Network Working Group R. Aggarwal Internet Draft Juniper Networks
Expiration Date: August 2006Y. Rekhter Juniper Networks E. Rosen Cisco Systems, Inc. FebruaryDecember 2006 MPLS Upstream Label Assignment and Context Specific Label Space draft-ietf-mpls-upstream-label-00.txtdraft-ietf-mpls-upstream-label-01.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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 RFC 3031 limits the MPLS architecture to downstream assigned MPLS labels. This document introduces the notion of upstream assigned MPLS labels. It describes the procedures for upstream MPLS label assignment and introduces the concept of a "Context-Specific Label Space". Table of Contents 1 Specification of requirements ......................... 2 2 Introduction .......................................... 2 3 Context-Specific Label Space .......................... 3 4 Upstream Label Assignment ............................. 4 4.1 Upstream Assigned and Downstream Assigned Labels ...... 4 5 Assigning Upstream Assigned Labels .................... 5 6 Distributing Upstream Assigned Labels ................. 5 7 Upstream Neighbor Label Space and Tunnel Label Space .. 6 8 Usage of Upstream Assigned Labels ..................... 7 9 References ............................................ 8 9.1 Normative References .................................. 8 9.2 Informative References ................................ 8 10 Author Information .................................... 8 11 Intellectual Property Statement ....................... 9 12 Full Copyright Statement .............................. 9 1. Specification of requirements 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 [RFC2119]. 2. Introduction RFC 3031 [RFC3031] limits the MPLS architecture to downstream assigned MPLS labels. To quote from RFC 3031: "In the MPLS architecture, the decision to bind a particular label L to a particular FEC F is made by the LSR which is DOWNSTREAM with respect to that binding. The downstream LSR then informs the upstream LSR of the binding. Thus labels are "downstream-assigned", and label bindings are distributed in the "downstream to upstream" direction." MPLS upstream label assignment has been discussed and mentioned before [RFC3353, MVPN]. However the architecture for MPLS upstream label assignment and the associated procedures have not been described. This document introduces the notion of upstream assigned MPLS labels to the MPLS architecture. The procedures for upstream MPLS label assignment are described. RFC 3031 describes per-platform and per-interface label space. This document generalizes the latter to a "Context-Specific Label Space" and describes a "Neighbor Label Space" as an example of this. Upstream assigned labels are always looked up in a context-specific label space. 3. Context-Specific Label Space RFC 3031 describes per-platform and per-interface label spaces. This document introduces the more general concept of a "Context-Specific Label Space". A LSR may contain one or more context-specific label spaces. In general, labels are looked up in the per-platform label space unless something about the context determines that a label be looked up in a particular context-specific label space. One example of a context-specific label space is the per-interface label space discussed in RFC 3031. When a MPLS packet is received over a particular interface the top label of the packet may need to be looked up in the receiving interface's per-interface label space. In this case the receiving interface determines the context of the packet. Whether MPLS packets received over a particular interface need to have their top labels looked up in a per-interface label space depends on some characteristic or configuration of the interface. There may be more than one kind of context-specific label space. Context-specific label spaces can be used for downstream assigned labels or upstream assigned labels. Per-interface label space [RFC3031] is an example of a context-specific label space used for downstream assigned labels. When MPLS labels are upstream assigned the context of a MPLS label L is provided by the LSR that assigns the label and binds the label to a FEC F for a LSP LSP1. The LSR that assigns the label distributes the binding and context to a LSR Lr that then receives MPLS packets on LSP1 with label L. When Lr receives a MPLS packet on LSP1 it MUST be able to determine the context of this packet. An example of such a context is a Tunnel over which MPLS packets on LSP1 may be received and in this case the top label of the MPLS packet is looked up in a "Tunnel Specific Label Space". This does imply that Lr be able to determine the tunnel over which the packet was received. If the tunnel is a MPLS tunnel, penultimate-hop-popping (PHP) must be disabled for the tunnel. Another example of such a con- textcontext is the neighbor from which MPLS packets on LSP1 may be received and in this case the top label of the MPLS packet is looked up in a "Neighbor Specific Label Space". These are further described in sec- tionsection 7. There may be other sorts of contexts as well. For instance, we define the notion of a MPLS label being used to establish a context, i.e. identify a label space. 