One document matched: draft-ietf-l2tpext-l2tphc-00.txt
Network Working Group Andrew J. Valencia
Request for Comments: DRAFT Cisco Systems
Category: Internet Draft
Title: draft-ietf-l2tpext-l2tphc-00.txt
Date: January 2000
L2TP Header Compression (``L2TPHC'')
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,
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Abstract
The Layer 2 Tunneling Protocol (``L2TP'') [RFC2661] 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 [RFC2661] 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''
[1]. 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.
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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 uses a single IP address for the life of the tunnel
- The client has only one public IP network interface
- There may be only one tunnel between the client and its LNS
- There will be only one session within a 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 changing its IP address
as it moves among IP egress points, 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
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.
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,
possibly followed by an additional octet, 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
L2TPHC is negotiated by an optional AVP ``L2TPHC-Protocol'' 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
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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-Protocol AVP in the SCCRQ/SCCRP exchange. This optional
AVP, ``L2TPHC-Assigned-Tunnel-ID'', 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. Note that this ID is used only in the L2TPHC header, and
is a distinct value from the tunnel ID used in L2TP headers.
Since all control passes over the parallel L2TP tunnel
corresponding to the L2TPHC one, the L2TP tunnel is always the one
explicitly terminated, and the associated L2TPHC tunnel is
implicitly terminated.
3.2 AVP Format
The AVP L2TPHC-Protocol is encoded as Vendor ID 9, Attribute is
the 16-bit quantity 1 (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. The overall length of this AVP is thus 7. Unless and
until an official protocol number is allocated, the value 251 is
recommended. The M, H, and "rsvd" bits MUST all be set to 0.
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| rsvd | 7 | 9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 1 | 251 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The L2TPHC-Assigned-Tunnel-ID AVP MUST also be sent with the M, H,
and "rsvd" bits all set to 0. It MUST NOT be present except when
immediately following an L2TPHC-Protocol AVP. The Attribute is
the 16-bit value 2, encoded in network byte order. The single
octet Value is a Tunnel ID to be used in the L2TPHC encapsulation.
The overall length of this AVP is thus 7. 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 I bit
(see below), 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.
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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| rsvd | 7 | 9 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 2 | 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 is always sent 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|T|L|x|x|S|I|O|P| Tunnel ID | PPP packet... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
x bits indicate reserved bit fields and MUST be set to 0. A packet
received with a reserved bit set to 1 MUST be silently discarded,
unless the bit is defined for an extension that is known to the
implementation.
The T bit MUST be set to 0, indicating payload. Control messages are
never sent over L2TPHC.
The L bit MUST be set to 0, indicating no length field. No length
field is ever present in L2TPHC.
The S bit MUST be set to 0, indicating no Nr/Ns fields. Control
packets are not passed via L2TPHC. If sequencing of data packets is
required, L2TPHC MUST NOT be used for those data packets. However,
it is legal for data traffic to be mixed, with order-sensitive
packets using L2TP/UDP, and other data packets using L2TPHC. In this
situation, data packets passed using L2TPHC have no impact on L2TP
sequence numbers.
The I bit MUST be set to 1, and the Tunnel ID MUST be present, if the
L2TPHC-Assigned-Tunnel-ID AVP was both sent and received during
tunnel setup. Otherwise I MUST be set to 0, and the Tunnel ID octet
MUST be omitted from the packet.
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The O bit MUST be set to 0, indicating no offset field. Offsets are
never used in L2TPHC.
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, and optionally one additional octet if the I bit is set to
1.
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.
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 header 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.
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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 [RFC1968] 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 of Redback Networks 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
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9. References
[RFC2661] M. Townsley, ``Layer 2 Tunnel Protocol (L2TP)'', RFC 2661,
August 1999
[1] G. Zorn, ``RADIUS Attributes for Tunnel Protocol Support'', Internet
draft, August 1999
[RFC1968] G. Meyer, ``PPP Encryption Control Protocol (ECP)'', RFC 1968,
June 1996
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