L2TP diffserv extension: Layer Two Tunneling Protocol (L2TP) Differentiated Services Extension
How two L2TP peers agree on the forwarding treatment for a tunnel's control messages and for each session inside it.
Calhoun, P., Luo, W., McPherson, D., and K. Peirce, "Layer Two Tunneling Protocol (L2TP) Differentiated Services Extension", RFC 3308, DOI 10.17487/RFC3308, November 2002, <https://www.rfc-editor.org/info/rfc3308>.
@misc{rfc3308,
series = {Request for Comments},
number = 3308,
howpublished = {RFC 3308},
publisher = {RFC Editor},
doi = {10.17487/RFC3308},
url = {https://www.rfc-editor.org/info/rfc3308},
author = {P. Calhoun and W. Luo and D. McPherson and K. Peirce},
title = {{Layer Two Tunneling Protocol (L2TP) Differentiated Services Extension}},
pagetotal = 10,
year = 2002,
month = nov,
} RFC text
Contents
Network Working Group P. Calhoun
Request for Comments: 3308 Black Storm Networks
Category: Standards Track W. Luo
Cisco Systems, Inc.
D. McPherson
TCB
K. Peirce
Malibu Networks, Inc.
November 2002
Layer Two Tunneling Protocol (L2TP)
Differentiated Services Extension
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document describes mechanisms which enable the Layer Two
Tunneling Protocol (L2TP) to negotiate desired Per Hop Behavior (PHB)
code for the L2TP control connection, as well as individual sessions
within an L2TP tunnel.
L2TP provides a standard method for tunneling PPP packets. The
current specification provides no provisions for supporting
Differentiated Services (diffserv) over the L2TP control connection
or subsequent data sessions. As a result, no standard mechanism
currently exists within L2TP to provide L2TP protocol negotiations
for service discrimination.
Table of Contents
1. Specification of Requirements ............................... 2
2. Introduction ................................................ 2
3. Control Connection Operation ................................ 3
3.1. Control Connection DS AVP (SCCRQ, SCCRP) .................... 4
4. Session Operation ........................................... 4
4.1. Session DS AVP (ICRQ, ICRP, OCRQ, OCRP) ..................... 6
5. DS AVPs Correlation ......................................... 6
6. PHB Encoding ................................................ 6
7. DSCP Selection .............................................. 7
8. Packet Reordering and Sequence Numbers ...................... 7
9. Crossing Differentiated Services Boundaries ................. 7
10. IANA Considerations ......................................... 8
11. Security Considerations ..................................... 8
12. Acknowledgements ............................................ 8
13. References .................................................. 8
14. Authors' Addresses .......................................... 9
15. Full Copyright Statement .................................... 10
1. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC 2119].
2. Introduction
The L2TP specification currently provides no mechanism for supporting
diffserv (DS). This document describes mechanisms that enable L2TP
to indicate desired PHB code, as defined in [RFC 3140], to be
associated with an L2TP control connection, as well as individual
sessions within an L2TP tunnel.
The actual bit interpretation of the DS field is beyond the scope of
this document, and is purposefully omitted. This document is
concerned only with defining a uniform exchange and subsequent
mapping mechanism for the DS AVPs.
3. Control Connection Operation
As defined in [RFC 2661], a control connection operates in-band over
a tunnel to control the establishment, release, and maintenance of
sessions and of the tunnel itself. As such, this document provides a
mechanism to enable discrimination of L2TP control messages from
other packets. For this purpose, we introduce the Control Connection
DS (CCDS) AVP.
The presence of the CCDS AVP serves as an indication to the peer (LAC
or LNS) that the tunnel initiator wishes both the tunnel initiator
and terminator to use the per-hop behavior(s) (PHB(s)) indicated by
the AVP's PHB code for all packets within the tunnel's control
connection. A PHB is a description of the externally observable
forwarding behavior of a DS node applied to a particular DS behavior
aggregate, as defined in [RFC 2475]. The most simple example of a
PHB is one which guarantees a minimal bandwidth allocation of a link
to a behavior aggregate.
Upon receipt of a Start-Control-Connection-Request (SCCRQ) containing
the CCDS AVP, if the tunnel terminator provides no support for the
CCDS AVP it MUST ignore the AVP and send an SCCRP to the tunnel
initiator without the CCDS AVP. The tunnel initiator interprets the
absence of the CCDS AVP in the SCCRP as an indication that the tunnel
terminator is incapable of supporting CCDS.
