: 4466 | 1496
--------------------------------------------------
A | L | 9212 | B: 1496 | 1496
--------------------------------------------------
Table 1
LSR | Link | Hop MTU | Recvd MTU | LSP MTU
--------------------------------------------------
F | - | 65535 | - | 65535
--------------------------------------------------
E | R | 4466 | F: 65535 | 4466
--------------------------------------------------
D | Q | 4466 | E: 4466 | 4466
--------------------------------------------------
C | P | 1496 | E: 4466 | 1496
--------------------------------------------------
B | T | 1492 | E: 4466 |
| N | 1496 | D: 4466 | 1492
--------------------------------------------------
A | L | 9212 | B: 1492 | 1492
--------------------------------------------------
Table 2
4. Using the LSP MTU
An ingress LSR that forwards an IP packet into an LSP whose MTU it
knows MUST either fragment the IP packet to the LSP's MTU (if the
Don't Fragment bit is clear) or drop the packet and respond with an
ICMP Destination Unreachable message to the source of the packet,
with the Code indicating "fragmentation needed and DF set", and the
Next-Hop MTU set to the LSP MTU. In other words, the LSR behaves as
RFC 1191 says, except that it treats the LSP as the next hop
"network".
If the payload for the LSP is not an IP packet, the LSR MUST forward
the packet if it fits (size <= LSP MTU) and SHOULD drop it if it
doesn't.
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RFC 3988 MTU Signalling Extensions for LDP January 2005
5. Protocol Interaction
5.1. Interaction with LSRs that 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 that do not support MTU signalling will, due to message and TLV
processing mechanisms specified in RFC 3036 [2], accept the messages
carrying the MTU TLV but will ignore the TLV and forward the TLV to
the upstream nodes (see Section 2.4).
5.2. 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 with CR-LDP, as specified in [5].
Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled
using LDP, CR-LDP, or RSVP-TE [6]; the mechanism suggested here
applies in all of these cases, essentially by treating the tunnel
LSPs as links.
6. References
6.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Andersson, L., Doolan, P., Feldman, N., Fredette, A., and B.
Thomas, "LDP Specification", RFC 3036, January 2001.
[3] Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
November 1990.
[4] Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y., Farinacci, D.,
Li, T., and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
January 2001.
[6] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G.
Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC
3209, December 2001.
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RFC 3988 MTU Signalling Extensions for LDP January 2005
6.2. Informative References
[5] Jamoussi, B., Andersson, L., Callon, R., Dantu, R., Wu, L.,
Doolan, P., Worster, T., Feldman, N., Fredette, A., Girish, M.,
Gray, E., Heinanen, J., Kilty, T., and A. Malis, "Constraint-
Based LSP Setup using LDP", RFC 3212, January 2002.
7. 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 LDP has mechanisms to thwart these types
of attacks. See Section 5 of [2] for more information on security
aspects of LDP.
8. IANA Considerations
IANA has allocated 0x0601 as a new LDP TLV Type, defined in Section
2.4. See: http://www.iana.org/assignments/ldp-namespaces
9. Acknowledgements
We would like to thank Andre Fredette for a number of detailed
comments on earlier versions of the signalling mechanism. Eric Gray,
Giles Heron, and Mark Duffy have contributed numerous useful
suggestions.
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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RFC 3988 MTU Signalling Extensions for LDP January 2005
Full Copyright Statement
Copyright © The Internet Society (2005).
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Acknowledgement
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Maximum Transmission Unit Signalling Extensions for the Label Distribution Protocol
RFC TOTAL SIZE: 18841 bytes
PUBLICATION DATE: Monday, January 31st, 2005
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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