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(See Section 2.2 of https://www.ietf.org/id-info/checklist for how to handle the case when there are no actions for IANA.) ** The document seems to lack an Authors' Addresses Section. ** The document seems to lack separate sections for Informative/Normative References. All references will be assumed normative when checking for downward references. ** There are 4 instances of too long lines in the document, the longest one being 2 characters in excess of 72. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 115: '...ng to use L2TPHC MUST NOT do so until ...' RFC 2119 keyword, line 120: '...an L2TPHC tunnel MUST NOT be initiated...' RFC 2119 keyword, line 123: '...and such payload MUST NOT be forwarded...' RFC 2119 keyword, line 126: '...n is that a peer MAY initiate L2TPHC o...' RFC 2119 keyword, line 129: '...entation by default MUST disregard the...' (4 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == Line 259 has weird spacing: '... bytes to th...' == Unrecognized Status in 'Category: Internet Draft', assuming Proposed Standard (Expected one of 'Standards Track', 'Full Standard', 'Draft Standard', 'Proposed Standard', 'Best Current Practice', 'Informational', 'Experimental', 'Informational', 'Historic'.) -- The document seems to lack a disclaimer for pre-RFC5378 work, but may have content which was first submitted before 10 November 2008. If you have contacted all the original authors and they are all willing to grant the BCP78 rights to the IETF Trust, then this is fine, and you can ignore this comment. If not, you may need to add the pre-RFC5378 disclaimer. (See the Legal Provisions document at https://trustee.ietf.org/license-info for more information.) -- The document date (November 1997) is 9659 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' -- Possible downref: Non-RFC (?) normative reference: ref. '2' Summary: 12 errors (**), 0 flaws (~~), 4 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 PPP Working Group Andrew J. Valencia 3 Request for Comments: DRAFT Cisco Systems 4 Category: Internet Draft 5 Title: draft-ietf-pppext-l2tphc-00.txt 6 Date: November 1997 8 L2TP Header Compression (''L2TPHC'') 10 Status of this Memo 12 This document is an Internet-Draft. 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 18 months. Internet-Drafts may be updated, replaced, or obsoleted by 19 other documents at any time. It is not appropriate to use Internet- 20 Drafts as reference material or to cite them other than as a 21 ``working draft'' or ``work in progress.'' 23 To learn the current status of any Internet-Draft, please check the 24 1id-abstracts.txt listing contained in the Internet-Drafts Shadow 25 Directories on ds.internic.net, nic.nordu.net, ftp.nisc.sri.com, or 26 munnari.oz.au. 28 Abstract 30 The Layer 2 Tunneling Protocol (''L2TP'') defines a mechanism for 31 tunneling PPP sessions over arbitrary media. There exists a class of 32 specific media and applications for which protocol overhead may be 33 optimized, and where such reduction results in improved operation. 34 This document describes the solution space addressed, its underlying 35 motivations, and the protocol modifications required. The 36 enhancement to the L2TP protocol is called L2TP Header Compression, 37 or ''L2TPHC''. 39 1. Introduction 41 L2TP [1] defines a general purpose mechanism for tunneling PPP over 42 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" [2]. 45 In this environment, the L2TP tunnel runs from the dial-up client 46 itself, through a public IP infrastructure, and then terminating at 47 the target LNS. Because this mode of operation results in the L2TP 48 header traversing the slow, high-latency dial-up link, each byte of 49 tunnel overhead can have a measurable impact on the operation of the 50 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 will not roam (i.e., change its IP address) 59 - The client has only one public IP network interface 60 - There will be only one tunnel between the client and its LNS 61 - There will be only one session within this 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 roaming or switching to a 69 distinct backup IP interface), the identity of the client may be 70 determined by its source IP address, rather than the Tunnel ID. 71 Because there is only one session within the tunnel, it is trivial to 72 determine the Session ID. Because each byte is a measurable 73 component of overhead, it is better to send fields on unaligned 74 boundaries rather than ever pad. Because IP will preserve the packet 75 length end-to-end, there is no need to communicate this in the header 76 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 83 - Rate pacing may be determined outside the main payload exchange 84 - Priority packets do not need to be optimized 85 - If there are no protocol fields, a protocol header is not required 87 The first two bytes of an L2TP payload header determined the presence 88 of further, optional, fields. It also contains a Version field, used 89 to detect compatible version operation. Realistically, these may all 90 be determined in advance of payload exchange. Similarly, the 91 optional rate pacing of L2TP can be determined outside of the core 92 payload packet path. 94 If rate pacing is not used, Priority flagged packets will probably be 95 present to guarantee the timely exchange of PPP keepalives, routing 96 adjacency packets, and so forth. However, by their nature, these 97 packets are a statistically insignificant fraction of the overall 98 packet flow, and do not need to be optimized. 