draft-ietf-mpls-ldp-mtu-extensions-01.txt   draft-ietf-mpls-ldp-mtu-extensions-02.txt 
Network Working Group B. Black Network Working Group B. Black
Internet Draft Layer8 Networks Internet Draft Layer8 Networks
Updates: 3036 K. Kompella Updates: 3036 K. Kompella
Category: Standards Track Juniper Networks Category: Standards Track Juniper Networks
Expires: December 2003 June 2003 Expires: April 2004 October 2003
MTU Signalling Extensions for LDP Maximum Transmission Unit Signalling Extensions
draft-ietf-mpls-ldp-mtu-extensions-01.txt for the Label Distribution Protocol
draft-ietf-mpls-ldp-mtu-extensions-02.txt
Status of this Memo Status of this Memo
This document is an Internet-Draft and is in full conformance with This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
Proper functioning of RFC 1191 path MTU discovery requires that IP Proper functioning of RFC 1191 path Maximum Transmission Unit (MTU)
routers have knowledge of the MTU for each link to which they are discovery requires that IP routers have knowledge of the MTU for each
connected. As currently specified, the Label Distribution Protocol link to which they are connected. As currently specified, the Label
(LDP) does not have the ability to signal the MTU for a Label Distribution Protocol (LDP) does not have the ability to signal the
Switched Path (LSP) to the ingress Label Switching Router (LSR). In MTU for a Label Switched Path (LSP) to the ingress Label Switching
the absence of this functionality, the MTU for each LSP must be Router (LSR). In the absence of this functionality, the MTU for each
statically configured by network operators or by equivalent, off-line LSP must be statically configured by network operators or by
mechanisms. equivalent, off-line mechanisms.
This document specifies extensions to LDP in support of LSP MTU This document specifies extensions to LDP in support of LSP MTU
discovery. discovery.
Conventions used in this document Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [1]. document are to be interpreted as described in RFC-2119 [1].
Changes from last version
[Note to RFC Editor: please remove this section before publishing.]
Incorporated suggestions received on the MPLS WG mailing list, mostly
clarifications and editorial nits.
Expanded examples. Made slight formatting changes.
Updated references.
1. Introduction 1. Introduction
As currently specified in [2], the LDP protocol for MPLS does not As currently specified in [2], the LDP protocol for MPLS does not
support signalling of the MTU for LSPs to ingress LSRs. This support signalling of the MTU for LSPs to ingress LSRs. This
functionality is essential to the proper functioning of RFC 1191 path functionality is essential to the proper functioning of RFC 1191 path
MTU detection [3]. Without knowledge of the MTU for an LSP, edge MTU detection [3]. Without knowledge of the MTU for an LSP, edge
LSRs may transmit packets along that LSP which are, according to [4], LSRs may transmit packets along that LSP which are, according to [4],
too big. Such packets may be silently discarded by LSRs along the too big. Such packets may be silently discarded by LSRs along the
LSP, effectively preventing communication between certain end hosts. LSP, effectively preventing communication between certain end hosts.
The solution proposed in this document enables automatic The solution proposed in this document enables automatic
determination of the MTU for an LSP with the addition of a TLV to 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 carry MTU information for a FEC between adjacent LSRs in LDP Label
Mapping messages. This information is sufficient for a set of LSRs Mapping messages. This information is sufficient for a set of LSRs
along the path followed by an LSP to discover either the exact MTU 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 for that LSP, or an approximation which is no worse than could be
generated with local information on the ingress LSR. generated with local information on the ingress LSR.
1.1. Changes from last version
The biggest change, protocol-wise is that the notion of 'egress'
interface has been removed. The LSP MTU at the egress is now 65535.
This has repurcussions on the processing of the MTU TLV. Also, the
MTU TLV now has both the U and F bits set.
A number of definitions have been introduced to clarify the
exposition. Also, the examples have been changed significantly.
2. MTU Signalling 2. MTU Signalling
The signalling procedure described in this document employs the The signalling procedure described in this document employs the
addition of a single TLV to LDP Label Mapping messages and a simple addition of a single TLV to LDP Label Mapping messages and a simple
algorithm for LSP MTU calculation. algorithm for LSP MTU calculation.
