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(See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (January 2000) is 8858 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) -- Possible downref: Non-RFC (?) normative reference: ref. '1' Summary: 9 errors (**), 0 flaws (~~), 4 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 Network Working Group Andrew J. Valencia 2 Request for Comments: DRAFT Cisco Systems 3 Category: Internet Draft 4 Title: draft-ietf-l2tpext-l2tphc-00.txt 5 Date: January 2000 7 L2TP Header Compression (``L2TPHC'') 9 Status of this Memo 11 This document is an Internet-Draft and is in full conformance with 12 all provisions of Section 10 of RFC2026. Internet-Drafts are working 13 documents of the Internet Engineering Task Force (IETF), its areas, 14 and its working groups. Note that other groups may also distribute 15 working documents as Internet-Drafts. 17 Internet-Drafts are draft documents valid for a maximum of six months 18 and may be updated, replaced, or obsoleted by other documents at any 19 time. It is inappropriate to use Internet-Drafts as reference 20 material or to cite them other than as "work in progress." 22 The list of current Internet-Drafts can be accessed at 23 http://www.ietf.org/ietf/1id-abstracts.txt 25 The list of Internet-Draft Shadow Directories can be accessed at 26 http://www.ietf.org/shadow.html. 28 Abstract 30 The Layer 2 Tunneling Protocol (``L2TP'') [RFC2661] defines a 31 mechanism for tunneling PPP sessions over arbitrary media. There 32 exists a class of specific media and applications for which protocol 33 overhead may be optimized, and where such reduction results in 34 improved operation. This document describes the solution space 35 addressed, its underlying motivations, and the protocol modifications 36 required. The enhancement to the L2TP protocol is called L2TP Header 37 Compression, or ``L2TPHC''. 39 1. Introduction 41 L2TP [RFC2661] defines a general purpose mechanism for tunneling PPP 42 over various media. By design, it insulates L2TP operation from the 43 details of the media over which it operates. A significant 44 application of L2TP has emerged, known as ``voluntary tunneling'' 45 [1]. In this environment, the L2TP tunnel runs from the dial-up 46 client itself, through a public IP infrastructure, and then 47 terminating at the target LNS. Because this mode of operation 48 results in the L2TP header traversing the slow, high-latency dial-up 49 link, each byte of tunnel overhead can have a measurable impact on 50 the operation of the carried protocols. 52 2. Simplifying Assumptions 54 Fortunately, several simplifying assumptions may be made in the 55 voluntary tunneling environment: 57 - The client will not operate through a NAT interface 58 - The client uses a single IP address for the life of the tunnel 59 - The client has only one public IP network interface 60 - There may be only one tunnel between the client and its LNS 61 - There will be only one session within a tunnel 62 - Alignment is not required 63 - Packet length is preserved by the IP header 65 Each of these simplifying assumptions directly relates to an L2TP 66 protocol header field's function. Because NAT functionality is not 67 needed, the UDP header is not required. Because the client will not 68 change its source IP address (due to either changing its IP address 69 as it moves among IP egress points, or switching to a distinct backup 70 IP interface), the identity of the client may be determined by its 71 source IP address, rather than the Tunnel ID. Because there is only 72 one session within the tunnel, it is trivial to determine the Session 73 ID. Because each byte is a measurable component of overhead, it is 74 better to send fields on unaligned boundaries rather than ever pad. 75 Because IP will preserve the packet length end-to-end, there is no 76 need to communicate this in the header itself. 78 In addition, several operational considerations permit further 79 simplification: 81 - There is no need to optimize control packet overhead 82 - Version compatibility may be determined by control packets 84 The first two bytes of an L2TP payload header determined the presence 85 of further, optional, fields. It also contains a Version field, used 86 to detect compatible version operation. Realistically, these may all 87 be determined in advance of payload exchange. 89 Thus, by choosing very reasonable simplifying assumptions, it is 90 possible to minimize the L2TP fields from the header of a payload 91 packet. The resulting protocol is a one octet mandatory header, 92 possibly followed by an additional octet, followed by PPP frames, all 93 encapsulated within a raw IP protocol header. These packets are 94 exchanged in parallel with the regular UDP-based L2TP tunnel which 95 provides all management and related functions. 