PPP Working Group Andrew J. Valencia Request for Comments: DRAFT Cisco Systems Category: Internet Draft Title: draft-ietf-pppext-l2tphc-01.txt Date: December 1997 L2TP Header Compression (``L2TPHC'') Status of this Memo This document is an Internet-Draft. 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. Internet-Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet- Drafts as reference material or to cite them other than as a ``working draft'' or ``work in progress.'' To learn the current status of any Internet-Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com, or munnari.oz.au. Abstract The Layer 2 Tunneling Protocol (``L2TP'') defines a mechanism for tunneling PPP sessions over arbitrary media. There exists a class of specific media and applications for which protocol overhead may be optimized, and where such reduction results in improved operation. This document describes the solution space addressed, its underlying motivations, and the protocol modifications required. The enhancement to the L2TP protocol is called L2TP Header Compression, or ``L2TPHC''. 1. Introduction L2TP [1] defines a general purpose mechanism for tunneling PPP over various media. By design, it insulates L2TP operation from the details of the media over which it operates. A significant application of L2TP has emerged, known as ``voluntary tunneling'' [2]. In this environment, the L2TP tunnel runs from the dial-up client itself, through a public IP infrastructure, and then terminating at the target LNS. Because this mode of operation results in the L2TP header traversing the slow, high-latency dial-up link, each byte of tunnel overhead can have a measurable impact on the operation of the carried protocols. Valencia expires June 1998 [Page 1]
INTERNET DRAFT December 1997 2. Simplifying Assumptions Fortunately, several simplifying assumptions may be made in the voluntary tunneling environment: - The client will not operate through a NAT interface - The client will not roam (i.e., change its IP address) - The client has only one public IP network interface - There will be only one tunnel between the client and its LNS - There will be only one session within this tunnel - Alignment is not required - Packet length is preserved by the IP header Each of these simplifying assumptions directly relates to an L2TP protocol header field's function. Because NAT functionality is not needed, the UDP header is not required. Because the client will not change its source IP address (due to either roaming or switching to a distinct backup IP interface), the identity of the client may be determined by its source IP address, rather than the Tunnel ID. Because there is only one session within the tunnel, it is trivial to determine the Session ID. Because each byte is a measurable component of overhead, it is better to send fields on unaligned boundaries rather than ever pad. Because IP will preserve the packet length end-to-end, there is no need to communicate this in the header itself. In addition, several operational considerations permit further simplification: - There is no need to optimize control packet overhead - Version compatibility may be determined by control packets - Rate pacing may be determined outside the main payload exchange The first two bytes of an L2TP payload header determined the presence of further, optional, fields. It also contains a Version field, used to detect compatible version operation. Realistically, these may all be determined in advance of payload exchange. Similarly, the optional rate pacing of L2TP could determined outside of the core payload packet path, or the Priority bit facility could be used instead. Thus, by choosing very reasonable simplifying assumptions, it is possible to minimize the L2TP fields from the header of a payload packet. The resulting protocol is a one octet mandatory header, followed by 0, 1, or 2 additional octets, followed by PPP frames, all encapsulated within a raw IP protocol header. These packets are exchanged in parallel with the regular UDP-based L2TP tunnel which provides all management and related functions. 3. Tunnel Establishment 3.1 Negotiation Valencia expires June 1998 [Page 2]