Upon receipt of an SCCRP that contains no CCDS AVP in response to a
SCCRQ that contained a CCDS AVP, if the tunnel initiator wants to
continue tunnel establishment it sends an SCCCN. Otherwise, it sends
a StopCCN to the tunnel terminator to end the connection. The
StopCCN control message MUST contain the Result Code 8 that indicates
CCDS AVP value (47) as the reason for sending the StopCCN.
If the tunnel terminator provides support for CCDS, it SHOULD use the
Host Name AVP embedded in SCCRQ to consult its local policy, and to
determine whether local policy permits the requested PHB code to be
used on this control connection. If it is unwilling or unable to
support the requested PHB code after consulting the local policy, the
tunnel terminator MUST send an SCCRP control message containing a
CCDS AVP indicating the value it is willing to use. If the CCDS AVP
value is the same as the one in the SCCRQ, it signals the acceptance
of the requested PHB code. If the value is different it serves as a
counter-offer by the tunnel terminator.
If the tunnel initiator receives an SCCRP that contains a CCDS AVP
with a value other than that requested in the SCCRQ, the tunnel
initiator SHOULD check the PHB code against its own policy. If it is
unwilling to use the value, the tunnel initiator MUST send a StopCCN
control message containing the Result Code 8 that indicates CCDS AVP
value (47) as the reason for sending the StopCCN.
3.1. Control Connection DS AVP (SCCRQ, SCCRP)
The CCDS AVP is encoded as Vendor ID 0, and the Attribute Type is 47.
Each CCDS AVP is encoded as follows:
Vendor ID = 0
Attribute = 47
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|0|0|0|0| Length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 47 | PHB Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP MAY be present in the following message types: SCCRQ and
SCCRP. This AVP MAY be hidden (the H-bit set to 0 or 1) and is
optional (M-bit not set). The length (before hiding) of this AVP
MUST be 8 octets. The encoding of the PHB code is described in
Section 6.
4. Session Operation
As defined in [RFC 2661], an L2TP session is connection-oriented. The
LAC and LNS maintain states for each call that is initiated or
answered by an LAC. An L2TP session is created between the LAC and
LNS when an end-to-end connection is established between a Remote
System and the LNS. Datagrams related to the connection are sent
over the tunnel between the LAC and LNS. As such, this document
provides a mechanism to enable discrimination for packets within a
particular session from those in other sessions. For this purpose,
we introduce the Session DS (SDS) AVP.
The presence of the SDS AVP serves as an indication to the peer (LAC
or LNS) that the session initiator wishes both the session initiator
and terminator to use the per-hop behavior(s) (PHB(s)) indicated by
the AVP's PHB code for all packets within the session.
Upon receipt of an Incoming-Call-Request (ICRQ) or Outgoing-Call-
Request (OCRQ) containing the SDS AVP if the session terminator
provides no support for the requested PHB code, the session
terminator MUST ignore the SDS AVP and send an ICRP or OCRP to the
session initiator without the SDS AVP. The session initiator
interprets the absence of the SDS AVP in the ICRP or OCRP as an
indication that the session terminator is incapable of supporting
SDS.
Upon receipt of an ICRP or OCRP that contains no SDS AVP in response
to an ICRQ or OCRQ that contained an SDS AVP, if the session
initiator is willing to omit employing SDS AVP it continues session
establishment as defined in [RFC 2661]. Otherwise, it sends a CDN to
the session terminator to end the connection. The CDN control
message MUST contain the Result Code 12 that indicates SDS AVP value
(48) as the reason for sending the CDN.
In order to help the session terminator to distinguish one session
from another when consulting the local policy of the PHB code, the
session initiator MAY use the identifier or a combination of
identifiers embedded in AVPs such as Proxy Authen Name AVP, Calling
Number AVP, Called Number AVP, and Sub-Address AVP. When Proxy
Authen Name AVP is used as a distinguishor, it SHOULD be present in
the ICRQ or OCRQ. The designated DS identifier(s) used for looking
up the PHB code SHOULD be configurable.
If the session terminator provides support for SDS, it SHOULD use the
the designated DS identification AVP (via out-of-band agreement
between the administrators of the LAC and LNS) to consult the local
policy and determinate whether the local policy permits the requested
PHB code to be used on this session. If it is unwilling or unable to
support the requested PHB code the session terminator MUST do one of
the following:
1) Send a CDN message containing the Result Code 12 that indicates
SDS AVP value (48) as the reason for sending the CDN.
