Network Working Group Pat R. Calhoun INTERNET DRAFT Sun Microsystems, Inc. Wei Luo Cisco Systems, Inc. Danny McPherson Amber Networks, Inc. Ken Peirce Malibu Networks, Inc. March 2001 L2TP Differentiated Services Extension 1. 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. Calhoun, Luo, McPherson, Peirce [Page 1] INTERNET DRAFT March 2001 2. Abstract The Layer Two Tunneling Protocol (L2TP) [RFC 2661] provides a standard method for tunneling PPP [RFC 1661] packets. The current specification provides no provisions for supporting Differentiated Services (diffserv) [RFC 2474, RFC 2475] over the L2TP control connection or subsequent data sessions. As a result, no standard mechanism currently exists within L2TP to provide L2TP protocol negotiations for service discrimination. This document describes mechanisms which enable L2TP to negotiate desired DS values for the L2TP control connection, as well as individual sessions within an L2TP tunnel. 3. 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 [RFC 2119]. 4. Introduction The L2TP specification currently provides no mechanism for supporting diffserv (DS). This document describes mechanisms that enable L2TP to indicate desired DS values to be associated with an L2TP control connection, as well as individual sessions within an L2TP tunnel. This document will describe how a set of L2TP peers MAY negotiate a set of differential services indicators for a tunnel control connection, as well as for individual sessions within the tunnel. The actual bit interpretation of the DS field is beyond the scope of this document, and is purposefully omitted. This document is concerned only with defining a uniform exchange and subsequent mapping mechanism for the DS AVPs. Calhoun, Luo, McPherson, Peirce [Page 2] INTERNET DRAFT March 2001 5. Control Connection Operation As defined in [RFC 2661], a control connection operates in-band over a tunnel to control the establishment, release, and maintenance of sessions and of the tunnel itself. As such, this document provides a mechanism to enable discrimination of L2TP control messages from other packets. For this purpose, we introduce the Control Connection DS (CCDS) AVP. The presence of the CCDS AVP serves as an indication to the peer (LAC or LNS) that the tunnel initiator wishes both the tunnel initiator and terminator to use the per-hop behavior(s) (PHB(s)) indicated by the AVP's DS value for all packets within the tunnel's control connection. A PHB is a description of the externally observable forwarding behavior of a DS node applied to a particular DS behavior aggregate, as defined in [RFC 2475]. The most simple example of a PHB is one which guarantees a minimal bandwidth allocation of a link to a behavior aggregate. Upon receipt of a Start-Control-Connection-Request (SCCRQ) containing the CCDS AVP, if the tunnel terminator provides no support for the CCDS AVP it MUST ignore the AVP and send a SCCRP to the tunnel initiator without the CCDS AVP. The tunnel initiator interprets the absence of the CCDS AVP in the SCCRP as as an indication that the tunnel terminator is incapable of supporting CCDS. Upon receipt of a SCCRP that contains no CCDS AVP in response to a SCCRQ that contained a CCDS AVP, if the tunnel initiator wants to continue tunnel establishment it sends a SCCCN. Otherwise, it sends a StopCCN to the tunnel terminator to end the connection. The StopCCN control message MUST contain a Result Code AVP that indicates CCDS AVP value [TBD] as the reason for sending the StopCCN. If the tunnel terminator provides support for CCDS, it SHOULD use the Host Name AVP embedded in SCCRQ to consult its local policy, and to determine whether local policy permits the requested DS value to be used on this control connection. If it is unwilling or unable to support the requested DS value after consulting the local policy, the tunnel terminator MUST send a SCCRP control message containing a CCDS AVP indicating the value it is willing to use. If the CCDS AVP value is the same as the one in the SCCRQ, it signals the acceptence of the requested DS value. If the value is different it serves as a counter-offer by the tunnel terminator. If the tunnel initiator receives an SCCRP that contains a CCDS AVP with a value other than that requested in the SCCRQ, the tunnel initiator SHOULD check the DS value against its own policy. If it is unwilling to use the value, the tunnel initiator MUST send a StopCCN Calhoun, Luo, McPherson, Peirce [Page 3] INTERNET DRAFT March 2001 control message containing a Result Code AVP that indicates CCDS AVP value [TBD] as the reason for sending the StopCCN. 