INTERNET DRAFT December 1997 L2TPHC is negotiated by an optional AVP ``L2TPHC-Proto'' which is placed in the SCCRQ/SCCRP tunnel establishment messages. The effect of this AVP will never occur until L2TP reaches a state where payload data may be forwarded within the session in the tunnel. Additionally, each side intending to use L2TPHC MUST NOT do so until it both sends and receives this AVP. Thus, unless both sides support L2TPHC, the optional AVP will be ignored by one side, and not sent to the other side, and L2TP will operate in its regular mode. Further sessions within an L2TPHC tunnel MUST NOT be initiated. However, L2TPHC permits multiple tunnels if a second AVP, indicating a special Tunnel ID, is included immediately following L2TPHC-Proto AVP in the SCCRQ/SCCRP exchange. This optional AVP, ``L2TPHC-Tunnel'', is ignored unless it is both sent and received. In this case, the Value indicates the octet value which will be included as the Tunnel ID within the L2TPHC header. Once the tunnel associated with a given L2TPHC context has been terminated, the L2TPHC context is considered free, and may be used in future L2TP connections. Because all control passes over the parallel L2TP session corresponding to the L2TPHC one, the L2TP tunnel terminates, and the L2TPHC tunnel is implicitly terminated. 3.2 AVP Format The AVP L2TPHC-Proto is encoded as Vendor ID 9, Attribute is the 16-bit quantity 0 (the ID 9 reflects Cisco Systems, the initial developer of this specification, and it SHOULD be changed to 0 and an official Attribute value chosen if this specification advances on a standards track). The Value is a single octet, encoding the IP protocol number to use for the exchange of payload. Unless and until an official protocol number is allocated, the value 251 is recommended. The AVP is marked optional, permitting interoperability with peers not implementing L2TPHC. 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|0|0|0| 7 | 9 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 251 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ The L2TPHC-Tunnel AVP is also marked optional. It MUST NOT be present except when immediately following an L2TPHC-Proto AVP. The Attribute is the 16-bit value 1, encoded in network byte order. The single octet Value is a Tunnel ID to be used in the L2TPHC encapsulation. If this AVP is both sent and received, up to 256 parallel tunnels may be supported between the peers, and all L2TPHC packets MUST include the T bit, and the Tunnel ID specified by the peer MUST be used as the Tunnel ID in all packets sent to that peer for a given tunnel. Valencia expires June 1998 [Page 3]
INTERNET DRAFT December 1997 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|0|0|0| 7 | 9 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 1 | Tunnel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 4. Payload Exchange If the L2TPHC AVP is sent to and received from the peer, PPP payload packets may be sent to the peer's IP address as raw IP packets, with the IP protocol number set as indicated from the peer. Note that it is legal for each peer to have specified a different protocol number; traffic sent is always to the number indicated in the peer's AVP. Such payload may be sent any time it would have been legal to send such payload over the regular UDP-based L2TP tunnel. Similarly, payload over the UDP tunnel MUST always be accepted, even after payload has flowed using the header compressed raw IP packet format. The payload so exchanged is always associated with the tunnel on which the AVP was received, and with the single session within that tunnel. Each L2TPHC payload packet is encoded as: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |F|I|P|0|0|0|0|0| Nr | Ns | Tunnel ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | PPP packet... | +-+-+-+-+-+-+-+-+ The Nr/Ns fields MUST be present if the F bit is 1, otherwise they MUST be omitted. Their use is identical to that of the fields of the same name in L2TP payload packets, except that their numerical range is only 8 bits, rather than 16 in L2TP. A regular L2TP tunnel MUST be used in any situation where the speed of a link and the latency of the path result in more packets outstanding than can be accounted with a byte numbering space. The I bit MUST be set, and the Tunnel ID MUST be present, if the L2TPHC-Tunnel AVP was both sent and received during tunnel setup. Otherwise I must be sent 0, and the Tunnel ID octet omitted from the packet. The P bit is the Priority bit, and serves the same function as the bit of the same name in an L2TP packet. Priority packets MUST enjoy priority over traffic queued on both the UDP tunnel as well as the corresponding L2TPHC raw IP tunnel. Therefore, an L2TPHC packet will have an L2TPHC header of at least one octet, with up to three more octets as indicated by the flags in this first octet. Valencia expires June 1998 [Page 4]