97 3. Tunnel Establishment 99 3.1 Negotiation 101 L2TPHC is negotiated by an optional AVP ``L2TPHC-Protocol'' which 102 is placed in the SCCRQ/SCCRP tunnel establishment messages. The 103 effect of this AVP will never occur until L2TP reaches a state 104 where payload data may be forwarded within the session in the 105 tunnel. Additionally, each side intending to use L2TPHC MUST NOT 106 do so until it both sends and receives this AVP. Thus, unless 107 both sides support L2TPHC, the optional AVP will be ignored by one 108 side, and not sent to the other side, and L2TP will operate in its 109 regular mode. 111 Further sessions within an L2TPHC tunnel MUST NOT be initiated. 112 However, L2TPHC permits multiple tunnels if a second AVP, 113 indicating a special Tunnel ID, is included immediately following 114 L2TPHC-Protocol AVP in the SCCRQ/SCCRP exchange. This optional 115 AVP, ``L2TPHC-Assigned-Tunnel-ID'', is ignored unless it is both 116 sent and received. In this case, the Value indicates the octet 117 value which will be included as the Tunnel ID within the L2TPHC 118 header. Note that this ID is used only in the L2TPHC header, and 119 is a distinct value from the tunnel ID used in L2TP headers. 121 Since all control passes over the parallel L2TP tunnel 122 corresponding to the L2TPHC one, the L2TP tunnel is always the one 123 explicitly terminated, and the associated L2TPHC tunnel is 124 implicitly terminated. 126 3.2 AVP Format 128 The AVP L2TPHC-Protocol is encoded as Vendor ID 9, Attribute is 129 the 16-bit quantity 1 (the ID 9 reflects Cisco Systems, the 130 initial developer of this specification, and it SHOULD be changed 131 to 0 and an official Attribute value chosen if this specification 132 advances on a standards track). The Value is a single octet, 133 encoding the IP protocol number to use for the exchange of 134 payload. The overall length of this AVP is thus 7. Unless and 135 until an official protocol number is allocated, the value 251 is 136 recommended. The M, H, and "rsvd" bits MUST all be set to 0. 138 0 1 2 3 139 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 1 140 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 141 |M|H| rsvd | 7 | 9 | 142 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 143 | 1 | 251 | 144 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 146 The L2TPHC-Assigned-Tunnel-ID AVP MUST also be sent with the M, H, 147 and "rsvd" bits all set to 0. It MUST NOT be present except when 148 immediately following an L2TPHC-Protocol AVP. The Attribute is 149 the 16-bit value 2, encoded in network byte order. The single 150 octet Value is a Tunnel ID to be used in the L2TPHC encapsulation. 151 The overall length of this AVP is thus 7. If this AVP is both 152 sent and received, up to 256 parallel tunnels may be supported 153 between the peers, and all L2TPHC packets MUST include the I bit 154 (see below), and the Tunnel ID specified by the peer MUST be used 155 as the Tunnel ID in all packets sent to that peer for a given 156 tunnel. 158 0 1 2 3 159 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 1 160 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 161 |M|H| rsvd | 7 | 9 | 162 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 163 | 2 | Tunnel ID | 164 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 166 4. Payload Exchange 168 If the L2TPHC AVP is sent to and received from the peer, PPP payload 169 packets may be sent to the peer's IP address as raw IP packets, with 170 the IP protocol number set as indicated from the peer. Note that it 171 is legal for each peer to have specified a different protocol number; 172 traffic is always sent to the number indicated in the peer's AVP. 173 Such payload may be sent any time it would have been legal to send 174 such payload over the regular UDP-based L2TP tunnel. Similarly, 175 payload over the UDP tunnel MUST always be accepted, even after 176 payload has flowed using the header compressed raw IP packet format. 177 The payload so exchanged is always associated with the tunnel on 178 which the AVP was received, and with the single session within that 179 tunnel. 181 Each L2TPHC payload packet is encoded as: 183 0 1 2 3 184 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 185 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 186 |T|L|x|x|S|I|O|P| Tunnel ID | PPP packet... | 187 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 189 x bits indicate reserved bit fields and MUST be set to 0. A packet 190 received with a reserved bit set to 1 MUST be silently discarded, 191 unless the bit is defined for an extension that is known to the 192 implementation. 194 The T bit MUST be set to 0, indicating payload. Control messages are 195 never sent over L2TPHC. 197 The L bit MUST be set to 0, indicating no length field. No length 198 field is ever present in L2TPHC. 200 The S bit MUST be set to 0, indicating no Nr/Ns fields. Control 201 packets are not passed via L2TPHC. If sequencing of data packets is 202 required, L2TPHC MUST NOT be used for those data packets. However, 203 it is legal for data traffic to be mixed, with order-sensitive 204 packets using L2TP/UDP, and other data packets using L2TPHC. In this 205 situation, data packets passed using L2TPHC have no impact on L2TP 206 sequence numbers. 