100 Thus, by choosing very reasonable simplifying assumptions, it is 101 possible to entirely remove all L2TP fields from the header of a 102 payload packet. The resulting protocol is simply PPP frames 103 encapsulated inside a raw IP protocol header, running in parallel 104 with the regular UDP-based L2TP tunnel which provides all management 105 and related functions. 107 3. Tunnel Establishment 109 3.1 Negotiation 111 L2TPHC is negotiated by an optional AVP which is placed in the 112 SCCRQ/SCCRP tunnel establishment messages. The effect of this AVP 113 will never occur until L2TP reaches a state where payload data may 114 be forwarded within the session in the tunnel. Additionally, each 115 side intending to use L2TPHC MUST NOT do so until it both sends 116 and receives this AVP. Thus, unless both sides support L2TPHC, 117 the optional AVP will be ignored by one side, and not sent to the 118 other side, and L2TP will operate in its regular mode. 120 Further sessions within an L2TPHC tunnel MUST NOT be initiated. 121 By default, further tunnels may be established, but the operation 122 of these tunnels will be using standard UDP-based L2TP tunneling, 123 and such payload MUST NOT be forwarded over the header compression 124 channel. 126 A single exception is that a peer MAY initiate L2TPHC on further 127 tunnels if its L2TPHC AVP specifies a distinct IP protocol number. 128 If an SCCRQ holds the L2TPHC AVP from a peer with which L2TPHC is 129 already active, an implementation by default MUST disregard the 130 AVP and bring up a standard UDP L2TP tunnel. The implementation 131 MAY recognize that the AVP specifies a new IP protocol number, and 132 choose a new IP protocol number on its side, and bring up further 133 L2TPHC tunnels, with the Tunnel ID being determined by the 134 distinct IP protocol numbers of the payload packets. It is 135 recommended that protocol numbers so used start at one greater 136 than the default L2TPHC protocol number (see section 3.2), and 137 count upwards, using the first number determined to be available 138 on a given implementation. 140 Once the tunnel associated with a given L2TPHC context has been 141 terminated, the L2TPHC context is considered free, and may be used 142 in future L2TP connections. 144 3.2 AVP Format 146 The AVP is encoded as Vendor ID 9, Attribute is the 16-bit 147 quantity 0 (the ID 9 reflects Cisco Systems, the initial developer 148 of this specification, and it SHOULD be changed to 0 and an 149 official Attribute value chosen if this specification advances on 150 a standards track). The Value is a single octet, encoding the IP 151 protocol number to use for the exchange of payload. Unless and 152 until an official protocol number is allocated, the value 251 is 153 recommended. The AVP is marked optional, permitting 154 interoperability with peers not implementing L2TPHC. 156 0 1 2 3 157 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 1 158 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 159 |0|0|0|0| 7 | 9 | 160 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 161 | 0 | 251 | 162 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ 164 4. Payload Exchange 166 If the L2TPHC AVP is sent to and received from the peer, PPP payload 167 packets may be sent to the peer's IP address as raw IP packets, with 168 the IP protocol number set as indicated from the peer. Note that it 169 is legal for each peer to have specified a different protocol number; 170 traffic sent is always to the number indicated in the peer's AVP. 171 Such payload may be sent any time it would have been legal to send 172 such payload over the regular UDP-based L2TP tunnel. Similarly, 173 payload over the UDP tunnel MUST always be accepted, even after 174 payload has flowed using the header compressed raw IP packet format. 175 The payload so exchanged is always associated with the tunnel on 176 which the AVP was received, and with the single session within that 177 tunnel. 179 When a packet with the Priority bit is to be sent, it is sent via the 180 regular UDP-based tunnel. Priority packets MUST enjoy priority over 181 traffic queued on both the UDP tunnel as well as the corresponding 182 raw IP tunnel. 