5.1. Control Connection DS AVP (SCCRQ, SCCRP) The CCDS AVP is encoded as Vendor ID 43, and the Attribute Value is the 16-bit quantity 1 (the ID 43 reflects 3Com Corporation, it should be changed to 0 and an official Attribute Value chosen should this specification advance on as standards track). Each CCDS AVP is encoded as follows: Vendor ID = 43 Attribute = 1 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H|0|0|0|0| Length | 43 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | DS Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This AVP MAY be present in the following message types: SCCRQ and SCCRP. This AVP MAY be hidden (the H-bit set to 0 or 1) and is optional (M-bit not set). The length (before hiding) of this AVP MUST be 8 octets. The encoding of the DS value is described in Section 7. 6. Session Operation As defined in [RFC 2661], a L2TP session is connection-oriented. The LAC and LNS maintain states for each call that is initiated or answered by an LAC. An L2TP session is created between the LAC and LNS when an end-to-end connection is established between a Remote System and the LNS. Datagrams related to the connection are sent over the tunnel between the LAC and LNS. As such, this document provides a mechanism to enable discrimination for packets within an particular session from those in other sessions. For this purpose, we introduce the Session DS (SDS) AVP. The presence of the SDS AVP serves as an indication to the peer (LAC or LNS) that the session initiator wishes both the session initiator and terminator to use the per-hop behavior(s) (PHB(s)) indicated by the AVP's DS value for all packets within the session. Calhoun, Luo, McPherson, Peirce [Page 4] INTERNET DRAFT March 2001 Upon receipt of a Incoming-Call-Request (ICRQ) or Outgoing-Call- Request (OCRQ) containing the SDS AVP if the session terminator provides no support for the requested DS value, the session terminator MUST ignore the SDS AVP and send a ICRP or OCRP to the session initiator without the SDS AVP. The session initiator interprets the absence of the SDS AVP in the ICRP or OCRP as an indication that the session terminator is incapable of supporting SDS. Upon receipt of a ICRP or OCRP that contains no SDS AVP in response to a ICRQ or OCRQ that contained a SDS AVP, if the session initiator is willing to omit employing SDS AVP it continues session establishment as defined in [RFC 2661]. Otherwise, it sends a CDN to the session terminator to end the connection. The CDN control message MUST contain a Result Code AVP that indicates SDS AVP value [TBD] as the reason for sending the CDN. In order to help the session terminator to distinguish one session from another when looking up the DS value in its local or remote policy database, the session initiator MAY use the Sub-Address AVP to carry other session information in addition to the Calling Number AVP (ICRQ), the Called Number AVP (ICRQ, OCRQ). As described in [RFC 2661], the Sub-Address AVP is an ASCII string for encoding additional information, and it may be necessary for the administrators of the LAC and the LNS to coordinate interpretation of the value in this AVP. For instance, the administrators may agree upon using user name in this AVP when carrying PPP in L2TP payload. If the session terminator provides support for SDS, it SHOULD use the the designated DS identification AVP (via out-of-band agreement between the administrators of the LAC and LNS), e.g. Sub-Address, Calling number AVP, to consult local policy and determinate whether local policy permits the requested DS value to be used on this session. If it is unwilling or unable to support the requested DS value the session terminator MUST do one of the following: 1) Send a CDN message containing a Result Code AVP that indicates SDS AVP value [TBD] as the