4. Upstream Label Assignment When two MPLS LSRs are adjacent in a MPLS label switched path (LSP) one of them can be termed an "upstream LSR" and the other a "down- stream"downstream LSR" [RFC3031]. Consider two LSRs, Ru and Rd that have agreed to bind Label L to a Forwarding Equivalence Class (FEC), F, for pack- etspackets sent from Ru to Rd. Then with respect to this binding, Ru is the "upstream LSR", and Rd is the "downstream LSR"." When the label binding for F is first made by Rd and distributed by Rd to Ru, the binding is said to be "downstream assigned". When the label binding for F is first made by Ru and distributed by Ru to Rd, the binding is said to be "upstream assigned". An important observation is that the downstream LSR Rd that receives MPLS packets with a top label L is not the LSR that assigns and dis- tributesdistributes label L. Rd must use a context-specific label space to lookup label L as described in section 7. 4.1. Upstream Assigned and Downstream Assigned Labels It is possible that some LSRs on a LSP for FEC F, distribute down- streamdownstream assigned label bindings for FEC F, while other LSRs distribute upstream assigned label bindings. It is possible for a LSR to dis- tributedistribute a downstream assigned label binding for FEC F to its upstream adjacent LSR AND distribute an upstream assigned label binding for FEC F to its downstream adjacent LSR. Two adjacent LSRs for a LSP that is bound to FEC F, MUST use either downstream assigned label distribution or upstream assigned label distribution, for FEC F, but NOT both. How these LSRs will determine which of the two is to be used is outside the scope of this document. 5. Assigning Upstream Assigned Labels The only requirement on an upstream LSR assigning upstream assigned labels is that an upstream assigned label must be unambiguous in the context-specific label space in which the downstream LSR will look it up. An upstream LSR which is the head end of multiple tunnels SHOULD by default assign the upstream-assigned labels from a single label space which is common to all those tunnels. Further an upstream LSR which is the head of multiple tunnels SHOULD use the same IP address as the head identifier of these tunnels, provided that the head iden- tifieridentifier of these tunnels is an IP address. The LSR could assign the same label value to both a downstream-assigned and an upstream- assigned label. The downstream LSR always looks up upstream assigned MPLS labels in a context-specific label space as described in section 7. An entry for the upstream assigned labels is not created in the Incoming Label Map (ILM) [RFC3031] at the upstream LSR as these labels are not incoming labels. Instead an upstream label is an out- goingoutgoing label, with respect to the upstream LSR, for MPLS packets transmitted on the MPLS LSP in which the upstream LSR is adjacent to the downstream LSR. Hence an upstream label is part of a Next Hop Label Forwarding Entry (NHLFE) at the upstream LSR. When Ru advertises a binding of label L for FEC F to Rd, it creates a NHLFE entry corresponding to L. This NHLFE entry results in imposing the label L on the MPLS label stack of the packet forwarded using the NHLFE entry. If Ru is a transit router on the LSP for FEC F, it binds the ILM for the LSP to this NHLFE. If Ru is an ingress router on the LSP for FEC F, it binds the FEC to the NHLFE entry. 6. Distributing Upstream Assigned Labels Upstream-assigned label bindings MUST NOT be used unless it is known that the downstream LSR supports them. How this is known is outside the scope of this document. MPLS upstream label assignment requires a label distribution protocol to distribute the binding from the upstream LSR to the downstream LSR. Considerations that pertain to a label distribution protocol that are described in [RFC3031] apply. The distribution of the upstream-assigned labels is similar to either the ordered LSP control or independent LSP control of the downstream- assigned labels. In the former case a LSR distributes an upstream- assigned label binding for a FEC F if it is either (a) the ingress LSR for FEC F, or (b) if it has already received an upstream label binding for that FEC from its adjacent upstream LSR for FEC F, or (c) if it has received a request for a downstream label binding from its upstream adjacent LSR. In the latter case each LSR, upon noting that it recognizes a particular FEC, makes an independent decision to bind an upstream-assigned label to that FEC and to distribute that binding to its label distribution peers. 