2.1. Definitions 2.1. Definitions
Link MTU: the MTU of a given link. This size includes the IP header 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 and data (or other payload) and the label stack, but does not include
any lower-level headers. A link may be an interface (such as 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 Ethernet or Packet-over-SONET), a tunnel (such as GRE or IPsec) or an
LSP. LSP.
Peer LSRs: for LSR A and FEC F, this is the set of LSRs that sent a Peer LSRs: for LSR A and FEC F, this is the set of LSRs that sent a
Label Mapping for FEC F to A. Label Mapping for FEC F to A.
Downstream LSRs: for LSR A and FEC F, this is the subset of A's peer Downstream LSRs: for LSR A and FEC F, this is the subset of A's peer
LSRs for FEC F to whom A will forward packets for the FEC. LSRs for FEC F to whom A will forward packets for the FEC.
Typically, this subset is determined via the routing table. Typically, this subset is determined via the routing table.
Hop MTU: the MTU of an LSP hop between an upstream LSR A and a 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 downstream LSR B. This size includes the IP header and data (or
other payload) and the part of the label stack that is considered 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 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 headers. (Note: if there are multiple links between A and B, the Hop
MTU is the minimum of the MTU of those links used for forwarding.) 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 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 each valid (forwarding) path. This size includes the IP header and
data (or other payload) and any part of the label stack that was 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 received by the ingress LSR before it placed the packet into the LSP
(this part of the label stack is considered part of the payload for (this part of the label stack is considered part of the payload for
this LSP). The size does not include any lower-level headers. this LSP). The size does not include any lower-level headers.
2.2. Example 2.2. Example
Consider LSRs A-F interconnected as follows: Consider LSRs A-F interconnected as follows:
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2.2. Example 2.2. Example
Consider LSRs A-F interconnected as follows: Consider LSRs A-F interconnected as follows:
M P M P
_____ C ===== _____ C =====
/ | \ / | \
A ~~~~~ B ===== D ----- E ----- F A ~~~~~ B ===== D ----- E ----- F
L N Q R L N Q R
Say that the link MTU for link L is 9216, for links M, Q and R is Say that the link MTU for link L is 9216, for links M, Q and R is
4470, and for N and P is 1500. 4470, and for N and P is 1500.
Consider a FEC X for which F is the egress, and say that all LSRs Consider a FEC X for which F is the egress, and say that all LSRs
advertise X to their neighbors. advertise X to their neighbors.
Note that while 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 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 C chooses E as its C's peers for FEC X are B, D and E. Say C chooses E as its
downstream LSR for X. Similarly, A chooses B, B chooses C and D, D downstream LSR for X. Similarly, A chooses B, B chooses C and D
chooses E and E chooses F (respectively) as their downstream LSRs. (equal cost multi-path), D chooses E and E chooses F (respectively)
as their downstream LSRs.
C's Hop MTU to E for FEC X is 1496. B's Hop MTU to C is 4466, and to 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 D is 1496. A's LSP MTU for FEC X is 1496. If A has another LSP for
FEC Y to F (learned via targetted LDP) that rides over the LSP for FEC Y to F (learned via targetted LDP) that rides over the LSP for
FEC X, the MTU for that LSP would be 1492. FEC X, the MTU for that LSP would be 1492.
If B had a targetted LDP session to E over which B received a Mapping If B had a targetted LDP session to E, say over an RSVP-TE tunnel T,
for FEC X, then E would also be B's peer, and E may be chosen as a and B received a Mapping for FEC X over the targetted LDP session,
downstream LSR for B. 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 FEC X and Y. This memo describes how A determines its LSP MTU for FECs X and Y.
2.3. Signalling Procedure 2.3. Signalling Procedure
The procedure for signalling the MTU is performed hop-by-hop by each 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: LSR L along an LSP for a given FEC F. The steps are as follows:
1. First, L computes the its LSP MTU for FEC F: 1. First, L computes the its LSP MTU for FEC F:
A. If L is the egress for F, L sets the LSP MTU for F to 65535. A. If L is the egress for F, L sets the LSP MTU for F to 65535.
B. If L is not the egress LSR, L computes the LSP MTU for F as B. [OPTIONAL] If L's only downstream LSR is the egress for F
follows: (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.
a) L determines its downstream neighbors for FEC F. C. Otherwise (L is not the egress LSR), L computes the LSP MTU
for F as follows:
b) For each downstream neighbor Z, L computes the minimum of a) L determines its downstream LSRs for FEC F.
the Hop MTU to Z and the LSP MTU in the MTU TLV that Z
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 advertised to L. If Z did not include the MTU TLV in its
Label Mapping, then Z's LSP MTU is set to 65535. 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 c) L sets its LSP MTU to the minimum of the MTUs it computed
for its downstream neighbors. for its downstream LSRs.
