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Versions: 00 01 02 03 rfc4247                              Informational
Network Working Group                                        Jerry Ash
Internet Draft                                               Bur Goode
Category: Informational                                       Jim Hand
<draft-ietf-avt-hc-mpls-reqs-03.txt>                              AT&T

                                                         Raymond Zhang
                                          Infonet Services Corporation

                                                            June, 2004

            Requirements for Header Compression over MPLS

Status of this Memo:

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Internet Draft  Requirements for Header Compression over MPLS   June 2004


   VoIP typically uses the encapsulation voice/RTP/UDP/IP.  When MPLS
   labels are added, this becomes voice/RTP/UDP/IP/MPLS-labels, where,
   for example, the packet header is at least 48 bytes, while the voice
   payload is often no more than 30 bytes.  Header compression can
   significantly reduce the overhead through various compression
   mechanisms, such as enhanced compressed RTP (ECRTP) and robust header
   compression (ROHC). We consider using MPLS to route compressed
   packets over an MPLS LSP without compression/decompression cycles at
   each router.  This approach can increase the bandwidth efficiency as
   well as processing scalability of the maximum number of simultaneous
   flows that use header compression at each router.  In the draft we
   give a problem statement, goals and requirements, and an example

Table of Contents:

   1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2. Problem Statement  . . . . . . . . . . . . . . . . . . . . . . . 4
   3. Goals & Requirements . . . . . . . . . . . . . . . . . . . . . . 5
   4. Candidate Solution Methods & Needs . . . . . . . . . . . . . . . 6
   5. Example Scenario . . . . . . . . . . . . . . . . . . . . . . . . 7
   6. Security Considerations  . . . . . . . . . . . . . . . . . . . . 8
   7. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . . 8
   8. Normative References . . . . . . . . . . . . . . . . . . . . . . 8
   9. Informative References . . . . . . . . . . . . . . . . . . . . . 9
   10. Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . 9
   11. Intellectual Property Considerations. . . . . . . . . . . . . . 9
   12. Full Copyright Statement  . . . . . . . . . . . . . . . . . . . 10

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

1. Introduction

   Voice over IP (VoIP) typically uses the encapsulation
   voice/RTP/UDP/IP.  When MPLS labels [MPLS-ARCH] are added, this
   becomes voice/RTP/UDP/IP/MPLS-labels.  For an MPLS VPN (e.g.,
   [MPLS-VPN], the packet header is at least 48 bytes, while the voice
   payload is often no more than 30 bytes, for example.  The interest in
   header compression (HC) is to exploit the possibility of
   significantly reducing the overhead through various compression
   mechanisms, such as with enhanced compressed RTP [ECRTP] or robust
   header compression [ROHC], and also to increase scalability of HC.
   We consider using MPLS to route compressed packets over an MPLS LSP
   (label switched path) without compression/decompression cycles at
   each router.  Such an HC over MPLS capability can increase bandwidth
   efficiency as well as the processing scalability of the maximum
   number of simultaneous flows which use HC at each router.

   To implement HC over MPLS, the ingress router/gateway would have to
   apply the HC algorithm to the IP packet, the compressed packet routed
   on an MPLS LSP using MPLS labels, and the compressed header would be
   decompressed at the egress router/gateway where the HC session
   terminates.  Figure 1 illustrates an HC over MPLS session established
   on an LSP that crosses several routers, from R1/HC --> R2 --> R3 -->
   R4/HD, where R1/HC is the ingress router where HC is performed, and
   R4/HD is the egress router where header decompression (HD) is done.
   HC of the RTP/UDP/IP header is performed at R1/HC, and the compressed
   packets are routed using MPLS labels from R1/HC to R2, to R3, and
   finally to R4/HD, without further decompression/recompression cycles.
   The RTP/UDP/IP header is decompressed at R4/HD and can be forwarded
   to other routers, as needed.

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

                   |     |
                   |R1/HC| Header Compression (HC) Performed
                      | voice/compressed-header/MPLS-labels
                   |     |
                   | R2  |
                      | voice/compressed-header/MPLS-labels
                   |     |
                   | R3  |
                      | voice/compressed-header/MPLS-labels
                   |     |
                   |R4/HD| Header Decompression (HD) Performed

Figure 1. Example of Header Compression over MPLS over Routers R1-->R4

   In the example scenario, HC therefore takes place between R1 and R4,
   and the MPLS path transports voice/compressed-header/MPLS-labels
   instead of voice/RTP/UDP/IP/MPLS-labels, typically saving 30 octets
   or more per packet. The MPLS label stack and link-layer headers are
   not compressed. A signaling method is needed to set up a
   correspondence between the ingress and egress routers of the HC over
   MPLS session.

   In Section 2 we give a problem statement, in Section 3 we give goals
   and requirements, and in Section 4 we give an example scenario.

2. Problem Statement

   As described in the introduction, HC over MPLS can significantly
   reduce the header overhead through HC mechanisms.  The need for HC
   may be important on low-speed links where bandwidth is more scarce,
   but it could also be important on backbone facilities, especially
   where costs are high (e.g., some global cross-sections).  VoIP
   typically will use voice compression mechanisms (e.g., G.729) on
   low-speed and international routes, in order to conserve bandwidth.
   With HC, significantly more bandwidth could be saved. For example,
   carrying uncompressed headers for the entire voice load of a large
   domestic network with 300 million or more calls per day could
   consume on the order of about 20-40 gigabits-per-second on the
   backbone network for headers alone. This overhead could translate
   into considerable bandwidth capacity.