2) Send an Incoming-Call-Reply (ICRP) or Outgoing-Call-Reply (OCRP)
message containing an SDS AVP indicating the PHB code the
terminator is willing to use for the session.
If the session terminator supports the PHB code in the SDS AVP
session establishment MUST continue as defined in [RFC 2661].
If the session initiator receives an ICRP or OCRP that contains an
SDS AVP with a value other than that requested in the ICRQ or OCRQ,
and the session initiator is unwilling to use the value, the session
initiator MUST send a CDN message containing the Result Code 12 that
indicates SDS AVP value (48) as the reason for sending the CDN. If
the session initiator receives an ICRP or OCRP that contains an SDS
AVP with a value other than that requested in the ICRQ or OCRQ, and
the session initiator is willing to use the value, the session
initiator MUST proceed as indicated in [RFC 2661].
4.1. Session DS AVP (ICRQ, ICRP, OCRQ, OCRP)
The SDS AVP is encoded as Vendor ID 0, and the Attribute Value is 48.
Each SDS AVP is encoded as follows:
Vendor ID = 0
Attribute = 48
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|0|0|0|0| Length | 0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 48 | PHB Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
This AVP MAY be present in the following message types: ICRQ, ICRP,
OCRQ and OCRP. This AVP MAY be hidden (the H-bit set to 0 or 1) and
is optional (M-bit is not set 0). The length (before hiding) of this
AVP MUST be 8 octets. The encoding of the PHB code is described in
Section 6.
5. DS AVPs Correlation
CCDS AVP and SDS AVP are independent of each other. CCDS AVP is used
to signal diffserv for the control connection between two L2TP peers,
while SDS AVP is used for data connection. The PHB code signaled in
one AVP SHOULD NOT have any implication on the PHB code signaled in
the other AVP. Implementations MAY choose to implement either or
both DS AVPs, and operations MAY choose to enable diffserv on either
or both types of connections.
6. PHB Encoding
The PHB code is a left-justified 16-bit field using Per Hop Behavior
(PHB) encoding defined in [RFC 3140]. Note that [RFC 3140] and its
successor are the ultimate authority defining PHB encoding.
Upon successful establishment of an L2TP tunnel control connection or
individual L2TP session employing the appropriate DS AVP defined in
this document, both LAC and LNS MUST use their own PHB-to-DSCP
mappings of their present DS domains to map the PHB to a DSCP and
place it in the DS field of the outer IP header of packets
transmitted on the connection.
7. DSCP Selection
The requirements or rules of each service and DSCP mapping are set
through administrative policy mechanisms which are outside the scope
of this document.
8. Packet Reordering and Sequence Numbers
[RFC 2474] RECOMMENDS that PHB implementations not cause reordering
of packets within an individual connection. [RFC 3140] requires that
a set of PHBs signaled using a single PHB ID MUST NOT cause
additional packet reordering within an individual connection vs.
using a single PHB. Since the CCDS and SDS AVPs contain one PHB ID,
use of diffserv PHBs in accordance with this specification should not
cause additional packet reordering within an L2TP control or data
connection.
Sequence numbers are required to be present in all control messages
and are used to provide reliable delivery on the control connection,
as defined in [RFC 2661]. While packet reordering is inevitably as
much a function of the network as it is local traffic conditioning,
the probability of it occurring when employing the CCDS AVP is same
as when not employing the AVP. Data messages MAY use sequence
numbers to reorder packets and detect lost packets.
9. Crossing Differentiated Services Boundaries
With the potential that an L2TP connection traverses an arbitrary
number of DS domains, signaling PHBs via L2TP is more appropriate
than signaling DSCPs, because it maintains a consistent end-to-end
differentiated service for the L2TP connection. As per [RFC 2983],
the negotiated PHBs are mapped to locally defined DSCPs of the
current DS domain at the tunnel ingress node. At the DS domain
boundary nodes, the DSCPs can be rewritten in the DS field of the
outer IP header, so that the DSCPs are always with respect to
whatever DS domain the packet happens to be in.
As a result, it is perfectly acceptable that the outermost DS field
of packets arriving on a given control connection or session are not
marked with the same DSCP value that was used by the tunnel ingress
node.
10. IANA Considerations
This document defines 2 L2TP Differentiated Services Extension AVPs.
The IANA has assigned the value of 47 for the "CCDS AVP" defined in
section 5.1 and the value of 48 for SDS AVP defined in section 6.1.
IANA has also assigned L2TP Result Code values of 8 for disconnecting
control connection due to mismatching CCDS value (StopCCN), and 12
for disconnecting call due to mismatching SDS value (CDN).