INTERNET DRAFT December 1997 Since packet flow over this raw IP tunnel is distinct from the UDP based tunnel, it is possible that an asymmetry in the path (for instance, the unintentional presence of a NAT device) may disrupt one but not the other. It is recommended that at least during the time immediately following establishment of the session, that LCP echoes be used in tandem with the L2TP keepalive function so that connectivity of both paths may be verified. 5. Efficiency Considerations Some rough calculations will illustrate the environments in which L2TPHC may be beneficial. Overhead as a percentage of the carried traffic will be calculated for a typical packet size involved in bulk data transfer (700 bytes), and the canonical 64-byte ``small IP packet''. Percentages will be rounded to the nearest whole number. Overhead is tallied for an IP header of 20 bytes, a UDP header of 8 bytes, and an L2TP header of 8 bytes (4 bytes of rate pacing with the Nr/Ns fields will probably be avoided in favor of the more compact though less comprehensive Priority header bit). The worst case is a 64-byte packet carried within a UDP L2TP header. The 64 bytes of payload is carried by an overall header of 36 bytes, resulting in an overhead of 56%. With the larger payload size of 700 bytes, the header is amortized over many more bytes, reducing the overhead to 5%. With L2TPHC, the UDP is absent and the L2TPHC header is 1 byte for the most compact case. Overall size is thus one byte of L2TPHC and 20 bytes of IP header. The small packet now suffers an overhead of only 32%, and the larger packet 3%. Percentage overhead does not represent all the considerations involved in reducing overhead. The average modem connection is still only 14,400 bits per second, which translates to a per-byte real-time cost of 0.6 milliseconds (14400 divided by 8 bits, as async framing characters are not included in the modem-to-modem data transfer). Thus, a savings of 16 bytes per packet can also be viewed as a reduction of almost 10 milliseconds of latency per packet. While this latency is short enough to be unnoticeable by a human, it may impact real-time protocols such as streaming audio or video. Thus, L2TP Header Compression provides most of its benefits when carrying streams of small packets. In environments such as downloading of graphic files, or where human interaction is intermingled with the short packets, the benefits of L2TP Header Compression will probably be undetectable. 6. Security Considerations Because L2TPHC has no security facilities, it is critical that its operation be reconciled with the security policy of its environment. Since L2TPHC has no protocol header at all, it is trivial to spoof a source IP address and inject malicious packets into an ongoing session. There are several suitable techniques for controlling this Valencia expires June 1998 [Page 5]
INTERNET DRAFT December 1997 exposure. In the simplest case, L2TPHC operates across a private network. For instance, a remote user may dial into a private NAS located on this network, and use L2TP (with or without L2TPHC) to cross an IP-only portion of this network to establish a multi-protocol session connected at a convenient point in the network. In this environment, no additional security may be required, and L2TPHC would operate trusting to the integrity of this private network. If the weak protection of a difficult-to-guess protocol header is deemed sufficient, expanded protocol overhead has clearly been determined to be acceptable, and L2TP over UDP can be used without L2TPHC. If PPP encryption under ECP [3] is active, malicious PPP packets are trivially detected and discarded as they are received on the raw IP port number. Similarly, if an IPsec session is protecting the IP packets themselves, malicious packets will also be discarded. Note that in both cases, an expanded header is implicit in these security facilities, which will greatly reduce the overhead efficiencies gained by L2TPHC. For instance, an MD5 AH IPsec header will add 32 bytes to the packet. The 16 bytes saved by L2TPHC quickly approaches statistical insignificance. 7. Acknowledgments Thanks to Gurdeep Singh Pall of Microsoft for identifying and describing scenarios in which L2TP header size become a concern, and for help in designing the L2TPHC header. Thanks to Bill Palter and W. Mark Townsley of Cisco Systems for help in reviewing this draft. 8. Contacts Andrew J. Valencia Cisco Systems 170 West Tasman Drive San Jose CA 95134-1706 vandys@cisco.com 9. References [1] A. Valencia, ``Layer 2 Tunnel Protocol (L2TP)'', Internet Draft, October 1997 [2] G. Zorn, ``RADIUS Attributes for Tunnel Protocol Support'', Internet draft, July 1997 [3] G. Meyer, ``PPP Encryption Control Protocol (ECP)'', RFC 1968 Valencia expires June 1998 [Page 6]
