draft-ietf-mpls-ldp-mtu-extensions-00.txt   draft-ietf-mpls-ldp-mtu-extensions-01.txt 
MPLS B. Black Network Working Group B. Black
Internet-Draft Layer8 Networks Internet Draft Layer8 Networks
Expires: December 30, 2002 K. Kompella Updates: 3036 K. Kompella
Juniper Networks Category: Standards Track Juniper Networks
July 1, 2002 Expires: December 2003 June 2003
MTU Signalling Extensions for LDP MTU Signalling Extensions for LDP
draft-ietf-mpls-ldp-mtu-extensions-00 draft-ietf-mpls-ldp-mtu-extensions-01.txt
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved. Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract Abstract
Proper functioning of RFC 1191 path MTU detection requires that IP Proper functioning of RFC 1191 path MTU discovery requires that IP
routers have knowledge of the MTU for each link to which they are routers have knowledge of the MTU for each link to which they are
connected. As currently specified, LDP does not have the ability to connected. As currently specified, the Label Distribution Protocol
signal the MTU for an LSP to ingress LSRs. In the absence of this (LDP) does not have the ability to signal the MTU for a Label
functionality, the MTU for each LSP must be statically configured by Switched Path (LSP) to the ingress Label Switching Router (LSR). In
network operators or by equivalent, off-line mechanisms. 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 This document specifies extensions to LDP in support of LSP MTU
protocol in support of LSP MTU signalling. discovery.
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 RFC-2119 [1].
1. Introduction 1. Introduction
As currently specified in [3], 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 [1]. 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 Signalling Procedure 2.1. Definitions
The procedure for signalling the MTU is performed hop-by-hop by each Link MTU: the MTU of a given link. This size includes the IP header
LSR L along an LSP. The steps are as follows: 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
Ethernet or Packet-over-SONET), a tunnel (such as GRE or IPsec) or an
LSP.
1. First, L computes the MTU for each FEC: Peer LSRs: for LSR A and FEC F, this is the set of LSRs that sent a
Label Mapping for FEC F to A.
1. If L is the egress LSR for the FEC, L set the MTU to the MTU Downstream LSRs: for LSR A and FEC F, this is the subset of A's peer
of the egress interface, unless local policy specifies LSRs for FEC F to whom A will forward packets for the FEC.
otherwise. Typically, this subset is determined via the routing table.
2. If L is not the egress LSR for the FEC, L SHOULD set the MTU Hop MTU: the MTU of an LSP hop between an upstream LSR A and a
to 0xffff, indicating that it is not the egress LSR and has downstream LSR B. This size includes the IP header and data (or
not yet received an MTU other than 0xffff from downstream other payload) and the part of the label stack that is considered
LSRs. Local policy may dictate the selection of a value payload as far as this LSP goes. It does not include any lower-level
other than 0xffff, but the default in the absence of such headers. (Note: if there are multiple links between A and B, the Hop
policy should be 0xffff. MTU is the minimum of the MTU of those links used for forwarding.)
3. If L is not the egress LSR for a FEC, and L receives a LSP MTU: the MTU of an LSP from a given LSR to the egress(es), over
Mapping for a FEC which includes an MTU TLV with a value each valid (forwarding) path. This size includes the IP header and
other than 0xffff, L calculates the MTU according to the data (or other payload) and any part of the label stack that was
rules in Section 2.2. If L receives multiple Mapping received by the ingress LSR before it placed the packet into the LSP
messages for this FEC, it first chooses between them by some (this part of the label stack is considered part of the payload for
policy, then applies the calculation for the chosen Mapping. this LSP). The size does not include any lower-level headers.
This is the "active Mapping" for this FEC.
4. If L receives a Mapping for a FEC without an MTU TLV from a 2.2. Example
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 Consider LSRs A-F interconnected as follows:
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 M P
Mapping for a FEC, L attaches an MTU TLV with the MTU that it _____ C =====
computed for this FEC. Mapping messages sent to "remote" LDP / | \
neighbors need not have an MTU TLV. 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 is
4470, and for N and P is 1500.
3. When a new MTU is received for a label mapping from a downstream Consider a FEC X for which F is the egress, and say that all LSRs
LSR, or the active Mapping for a FEC changes, L returns to Step advertise X to their neighbors.
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 C's peers for FEC X are B, D and E. Say C chooses E as its
hop count, and the same rules apply, as specified in [3]. downstream LSR for X. Similarly, A chooses B, B chooses C and D, D
chooses E and E chooses F (respectively) as their downstream LSRs.
4. In some cases, a node may act as both an LER and an LSR for the C's Hop MTU to E for FEC X is 1496. B's Hop MTU to C is 4466, and to
same LSP. In these situations, the node will calculate multiple D is 1496. A's LSP MTU for FEC X is 1496. If A has another LSP for
MTUs: the MTU advertised to upstream LSRs for labelled traffic FEC Y to F (learned via targetted LDP) that rides over the LSP for
and the MTU used locally when processing unlabelled traffic. The FEC X, the MTU for that LSP would be 1492.
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 If B had a targetted LDP session to E over which B received a Mapping
for FEC X, then E would also be B's peer, and E may be chosen as a
downstream LSR for B.
There is a wide variety of policies which may be used in determining This memo describes how A determines its LSP MTU for FEC X and Y.
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 2.3. Signalling Procedure
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 The procedure for signalling the MTU is performed hop-by-hop by each
calculated MTU MUST be less than or equal to the MTU of the egress LSR L along an LSP for a given FEC F. The steps are as follows:
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 1. First, L computes the its LSP MTU for FEC F:
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 A. If L is the egress for F, L sets the LSP MTU for F to 65535.