11. Security Considerations
This encoding in itself raises no security issues. However, users of
this encoding should consider that modifying a DSCP MAY constitute
theft or denial of service, so protocols using this encoding MUST be
adequately protected. No new security issues beyond those discussed
in [RFC 2474] and [RFC 2475] are introduced here.
12. Acknowledgements
Many thanks to David Black, W. Mark Townsley, Nishit Vasavada, Andy
Koscinski and John Shriver for their review and insightful feedback.
13. References
[RFC 1661] Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
RFC 1661, July 1994.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2474] Nichols, K., Blake, S., Baker, F. and D. Black "Definition
of the Differentiated Services Field (DS Field) in the
IPv4 and IPv6 Headers", RFC 2474, December 1998.
[RFC 2475] Blake, S., Black, D., Carlson, Z., Davies, E., Wang, Z.
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC 2661] Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn, G.
and B. Palter, "Layer 2 Tunnel Protocol (L2TP)", RFC 2661,
August 1999.
[RFC 2983] Black, D., "Differentiated Services and Tunnels", RFC
2983, October 2000.
[RFC 3140] Black, D., Brim, S., Carpenter, B. and F. Le Faucheur,
"Per Hop Behavior Identification Codes", RFC 3140, June
2001.
14. Authors' Addresses
Pat R. Calhoun
110 Nortech Parkway
San Jose, CA 95134-2307
Phone: +1 408.941.0500
EMail: pcalhoun@bstormnetworks.com
Wei Luo
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134
Phone: +1 408.525.6906
EMail: luo@cisco.com
Danny McPherson
TCB
Phone: +1 303.470.9257
EMail: danny@tcb.net
Ken Peirce
Malibu Networks, Inc.
1107 Investment Blvd, Suite 250
El Dorado Hills, CA 95762
Phone: +1 916.941.8814
EMail: Ken@malibunetworks.com
15. Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
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and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
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English.
The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
AI ELI5
On this page
§2The Gap in L2TP
The Layer Two Tunneling Protocol (L2TP) tunnels PPP packets, but the L2TP specification (RFC 2661) has no mechanism for supporting Differentiated Services (diffserv, or DS). This document adds one so that L2TP can indicate the desired Per Hop Behavior (PHB) for the control connection and for individual sessions within a tunnel.
A PHB, as defined in RFC 2475, is the externally observable forwarding behavior a DS node applies to a particular behavior aggregate. The simplest example is guaranteeing a minimal bandwidth allocation of a link to that aggregate.
The RFC deliberately stays small. The actual bit interpretation of the DS field is out of scope; the document only defines a uniform exchange and a mapping mechanism for the new DS AVPs.
L2TP could build a tunnel but could not say which traffic was more important. This document adds a way to say it.
§3Negotiating for the Control Connection
The control connection runs in-band over the tunnel and handles setting up, maintaining and releasing the tunnel and its sessions. To give those control messages their own treatment, the RFC introduces the Control Connection DS (CCDS) AVP. Its presence in a Start-Control-Connection-Request (SCCRQ) tells the peer that the tunnel initiator wants both ends to use the PHB named by the AVP's PHB code for all packets of the control connection.
The terminator can respond in three ways:
- If it does not support CCDS, it
MUSTignore the AVP and send an SCCRP without it. The initiator reads the missing AVP as a lack of support. - If it supports CCDS, it
SHOULDuse the Host Name AVP from the SCCRQ to consult local policy. If it is unwilling or unable to use the requested code, itMUSTsend an SCCRP with a CCDS AVP holding the value it will use. - A matching value means acceptance; a different value is a counter-offer.
If the initiator does not want to continue after a missing or different answer, it sends a StopCCN carrying Result Code 8, which names the CCDS AVP (value 47) as the reason. If it still wants the tunnel after an SCCRP without CCDS, it sends an SCCCN.
One side asks, 'Can we send our tunnel's own messages in the fast lane?' The other side says yes, offers a different lane, or says it does not know how. If the answer is no good, the first side can hang up.
- SCCRQ + CCDS AVPRequests a PHB code for the control connection.
- SCCRP + CCDS AVPSame value accepts; a different value is a counter-offer.
- SCCCN or StopCCN (Result Code 8)Continue, or end the tunnel if the offered value is not acceptable.
§4Negotiating for a Session
A session is created between the LAC and LNS when an end-to-end connection is established between a Remote System and the LNS. The Session DS (SDS) AVP lets the session initiator ask both ends to apply a PHB to all packets of that one session. It travels in ICRQ, ICRP, OCRQ and OCRP messages.