184 Since packet flow over this raw IP tunnel is distinct from the UDP 185 based tunnel, it is possible that an asymmetry in the path (for 186 instance, the unintentional presence of a NAT device) may disrupt one 187 but not the other. It is recommended that at least during the time 188 immediately following establishment of the session, that LCP echoes 189 be used in tandem with the L2TP keepalive function so that 190 connectivity of both paths may be verified. 192 5. Efficiency Considerations 194 Some rough calculations will illustrate the environments in which 195 L2TPHC may be beneficial. Overhead as a percentage of the carried 196 traffic will be calculated for a typical packet size involved in bulk 197 data transfer (700 bytes), and the canonical 64-byte "small IP 198 packet". Percentages will be rounded to the nearest whole number. 199 Overhead is tallied for an IP header of 20 bytes, a UDP header of 8 200 bytes, and an L2TP header of 8 bytes (4 bytes of rate pacing with the 201 Nr/Ns fields will probably be avoided in favor of the more compact 202 though less comprehensive Priority header bit). 204 The worst case is a 64-byte packet carried within a UDP L2TP header. 205 The 64 bytes of payload is carried by an overall header of 36 bytes, 206 resulting in an overhead of 56%. With the larger payload size of 700 207 bytes, the header is amortized over many more bytes, reducing the 208 overhead to 5%. 210 With L2TPHC, the UDP and L2TP headers are absent, leaving only the 20 211 bytes of IP header. The small packet now suffers an overhead of only 212 31%, and the larger packet a little short of 3%. 214 Percentage overhead does not represent all the considerations 215 involved in reducing overhead. The average modem connection is still 216 only 14,400 bits per second, which translates to a per-byte real-time 217 cost of 0.6 milliseconds (14400 divided by 8 bits, as async framing 218 characters are not included in the modem-to-modem data transfer). 219 Thus, a savings of 16 bytes per packet can also be viewed as a 220 reduction of almost 10 milliseconds of latency per packet. While 221 this latency is short enough to be unnoticeable by a human, it may 222 impact real-time protocols such as streaming audio or video. 224 Thus, L2TP Header Compression provides most of its benefits when 225 carrying streams of small packets. In environments such as 226 downloading of graphic files, or where human interaction is 227 intermingled with the short packets, the benefits of L2TP Header 228 Compression will probably be undetectable. 230 6. Security Considerations 232 Because L2TPHC has no security facilities, it is critical that its 233 operation be reconciled with the security policy of its environment. 234 Since L2TPHC has no protocol header at all, it is trivial to spoof a 235 source IP address and inject malicious packets into an ongoing 236 session. There are several suitable techniques for controlling this 237 exposure. 239 In the simplest case, L2TPHC operates across a private network. For 240 instance, a remote user may dial into a private NAS located on this 241 network, and use L2TP (with or without L2TPHC) to cross an IP-only 242 portion of this network to establish a multi-protocol session 243 connected at a convenient point in the network. In this environment, 244 no additional security may be required, and L2TPHC would operate 245 trusting to the integrity of this private network. 247 If the weak protection of a difficult-to-guess protocol header is 248 deemed sufficient, expanded protocol overhead has clearly been 249 determined to be acceptable, and L2TP over UDP can be used without 250 L2TPHC. 252 If PPP encryption under ECP [3] is active, malicious PPP packets are 253 trivially detected and discarded as they are received on the raw IP 254 port number. Similarly, if an IPsec session is protecting the IP 255 packets themselves, malicious packets will also be discarded. Note 256 that in both cases, an expanded header is implicit in these security 257 facilities, which will greatly reduce the overhead efficiencies 258 gained by L2TPHC. For instance, an MD5 AH IPsec header will add 32 259 bytes to the packet. The 16 bytes saved by L2TPHC quickly 260 approaches statistical insignificance. 262 7. Acknowledgments 264 Thanks to Gurdeep Singh Pall of Microsoft for identifying and 265 describing scenarios in which L2TP header size become a concern. 267 Thanks to Bill Palter and W. Mark Townsley of Cisco Systems for help 268 in reviewing this draft. 270 8. Contacts 272 Andrew J. Valencia 273 Cisco Systems 274 170 West Tasman Drive 275 San Jose CA 95134-1706 276 vandys@cisco.com 278 9. References 280 [1] A. Valencia, "Layer 2 Tunnel Protocol ("L2TP")", Internet Draft, 281 October 1997 283 [2] G. Zorn, "RADIUS Attributes for Tunnel Protocol Support", Internet 284 draft, July 1997 286 [3] G. Meyer, "PPP Encryption Control Protocol (ECP)", RFC 1968