2. For each LDP neighbor (direct or targetted) of L to which L 2. For each LDP neighbor (direct or targetted) of L to which L
decides to send a Mapping for FEC F, L attaches an MTU TLV with decides to send a Mapping for FEC F, L attaches an MTU TLV with
the MTU that it computed for this FEC. L MAY (because of policy the LSP MTU that it computed for this FEC. L MAY (because of
or other reasons) advertise a smaller MTU than it has computed, policy or other reasons) advertise a smaller MTU than it has
but L MUST NOT advertise a larger MTU. computed, but L MUST NOT advertise a larger MTU.
3. When a new MTU is received for FEC F from a downstream LSR, or 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. the set of downstream LSRs for F changes, L returns to Step 1.
If the newly computed LSP MTU is unchanged, L SHOULD NOT If the newly computed LSP MTU is unchanged, L SHOULD NOT
advertise new information to its neighbors. Otherwise, L advertise new information to its neighbors. Otherwise, L
readvertises its Mappings for F to all its peers with an updated readvertises its Mappings for F to all its peers with an updated
MTU TLV. MTU TLV.
This behavior is standard for attributes such as path vector and This behavior is standard for attributes such as path vector and
hop count, and the same rules apply, as specified in [2]. hop count, and the same rules apply, as specified in [2].
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Note that the U and F bits are set. An LSR that doesn't recognize 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 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 message, and forward the TLV to its peers. This may result in the
incorrect computation of the LSP MTU; however, silently forwarding incorrect computation of the LSP MTU; however, silently forwarding
the MTU TLV preserves maximal amount of information about the LSP the MTU TLV preserves maximal amount of information about the LSP
MTU. MTU.
3. Example of Operation 3. Example of Operation
Consider the example network in section 2.2. The following table Consider the example network in section 2.2. Table 1 describes, for
describes, for each LSR, the links to its downstream LSRs, the Hop each LSR, the links to its downstream LSRs, the Hop MTU for the peer,
MTU for the peer, the LSP MTU received from the peer, and the LSR's the LSP MTU received from the peer, and the LSR's computed LSP MTU.
computed LSP MTU.
Now consider the same network with the following changes: there is an
LSP T from B to E, and a targetted 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.
LSR | Link | Hop MTU | Recvd MTU | LSP MTU LSR | Link | Hop MTU | Recvd MTU | LSP MTU
-------------------------------------------------- --------------------------------------------------
F | - | 65535 | - | 65535 F | - | 65535 | - | 65535
-------------------------------------------------- --------------------------------------------------
E | R | 4466 | F: 65535 | 4466 E | R | 4466 | F: 65535 | 4466
-------------------------------------------------- --------------------------------------------------
D | Q | 4466 | E: 4466 | 4466 D | Q | 4466 | E: 4466 | 4466
-------------------------------------------------- --------------------------------------------------
C | P | 1496 | E: 4466 | 1496 C | P | 1496 | E: 4466 | 1496
-------------------------------------------------- --------------------------------------------------
B | M | 4466 | C: 1496 | B | M | 4466 | C: 1496 |
| N | 1496 | D: 4466 | 1496 | N | 1496 | D: 4466 | 1496
-------------------------------------------------- --------------------------------------------------
A | L | 9212 | B: 1496 | 1496 A | L | 9212 | B: 1496 | 1496
-------------------------------------------------- --------------------------------------------------
Table 1
Now consider the same network with the following changes: there is an
LSP X from B to E, and a targetted 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 X. The LSP MTU for LSP X is 1496.
