Internet Draft Network Working Group Benjamin Black Internet Draft InterNAP Expiration Date: June 2001 Kireeti Kompella Juniper Networks MTU Signalling Extensions for LDP draft-black-ldp-mtu-extensions-00.txt Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. 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 in [LDP], 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. Black & Kompella [Page 1] Internet Draft draft-black-ldp-mtu-extensions-00.txt June 2001 1. Overview As currently specified in [LDP], 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 for TCP sessions. Without knowledge of the MTU for an LSP, edge LSRs may transmit packets along that LSP which are, according to [MPLS-ENC], too big. Such packets may be silently discarded by LSRs along the LSP, effectively preventing reliable TCP sessions for 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. 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. a) Suppose L is the egress for a FEC. L sets the MTU for this FEC to 0xffff. b) Suppose L receives a Mapping for a FEC with an MTU State TLV with MTU M and over an interface with MTU X. L sets its MTU for this FEC (in octets) to the smaller of M and (X - 4). If L receives multiple Mapping messages for this FEC, it first chooses between them by some policy, then applies the above calculation for the chosen Mapping. This is the "active Mapping" for this FEC. Black & Kompella [Page 2] Internet Draft draft-black-ldp-mtu-extensions-00.txt June 2001 c) 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 1b. Otherwise, L MUST send Mappings for this FEC without an MTU State TLV. d) 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.) Suppose that LSP has MTU M. The LSR then sets the MTU for the FEC to (M - 4). 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. 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. 2.2. MTU State TLV The MTU State 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 State 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 State TLV (0x0XXX) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MTU | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ MTU This is a 16-bit unsigned integer that represents the MTU in bytes for a link, from the perspective of the downstream LSR. Black & Kompella [Page 3] Internet Draft draft-black-ldp-mtu-extensions-00.txt June 2001 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 this F to 0xffff 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 0xffff. Ry compares 0xffff 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. Black & Kompella [Page 4] Internet Draft draft-black-ldp-mtu-extensions-00.txt June 2001 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 4462 to (9216 - 4), and sets the MTU for P1 to 4462. 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/RSVP-TE The MTU State 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 [CR-LDP]. Note that LDP/CR-LDP LSPs may tunnel through other LSPs signalled using LDP, CR-LDP or RSVP-TE [RSVP-TE]; the mechanism suggested here applies in all these cases. 5. Security Considerations The procedure and TLV proposed in this document do not raise any new security concerns. 6. Acknowledgements 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. 7. References [CR-LDP] Jamoussi, B., editor, "Constraint-Based LSP Setup using LDP", draft-ietf-mpls-cr-ldp-04.txt (work in progress) Black & Kompella [Page 5] Internet Draft draft-black-ldp-mtu-extensions-00.txt June 2001 [LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A., Thomas, B., "LDP Specification", draft-ietf-mpls-ldp-11.txt (work in progress) [MPLS-ENC] E. Rosen, Y. Rekhter, D. Tappan, D. Farinacci, G. Fedorkow, T. Li, A. Conta, "MPLS Label Stack Encoding", draft-ietf-mpls-label-encaps-08.txt (work in progress) [RSVP-TE] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., Swallow, G., "RSVP-TE: Extensions to RSVP for LSP Tunnels", draft-ietf-mpls-rsvp-lsp-tunnel-07.txt (work in progress) 10. Authors' Addresses Benjamin Black InterNAP Network Services Two Union Square, Suite 1000 601 Union Street Seattle, WA 98101 email: ben@internap.com Kireeti Kompella Juniper Networks, Inc. 1194 N. Mathilda Ave Sunnyvale, CA 94089 email: kireeti@juniper.net Black & Kompella [Page 6]