7. Upstream Neighbor Label Space and Tunnel Label Space If the top label of an MPLS packet is upstream assigned, when the packet is received by LSR Rd the label is looked up in a context-specific label space, not in a per-platform label space. Rd uses a context-specific label space that it maintains for Ru to "reserve" MPLS labels assigned by Ru. Hence if Ru distributes an upstream assigned label binding L for FEC F to Rd, then Rd reserves L in the separate ILM for Ru's context-specific label space. This is the ILM that Rd uses to lookup MPLS packets received from Ru, the top label of which is upstream assigned by Ru. This implies that Rd MUST be able to determine whether the top label of a received MPLS packet is upstream assigned and if yes, the "con- text""context" of this packet. How this determination is made depends on the mechanism that is used by Ru to transmit the MPLS packet with an upstream assigned top label L, to Rd. Ru may transmit this packet to Rd by encapsulating it directly in a data link frame or by transmit- tingtransmitting it in an IP or MPLS tunnel. If Ru transmits this packet by encapsulating it in a data link frame, then whether L is upstream assigned or downstream assigned is deter- mineddetermined by Rd as described in [MPLS-MCAST-ENCAPS]. If L is upstream assigned then [MPLS-MCAST-ENCAPS] uses a different ether type in the data link frame. Rd maintains a separate "Upstream Neighbor Label Space" for Ru. The "context" of this packet i.e. the upstream neigh- borneighbor label space, in which L was reserved is determined by the data link header. For example if the data link header is ethernet, the source MAC address is used to determine the context. If Ru transmits this packet by encapsulating it in an IP or MPLS tun- nel,tunnel, then the fact that L is upstream assigned is determined by Rd by the tunnel on which the packet is received. A given tunnel can be used for transmitting either downstream assigned MPLS packets or upstream assigned MPLS packets, or both. There must be a mechanism for Ru to inform Rd that a particular tunnel from Ru to Rd will be used by Ru for transmitting MPLS packets with upstream assigned MPLS labels. The description of such a mechanism is outside the scope of this document. When Rd receives MPLS packets with a top label L on such a tunnel, it determines the "context" of this packet based on the tunnel that the packet is received on. Rd may maintain a separate "Tunnel Label Space" for the tunnel or it may maintain an Upstream Neighbor Label Space" for all tunnels that have the same root. If Rd uses the "Upstream Neighbor Label Space" for upstream assigned MPLS packets transmitted by Ru to Rd, over IP or MPLS tunnels, then Rd MUST be able to determine the root of these IP/MPLS tunnels. Rd would then use a separate label space for each unique root. If the tunnel on which Rd receives MPLS packets with a top label L is a MPLS tunnel, then Rd determines a) That L is upstream assigned and b) The context for L, from the labels above L in the label stack. Note that one or more of these labels may also be upstream assigned labels. If the tunnel on which Rd receives MPLS packets with a top label L is an IP/GRE tunnel then Rd determines a) That L is upstream assigned [MPLS-MCAST-ENCAPS] and b) The context for L, from the source address in the IP header. 8. Usage of Upstream Assigned Labels A typical usage of upstream assigned labels is when an upstream LSR Ru is adjacent to more than downstream LSRs <Rd1...Rdn> in a LSP LSP1 AND Ru is connected to <Rd1...Rdn> via a multi-access media or tunnel AND Ru wants to transmit a single copy of a MPLS packet on the LSP to <Rd1...Rdn>. In this case Ru can distribute an upstream assigned label L that is bound to the FEC for LSP1, to <Rd1..Rdn> and transmit a MPLS packet, the top label of which is L, on the multi-access media or tunnel. Each of <Rd1..Rdn> will then interpret this MPLS packet in the context of Ru and forward it appropriately. This implies that <Rd1..Rdn> MUST all be able to support an Upstream Neighbor Label Space for Ru and Ru MUST be able to determine this. The mechanisms for determining this are specific to the application that is using upstream assigned labels and is outside the scope of this document. 9. References 9.1. Normative References [RFC3031] "MPLS Architecture", E. Rosen, A. Viswanathan, R. Callon, RFC 3031. [RFC2119] "Key words for use in RFCs to Indicate Requirement Lev-[MPLS-MCAST-ENCAPS] T. Eckert, E. Rosen, R. Aggarwal, Y. Rekhter, draft-rosen-mpls-codepoint-00.txtdraft-ietf-mpls-codepoint-00.txt 9.2. Informative References [MVPN] E. Rosen, R. Aggarwal [Editors], Multicast in BGP/MPLS VPNs" 10. Author Information Rahul Aggarwal Juniper Networks 1194 North Mathilda Ave. Sunnyvale, CA 94089 Email: firstname.lastname@example.org Yakov Rekhter Juniper Networks 1194 North Mathilda Ave. Sunnyvale, CA 94089 Email: email@example.com Eric C. Rosen Cisco Systems, Inc. 1414 Massachusetts Avenue Boxborough, MA 01719 Email: firstname.lastname@example.org 11. 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