LSR | Link | Hop MTU | Recvd MTU | LSP MTU LSR | Link | Hop MTU | Recvd MTU | LSP MTU
-------------------------------------------------- --------------------------------------------------
F | - | 65535 | - | 65535 F | - | 65535 | - | 65535
-------------------------------------------------- --------------------------------------------------
E | R | 4466 | F: 65535 | 4466 E | R | 4466 | F: 65535 | 4466
-------------------------------------------------- --------------------------------------------------
D | Q | 4466 | E: 4466 | 4466 D | Q | 4466 | E: 4466 | 4466
-------------------------------------------------- --------------------------------------------------
C | P | 1496 | E: 4466 | 1496 C | P | 1496 | E: 4466 | 1496
-------------------------------------------------- --------------------------------------------------
B | X | 1492 | E: 4466 | B | T | 1492 | E: 4466 |
| N | 1496 | D: 4466 | 1492 | N | 1496 | D: 4466 | 1492
-------------------------------------------------- --------------------------------------------------
A | L | 9212 | B: 1492 | 1492 A | L | 9212 | B: 1492 | 1492
-------------------------------------------------- --------------------------------------------------
Table 2
4. Using the LSP MTU 4. Using the LSP MTU
An ingress LSR that forwards an IP packet into an LSP whose MTU it 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 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 Don't Fragment bit is clear) or drop the packet and respond with an
ICMP Destination Unreachable message to the source of the packet, ICMP Destination Unreachable message to the source of the packet,
with the Code indicating "fragmentation needed and DF set", and the 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 Next-Hop MTU set to the LSP MTU. In other words, the LSR behaves as
RFC 1191 says, except it treats the LSP as the next hop "network". RFC 1191 says, except it treats the LSP as the next hop "network".
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using LDP, CR-LDP or RSVP-TE [6]; the mechanism suggested here using LDP, CR-LDP or RSVP-TE [6]; the mechanism suggested here
applies in all these cases, essentially by treating the tunnel LSPs applies in all these cases, essentially by treating the tunnel LSPs
as links. as links.
Normative References Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997 Levels", BCP 14, RFC 2119, March 1997
[2] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B. [2] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
Thomas, "LDP Specification", RFC 3036, January 2001. Thomas, "LDP Specification", RFC 3036, January 2001
[3] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191, [3] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
November 1990. November 1990
[4] Rosen, E., Tappan, D., Federkow, G., Rekhter, Y., Farinacci, D., [4] Rosen, E., Tappan, D., Federkow, G., Rekhter, Y., Farinacci, D.,
Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032, Li, T. and A. Conta, "MPLS Label Stack Encoding", RFC 3032,
January 2001. January 2001
[5] Jamoussi, J., "Constraint-Based LSP Setup Using LDP", July 2000. [5] Jamoussi, B., Ed., "Constraint-Based LSP Setup Using LDP", RFC
3212, January 2002
[6] Awduche, D., Berger, L. and D. Gan, "RSVP-TE: Extensions to RSVP [6] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V. and G.
for LSP Tunnels", February 2001. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC
3209, December 2001
Security Considerations Security Considerations
This mechanism does not introduce any new weaknesses in LDP. It is This mechanism does not introduce any new weaknesses in LDP. It is
possible to spoof TCP packets belonging to an LDP session to 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. manipulate the LSP MTU, but LDP has mechanisms to thwart these types
of attacks.
IANA Considerations IANA Considerations
A new LDP TLV Type is defined in section 2.4. A Type has to be A new LDP TLV Type is defined in section 2.4. A Type has to be
allocated by IANA; a number from the range 0x0000 - 0x3DFF is allocated by IANA; a number from the range 0x0000 - 0x3DFF is
requested. requested.
Acknowledgments Acknowledgments
We would like to thank Andre Fredette for a number of detailed We would like to thank Andre Fredette for a number of detailed
comments on earlier versions of the signalling mechanism. Eric Gray comments on earlier versions of the signalling mechanism. Eric Gray,
and Giles Heron have contributed numerous useful suggestions. Giles Heron and Mark Duffy have contributed numerous useful
suggestions.
Authors' Addresses Authors' Addresses
Benjamin Black Benjamin Black
Layer8 Networks Layer8 Networks
EMail: ben@layer8.net EMail: ben@layer8.net
Kireeti Kompella Kireeti Kompella
Juniper Networks Juniper Networks
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obtain a general license or permission for the use of such obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat. be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive this standard. Please address the information to the IETF Executive
Director. Director.
Full Copyright Notice Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing document itself may not be modified in any way, such as by removing
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English. English.
The limited permissions granted above are perpetual and will not be The limited permissions granted above are perpetual and will not be
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Acknowledgement Acknowledgement:
Funding for the RFC Editor function is currently provided by the Funding for the RFC Editor function is currently provided by the
Internet Society. Internet Society.
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