reason for sending the CDN. 2) Send an Incoming-Call-Reply (ICRP) or Outgoing-Call-Reply (OCRP) message containing a SDS AVP indicating the DS value the terminator is willing to use for the session. If the session terminator supports the DS value in the SDS AVP session establishment MUST continue as defined in [RFC 2661]. If the session initiator receives an ICRP or OCRP that contains an SDS AVP with a value other than that requested in the ICRQ or OCRQ, Calhoun, Luo, McPherson, Peirce [Page 5] INTERNET DRAFT March 2001 and the session initiator is unwilling to use the value, the session initiator MUST send a CDN message containing a Result Code AVP that indicates SDS AVP value [TBD] as the reason for sending the CDN. If the session initiator receives an ICRP or OCRP that contains a SDS AVP with a value other than that requested in the ICRP or OCRP, and the session initiator is willing to use the value, the session initiator MUST proceed as indicated in [RFC 2661]. 6.1. Session DS AVP (ICRQ, ICRP, OCRQ, OCRP) The SDS AVP is encoded as Vendor ID 43, and the Attribute Value is a 16-bit quantity 2 (the ID 43 reflects 3Com Corporation, it should be changed to 0 and an official Attribute Value chosen should this specification advance on as standards track). Each SDS AVP is encoded as follows: Vendor ID = 43 Attribute = 2 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |M|H|0|0|0|0| Length | 43 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 2 | DS Value | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ This AVP MAY be present in the following message types: ICRQ, ICRP, OCRQ and OCRP. This AVP MAY be hidden (the H-bit set to 0 or 1) and is optional (M-bit is not set 0). The length (before hiding) of this AVP MUST be 8 octets. The encoding of the DS value is decribed in Section 7. Calhoun, Luo, McPherson, Peirce [Page 6] INTERNET DRAFT March 2001 7. DS AVPs Correlation CCDS AVP and SDS AVP are independent of each other. CCDS AVP is used to signal diffserv for the control connection between two L2TP peers, while SDS AVP is used for data connection. The DS value signaled in one AVP SHOULD NOT have any implication on the DS value signaled in the other AVP. Implementations MAY choose to implement either or both DS AVPs, and operations MAY choose to enable diffserv on either or both types of connections. 8. DS Value Encoding The DS value is a left-justified 16-bit field using Per Hop Behavior (PHB) encoding defined in [RFC 2836]. Note that [RFC 2836] and its successor is the ultimate authority defining PHB encoding, and governs if there is any conflict between it and the text reproduced in this section. When using PHBs defined by standards action, as per [RFC 2474], each DS value is encoded as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | DSCP |0|0|0|0|0|0|0|0|X|0| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ If the DS value comprises a single PHB, the encoding for the DS is the encoding for this single PHB. It is the recommended DSCP value for that PHB, left-justified in the 16-bit field, with bits 6 through 15 set to zero. If the DS value comprises multiple PHBs, the encoding for the DS is the encoding for this set of PHBs. It is the numerically smallest value of the recommended DSCP for the various PHBs, left-justified in the 16 bit field, with bits 6 through 13 and bit 15 set to zero and with bit 14 set to one. When using PHBs not defined by standards action, each DS value is encoded as follows: 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PHB id code |0|0|X|1| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Calhoun, Luo, McPherson, Peirce [Page 7] INTERNET DRAFT March 2001 An arbitrary 12 bit PHB identification code, assigned by the IANA, is placed left-justified in the 16 bit field. Bit 15 is set to 1, and bit 14 is zero for a single PHB or 1 for a set of PHBs. Bits 12 and 13 are zero. Upon successful establishment of an L2TP tunnel control connection or individual L2TP session employing the appropriate DS AVP defined in this document, both LAC and LNS MUST use their own PHB-to-DSCP mappings of their present DS domains to map the PHB to a DSCP and place it in the DS field of the outer IP header of packets transmitted on the connection. 