MTU.
2.3 MTU TLV B. If L is not the egress LSR, L computes the LSP MTU for F as
follows:
a) L determines its downstream neighbors for FEC F.
b) For each downstream neighbor 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 neighbors.
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
the MTU that it computed for this FEC. L MAY (because of policy
or 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.
2.4. MTU TLV
The MTU TLV encodes information on the maximum transmission unit for 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. an LSP, from the advertising LSR to the egress(es) over all valid
paths.
The encoding for the MTU TLV is: The encoding for the MTU TLV is:
0 1 2 3 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 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 | |1|1| MTU TLV (0x0XXX) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MTU | | MTU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
MTU MTU
This is a 16-bit unsigned integer that represents the MTU in bytes This is a 16-bit unsigned integer that represents the MTU in octets
for an LSP or segment of an LSP. for an LSP or segment of an LSP.
3. Example of Operation 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 figure and below describes a simple LSR topology. Ri and Re are message, and forward the TLV to its peers. This may result in the
the ingress and egress LSRs for LSP P1. Rx and Re are the ingress incorrect computation of the LSP MTU; however, silently forwarding
and egress LSRs for LSP P2. From Rx to Re, LSP P1 is encapsulated in the MTU TLV preserves maximal amount of information about the LSP
LSP P2. Ry is an intermediate LSR which does not act as ingress or MTU.
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]: 3. Example of Operation
1) Ry receives a Mapping for F from Re with an MTU of 9216. Ry Consider the example network in section 2.2. The following table
compares 9216 to 9216 (Ry does not push a label onto the stack for describes, for each LSR, the links to its downstream LSRs, the Hop
either P1 or P2), and sends a mapping message for F with an MTU of MTU for the peer, the LSP MTU received from the peer, and the LSR's
9216 to Rx. computed LSP MTU.
2) Ri and Rx have not received Mappings for F. 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
--------------------------------------------------
At t[2]: 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.
1) Rx receives a Mapping for F from Ry with an MTU of 9216. Rx LSR | Link | Hop MTU | Recvd MTU | LSP MTU
compares 9212 (9216 - 4) to 4466, and sends a Mapping message for F --------------------------------------------------
with an MTU of 4466 to Ri. F | - | 65535 | - | 65535
--------------------------------------------------
E | R | 4466 | F: 65535 | 4466
--------------------------------------------------
D | Q | 4466 | E: 4466 | 4466
--------------------------------------------------
C | P | 1496 | E: 4466 | 1496
--------------------------------------------------
B | X | 1492 | E: 4466 |
| N | 1496 | D: 4466 | 1492
--------------------------------------------------
A | L | 9212 | B: 1492 | 1492
--------------------------------------------------
2) Ri has not received Mappings for F. 4. Using the LSP MTU
At t[3]: 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 it treats the LSP as the next hop "network".
1) Ri receives a Mapping for F from Rx with an MTU of 4462. Ri If the payload for the LSP is not an IP packet, the LSR MUST forward
compares 4466 to 9216, and sets the MTU for P1 to 4462 (4466 minus the packet if it fits (size <= LSP MTU), and SHOULD drop it if it
the overhead of 1 label pushed onto the stack). doesn't fit.
4. Protocol Interaction 5. Protocol Interaction
4.1 Interaction With LSRs Which Do Not Support MTU Signalling 5.1. Interaction With LSRs Which Do Not Support MTU Signalling
Changes in MTU for sections of an LSP may cause intermediate LSRs to Changes in MTU for sections of an LSP may cause intermediate LSRs to
generate unsolicited label Mapping messages to advertise the new MTU. generate unsolicited label Mapping messages to advertise the new MTU.
LSRs which do not support MTU signalling MUST accept these messages, LSRs which do not support MTU signalling MUST accept these messages,
but MAY ignore them (see Section 2.1). but MAY ignore them (see Section 2.1).
4.2 Interaction with CR-LDP and RSVP-TE 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 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 CR-LDP LSPs. This proposal is not impacted in the presence of LSPs
created using CR-LDP, as specified in [2]. created using CR-LDP, as specified in [5].
Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled
using LDP, CR-LDP or RSVP-TE [5]; the mechanism suggested here using LDP, CR-LDP or RSVP-TE [6]; the mechanism suggested here
applies in all these cases. applies in all these cases, essentially by treating the tunnel LSPs
as links.
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.
6. Acknowledgments Normative References
We would like to thank Andre Fredette for a number of detailed [1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
comments on earlier versions of the signalling mechanism. Eric Gray Levels", BCP 14, RFC 2119, March 1997
and Giles Heron have contributed numerous useful suggestions.
References (Normative) [2] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and B.
Thomas, "LDP Specification", RFC 3036, January 2001.
[1] 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.
[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., [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] Awduche, D., Berger, L. and D. Gan, "RSVP-TE: Extensions to RSVP [5] Jamoussi, J., "Constraint-Based LSP Setup Using LDP", July 2000.
[6] Awduche, D., Berger, L. and D. Gan, "RSVP-TE: Extensions to RSVP
for LSP Tunnels", February 2001. for LSP Tunnels", February 2001.
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.
IANA Considerations
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
requested.
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.
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
1194 N. Mathilda Ave 1194 N. Mathilda Ave
Sunnyvale, CA 94089 Sunnyvale, CA 94089
US US
EMail: kireeti@juniper.net EMail: kireeti@juniper.net
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