Network Working Group B. Black Internet Draft Layer8 Networks Category: Experimental K. Kompella Juniper Networks Expires: October 2004 April 2004 Maximum Transmission Unit Signalling Extensions for the Label Distribution Protocol draft-ietf-mpls-ldp-mtu-extensions-03.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. Copyright Notice Copyright (C) The Internet Society (2004). All Rights Reserved. Black & Kompella Experimental [Page 1] Internet Draft MTU Signalling Extensions for LDP April 2004 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. 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]. Changes from last version [Note to RFC Editor: please remove this section before publishing.] - changed category to Experimental - incorporated suggestions from WG chairs and IESG Black & Kompella Experimental [Page 2] Internet Draft MTU Signalling Extensions for LDP April 2004 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. 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 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 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. 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 whom 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.) Black & Kompella Experimental [Page 3] Internet Draft MTU Signalling Extensions for LDP April 2004 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 (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 is 4470, and for N and P is 1500. Consider a FEC X for which F is the egress, and say that all LSRs 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 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 their 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 targetted LDP) that rides over the LSP for FEC X, the MTU for that LSP would be 1492. If B had a targetted LDP session to E, say over an RSVP-TE tunnel T, and B received a Mapping for FEC X over the targetted 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}. Black & Kompella Experimental [Page 4] Internet Draft MTU Signalling Extensions for LDP April 2004 This memo describes how A determines its LSP MTU for FECs X and Y. 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 the 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 targetted) 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 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 Black & Kompella Experimental [Page 5] Internet Draft MTU Signalling Extensions for LDP April 2004 readvertisement. 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: 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 (0x0XXX) | Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MTU | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ MTU This is a 16-bit unsigned integer that represents the MTU in octets for an LSP or 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 maximal amount of information about the LSP MTU. 3. Example of Operation Consider the example network in section 2.2. Table 1 describes, for each LSR, 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 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. Black & Kompella Experimental [Page 6] Internet Draft MTU Signalling Extensions for LDP April 2004 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 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 fit. Black & Kompella Experimental [Page 7] Internet Draft MTU Signalling Extensions for LDP April 2004 5. Protocol Interaction 5.1. 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 will accept these messages, but will ignore them (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 using 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 these cases, essentially by treating the tunnel LSPs as links. 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., Federkow, 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 Black & Kompella Experimental [Page 8] Internet Draft MTU Signalling Extensions for LDP April 2004 Informative References [5] Jamoussi, B., Ed., "Constraint-Based LSP Setup Using LDP", RFC 3212, January 2002 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 (see Section 5 of [2]) to thwart these types of attacks. 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, 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 Black & Kompella Experimental [Page 9] Internet Draft MTU Signalling Extensions for LDP April 2004 IPR Notice The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. 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