9. DSCP Selection The requirements or rules of each service and DSCP mapping are set through administrative policy mechanisms which are outside the scope of this document. 10. Packet Reordering and Sequence Numbers [RFC 2474] RECOMMENDS that PHB implementations not cause reordering of packets within an individual connection. For L2TP, a set of PHBs signaled using a single AVP SHOULD not cause additional packet reordering within an individual connection vs. using a single PHB. If a set of PHBs is capable of causing such reordering, then it SHOULD not be signaled with a single L2TP AVP. As a consequence, use of diffserv PHBs in accordance with this specification SHOULD not cause additional packet reordering within an L2TP control or data connection. Sequence numbers are required to be present in all control messages and are used to provide reliable delivery on the control connection, as defined in [RFC 2661]. While packet reordering is inevitably as much a function of the network as it is local traffic conditioning, the probability of it occuring when employing the CCDS AVP is same as when not employing the AVP. Data messages MAY use sequence numbers to reorder packets and detect lost packets. Calhoun, Luo, McPherson, Peirce [Page 8] INTERNET DRAFT March 2001 11. Crossing Differentiated Services Boundaries With the potential that an L2TP connection traverses an arbitrary number of DS domains, signaling PHBs via L2TP is more appropriate than signaling DSCPs, because it maintains a consistent end-to-end differentiated service for the L2TP connection. As per [RFC 2983], the negotiated PHBs are mapped to locally defined DSCPs of the current DS domain at the tunnel ingress node. At the DS domain boundary nodes, the DSCPs can be rewritten in the DS field of the outer IP header, so that the DSCPs are always with respect to whatever DS domain the packet happens to be in. As a result, it is perfectly acceptable that the outermost DS field of packets arriving on a given control connection or session are not marked with the same DSCP value that was used by the tunnel ingress node. 12. IANA Considerations Should this document advance on as standards track official Attribute Values need to be assigned for the CCDS and SDS AVPs. 13. Security Considerations This encoding in itself raises no security issues. However, users of this encoding should consider that modifying a DSCP MAY constitute theft or denial of service, so protocols using this encoding MUST be adequately protected. No new security issues beyond those discussed in [RFC 2474] and [RFC 2475] are introduced here. 14. Acknowledgements Many thanks to David Black, W. Mark Townsley, Nishit Vasavada, Andy Koscinski and John Shriver for their review and insightful feedback. Calhoun, Luo, McPherson, Peirce [Page 9] INTERNET DRAFT March 2001 15. References [RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51, RFC 1661, July 1994. [RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC 2474] Nichols et al., "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers", RFC 2474, December 1998. [RFC 2475] Blake et al., "An Architecture for Differentiated Services", RFC 2475, December 1998. [RFC 2661] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, B. Palter, "Layer 2 Tunnel Protocol (L2TP)", RFC 2661, August 1999. [RFC 2836] S. Brim, B. Carpenter, F. Le Faucheur, "Per Hop Behavior Identification Codes", RFC 2836, May 2000 [RFC 2983] D. Black, "Differentiated Services and Tunnels", RFC 2983, October 2000 16. Authors' Address Pat R. Calhoun Network and Security Research Center, Sun Labs Sun Microsystems, Inc. 15 Network Circle Menlo Park, California, 94025 Phone: +1 650.786.7733 Email: pcalhoun@eng.sun.com Wei Luo Cisco Systems, Inc. 170 West Tasman Drive San Jose, CA 95134 Phone: +1 408.525.6906 Email: luo@cisco.com Danny McPherson Amber Networks, Inc. 2465 Augustine Drive Santa Clara, CA 95054 Phone: +1 408.486.6336 Calhoun, Luo, McPherson, Peirce [Page 10] INTERNET DRAFT March 2001 Email: danny@ambernetworks.com Ken Peirce Malibu Networks, Inc. 1035 Suncast Lane, Suite 130 El Dorado Hills, CA 95762 Phone: +1 916.941.8814 Email: Ken@malibunetworks.com Calhoun, Luo, McPherson, Peirce [Page 11]