PPP Working Group Andrew J. Valencia Request for Comments: DRAFT Cisco Systems Category: Internet Draft Title: draft-ietf-pppext-l2tphc-00.txt Date: November 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 November 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 - Priority packets do not need to be optimized - If there are no protocol fields, a protocol header is not required 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 can be determined outside of the core payload packet path. If rate pacing is not used, Priority flagged packets will probably be present to guarantee the timely exchange of PPP keepalives, routing adjacency packets, and so forth. However, by their nature, these packets are a statistically insignificant fraction of the overall packet flow, and do not need to be optimized. Thus, by choosing very reasonable simplifying assumptions, it is possible to entirely remove all L2TP fields from the header of a payload packet. The resulting protocol is simply PPP frames encapsulated inside a raw IP protocol header, running in parallel with the regular UDP-based L2TP tunnel which provides all management Valencia expires June 1998 [Page 2]
INTERNET DRAFT November 1997 and related functions. 3. Tunnel Establishment 3.1 Negotiation L2TPHC is negotiated by an optional AVP 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. By default, further tunnels may be established, but the operation of these tunnels will be using standard UDP-based L2TP tunneling, and such payload MUST NOT be forwarded over the header compression channel. A single exception is that a peer MAY initiate L2TPHC on further tunnels if its L2TPHC AVP specifies a distinct IP protocol number. If an SCCRQ holds the L2TPHC AVP from a peer with which L2TPHC is already active, an implementation by default MUST disregard the AVP and bring up a standard UDP L2TP tunnel. The implementation MAY recognize that the AVP specifies a new IP protocol number, and choose a new IP protocol number on its side, and bring up further L2TPHC tunnels, with the Tunnel ID being determined by the distinct IP protocol numbers of the payload packets. It is recommended that protocol numbers so used start at one greater than the default L2TPHC protocol number (see section 3.2), and count upwards, using the first number determined to be available on a given implementation. 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. 3.2 AVP Format The AVP 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. Valencia expires June 1998 [Page 3]
INTERNET DRAFT November 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0 | 251 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 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. When a packet with the Priority bit is to be sent, it is sent via the regular UDP-based tunnel. Priority packets MUST enjoy priority over traffic queued on both the UDP tunnel as well as the corresponding raw IP tunnel. 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%. Valencia expires June 1998 [Page 4]
INTERNET DRAFT November 1997 With L2TPHC, the UDP and L2TP headers are absent, leaving only the 20 bytes of IP header. The small packet now suffers an overhead of only 31%, and the larger packet a little short of 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 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. Valencia expires June 1998 [Page 5]
INTERNET DRAFT November 1997 7. Acknowledgments Thanks to Gurdeep Singh Pall of Microsoft for identifying and describing scenarios in which L2TP header size become a concern. 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]