208 The I bit MUST be set to 1, and the Tunnel ID MUST be present, if the 209 L2TPHC-Assigned-Tunnel-ID AVP was both sent and received during 210 tunnel setup. Otherwise I MUST be set to 0, and the Tunnel ID octet 211 MUST be omitted from the packet. 213 The O bit MUST be set to 0, indicating no offset field. Offsets are 214 never used in L2TPHC. 216 The P bit is the Priority bit, and serves the same function as the 217 bit of the same name in an L2TP packet. Priority packets MUST enjoy 218 priority over traffic queued on both the UDP tunnel as well as the 219 corresponding L2TPHC raw IP tunnel. 221 Therefore, an L2TPHC packet will have an L2TPHC header of at least 222 one octet, and optionally one additional octet if the I bit is set to 223 1. 225 5. Efficiency Considerations 227 Some rough calculations will illustrate the environments in which 228 L2TPHC may be beneficial. Overhead as a percentage of the carried 229 traffic will be calculated for a typical packet size involved in bulk 230 data transfer (700 bytes), and the canonical 64-byte ``small IP 231 packet''. Percentages will be rounded to the nearest whole number. 232 Overhead is tallied for an IP header of 20 bytes, a UDP header of 8 233 bytes, and an L2TP header of 8 bytes. 235 The worst case is a 64-byte packet carried within a UDP L2TP header. 236 The 64 bytes of payload is carried by an overall header of 36 bytes, 237 resulting in an overhead of 56%. With the larger payload size of 700 238 bytes, the header is amortized over many more bytes, reducing the 239 overhead to 5%. 241 With L2TPHC, the UDP header is absent and the L2TPHC header is 1 byte 242 for the most compact case. Overall size is thus one byte of L2TPHC 243 and 20 bytes of IP header. The small packet now suffers an overhead 244 of only 32%, and the larger packet 3%. 246 Percentage overhead does not represent all the considerations 247 involved in reducing overhead. The average modem connection is still 248 only 14,400 bits per second, which translates to a per-byte real-time 249 cost of 0.6 milliseconds (14400 divided by 8 bits, as async framing 250 characters are not included in the modem-to-modem data transfer). 251 Thus, a savings of 16 bytes per packet can also be viewed as a 252 reduction of almost 10 milliseconds of latency per packet. While 253 this latency is short enough to be unnoticeable by a human, it may 254 impact real-time protocols such as streaming audio or video. 256 Thus, L2TP Header Compression provides most of its benefits when 257 carrying streams of small packets. In environments such as 258 downloading of graphic files, or where human interaction is 259 intermingled with the short packets, the benefits of L2TP Header 260 Compression will probably be undetectable. 262 6. Security Considerations 264 Because L2TPHC has no security facilities, it is critical that its 265 operation be reconciled with the security policy of its environment. 266 Since L2TPHC has no protocol header at all, it is trivial to spoof a 267 source IP address and inject malicious packets into an ongoing 268 session. There are several suitable techniques for controlling this 269 exposure. 271 In the simplest case, L2TPHC operates across a private network. For 272 instance, a remote user may dial into a private NAS located on this 273 network, and use L2TP (with or without L2TPHC) to cross an IP-only 274 portion of this network to establish a multi-protocol session 275 connected at a convenient point in the network. In this environment, 276 no additional security may be required, and L2TPHC would operate 277 trusting to the integrity of this private network. 279 If the weak protection of a difficult-to-guess protocol header is 280 deemed sufficient, expanded protocol overhead has clearly been 281 determined to be acceptable, and L2TP over UDP can be used without 282 L2TPHC. 284 If PPP encryption under ECP [RFC1968] is active, malicious PPP 285 packets are trivially detected and discarded as they are received on 286 the raw IP port number. Similarly, if an IPsec session is protecting 287 the IP packets themselves, malicious packets will also be discarded. 288 Note that in both cases, an expanded header is implicit in these 289 security facilities, which will greatly reduce the overhead 290 efficiencies gained by L2TPHC. For instance, an MD5 AH IPsec header 291 will add 32 bytes to the packet. The 16 bytes saved by L2TPHC 292 quickly approaches statistical insignificance. 294 7. Acknowledgments 296 Thanks to Gurdeep Singh Pall of Microsoft for identifying and 297 describing scenarios in which L2TP header size become a concern, and 298 for help in designing the L2TPHC header. 300 Thanks to Bill Palter of Redback Networks and W. Mark Townsley of 301 Cisco Systems for help in reviewing this draft. 303 8. Contacts 305 Andrew J. Valencia 306 Cisco Systems 307 170 West Tasman Drive 308 San Jose CA 95134-1706 309 vandys@cisco.com 311 9. References 313 [RFC2661] M. Townsley, ``Layer 2 Tunnel Protocol (L2TP)'', RFC 2661, 314 August 1999 316 [1] G. Zorn, ``RADIUS Attributes for Tunnel Protocol Support'', Internet 317 draft, August 1999 319 [RFC1968] G. Meyer, ``PPP Encryption Control Protocol (ECP)'', RFC 1968, 320 June 1996