The flow mirrors the control connection, with a few differences. A terminator that provides no support for the requested PHB code MUST ignore the SDS AVP and reply without it. A terminator that supports SDS but is unwilling or unable to use the requested code MUST either send a CDN with Result Code 12, which names the SDS AVP (value 48), or reply with an SDS AVP holding the code it will accept. An initiator that gets no SDS AVP back may carry on with ordinary setup if it is willing to go without; otherwise it sends a CDN with Result Code 12. The same applies if it receives a different value that it will not use.
Because many sessions share one tunnel, the terminator needs a way to tell them apart when it checks policy. The initiator MAY use identifiers carried in the Proxy Authen Name, Calling Number, Called Number or Sub-Address AVPs, and the identifiers used SHOULD be configurable. The administrators of the LAC and LNS agree on this out of band.
Inside the tunnel there can be many separate calls. Each call can ask for its own lane, and the other end can agree, offer something else, or end that call if the two cannot agree.
§5The CCDS and SDS AVPs
Both AVPs use Vendor ID 0. The CCDS AVP has attribute type 47 and the SDS AVP has attribute type 48. Each is 8 octets long before hiding and ends in a 16-bit PHB Code field.
CCDS may appear in SCCRQ and SCCRP. SDS may appear in ICRQ, ICRP, OCRQ and OCRP. Both AVPs may be hidden (the H bit may be 0 or 1) and are optional, so the M bit is not set.
The two AVPs are independent. The PHB signaled in one SHOULD NOT have any implication for the other, implementations MAY support either or both, and operators may enable diffserv on either or both kinds of connection.
Each request carries a tiny note with a lane code. There is one kind of note for the tunnel's own messages and another kind for each call.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
FLAGS | M, H and reserved bits | 6 bits | M bit not set; H bit may be 0 or 1. | |
LENGTH | Length | 10 bits | Must be 8 octets before hiding. | |
VENDOR-ID | Vendor ID | 2 bytes | 0 | Zero. |
ATTRIBUTE | Attribute | 2 bytes | 47 | Identifies the CCDS AVP. |
PHB-CODE | PHB Code | 2 bytes | Left-justified PHB code encoded as in RFC 3140. |
§6PHB Codes and DSCP Mapping
The PHB code is a left-justified 16-bit field using the PHB encoding defined in RFC 3140, which, along with its successor, is the ultimate authority on that encoding.
Once a control connection or session is established with the matching DS AVP, both LAC and LNS MUST use their own PHB-to-DSCP mappings for their present DS domains. They map the PHB to a DSCP and place it in the DS field of the outer IP header of packets sent on that connection. The rules for that mapping are set by administrative policy and are outside this document.
Signaling PHBs rather than DSCPs is deliberate. A connection may cross any number of DS domains, and a PHB keeps the end-to-end service consistent while each domain uses its own DSCP values. Boundary nodes may rewrite the DSCP, so it is acceptable for packets to arrive with an outer DS field different from the one the ingress node used (see RFC 2983).
The two ends agree on what kind of treatment they want, not on a particular sticker. Each network then uses its own sticker for that treatment, so the meaning survives the trip even if the sticker changes.
§8Reordering and Sequence Numbers
RFC 2474 recommends that PHB implementations not reorder packets within an individual connection, and RFC 3140 requires that a set of PHBs signaled with a single PHB ID must not cause additional reordering compared with a single PHB. Since the CCDS and SDS AVPs each carry one PHB ID, using them as specified should not add reordering within an L2TP control or data connection.
Control messages always carry sequence numbers for reliable delivery, so the chance of reordering with CCDS is the same as without it. Data messages MAY use sequence numbers to reorder packets and detect losses.
Giving a car a lane should not shuffle the order of cars in the same line, and numbered tickets let the receiver fix any mix-up.
§11IANA and Security Notes
IANA assigned AVP value 47 to CCDS and 48 to SDS. It also assigned L2TP Result Code 8 for disconnecting a control connection due to a mismatched CCDS value (StopCCN) and Result Code 12 for disconnecting a call due to a mismatched SDS value (CDN).
The encoding raises no security issues in itself. The RFC warns that modifying a DSCP may constitute theft or denial of service, so protocols using this encoding MUST be adequately protected. No new issues beyond those in RFC 2474 and RFC 2475 are introduced.
If someone could secretly change the lane stickers, they could steal fast-lane service or jam it for others, so the tunnel needs protection.