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

   The claim is often made that once fiber is in place, increasing the
   bandwidth capacity is inexpensive, nearly 'free'.  This may be true
   in some cases, however, on some international cross-sections,
   especially, facility/transport costs are very high and saving
   bandwidth on such backbone links is very worthwhile. Decreasing the
   backbone bandwidth is needed in some areas of the world where
   bandwidth is very expensive.  It is also important in almost all
   locations to decrease the bandwidth consumption on low-speed links.
   So although bandwidth is getting cheaper, the value of compression
   does not go away.  It should be further noted that IPv6 will increase
   the size of headers, and therefore increase the importance of HC for
   RTP flows.

   While hop-by-hop HC could be applied to decrease bandwidth
   requirements, that implies a processing requirement for
   compression-decompression cycles at every router hop, which does not
   scale well for large voice traffic loads.  The maximum number of cRTP
   flows is about 30-50 for a typical customer premise router, depending
   upon its uplink speed and processing power, while the need may exceed
   300-500 for a high-end case. Therefore, HC over MPLS seems to be a
   viable alternative to get the compression benefits without introducing
   costly processing demands on the intermediate nodes.  By using HC over
   MPLS, routers merely forward compressed packets without doing a
   decompression/recompression cycle, thereby increasing the maximum
   number of simultaneous compressed flows that routers can handle.

   Therefore the proposal is to use existing HC techniques, together
   with MPLS labels, to make the transport of the RTP/UDP/IP headers
   more efficient over an MPLS network.  However, at this time, there
   are no standards for HC over MPLS, and vendors have not implemented
   such techniques.

3. Goals & Requirements

Specification of Requirements

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC 2119].

   The goals of HC over MPLS are as follows:

   a. provide more efficient voice transport over MPLS networks,
   b. increase the scalability of HC to a large number of flows,
   c. not significantly increase packet delay, delay variation, or loss
   probability, and
   d. leverage existing work through use of standard protocols as much
   as possible.

   Therefore the requirements for HC over MPLS are as follows:

   a. MUST use existing protocols (e.g., [ECRTP], [ROHC]) to compress
   RTP/UDP/IP headers, in order to provide for efficient transport,
   tolerance to packet loss, and resistance to loss of session context.
   b. MUST allow HC over an MPLS LSP, and thereby avoid hop-by-hop
   compression/decompression cycles [e.g., ECRTP-MPLS-PROTO].

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

   c. MUST minimize incremental performance degradation due to increased
   delay, packet loss, and jitter.
   d. MUST use standard protocols to signal context identification and
   control information (e.g., [RSVP], [RSVP-TE], [LDP]).
   e. Packet reordering MUST NOT cause incorrectly decompressed packets
   to be forwarded from the decompressor.

   It is necessary that the HC method be able to handle out-of-sequence
   packets.  MPLS [MPLS-ARCH] enables 4-byte labels to be appended to IP
   packets to allow switching from the ingress label switched router
   (LSR) to the egress LSP on an LSP through an MPLS network.  However,
   MPLS does not guarantee that packets will arrive in order at the
   egress LSR, since a number of things could cause packets to be
   delivered out of sequence.  For example, a link failure could cause
   the LSP routing to change, due perhaps to an MPLS fast reroute taking
   place, or to the interior gateway protocol (IGP) and label
   distribution protocol (LDP) converging to another route, among other
   possible reasons.  Other causes could include IGP reroutes due to
   'loose hops' in the LSP, or BGP route changes reflecting back into
   IGP reroutes.  HC algorithms may be able to handle reordering
   magnitudes on the order of about 10 packets, which may make the time
   required for IGP reconvergence (typically on the order of seconds)
   untenable for the HC algorithm.  On the other hand, MPLS fast reroute
   may be fast enough (on the order of 50 ms. or less) for the HC
   algorithm to handle packet reordering.  The issue of reordering needs
   to be further considered in the development of the HC over MPLS

   Resynchronization and performance also needs to be considered, since
   HC over MPLS can sometimes have multiple routers in the LSP.
   Tunneling a HC session over an MPLS LSP with multiple routers in the
   path will increase the round trip delay and the chance of packet
   loss, and HC contexts are invalidated due to packet loss. The HC
   error recovery mechanism can compound the problem when long round
   trip delays are involved.

4. Candidate Solution Methods & Needs

   [cRTP] performs best with very low packet error rates on all hops of
   the path. When the cRTP decompressor context state gets out of synch
   with the compressor, it will drop packets associated with the context
   until the two states are resynchronized. To resynchronize context
   state at the two ends, the decompressor transmits the CONTEXT_STATE
   packet to the compressor, and the compressor transmits a FULL_HEADER
   packet to the decompressor.

   [ECRTP] uses mechanisms that make cRTP more tolerant to packet loss,
   and ECRTP thereby helps to minimize the use of feedback-based error
   recovery (CONTEXT_STATE packets).  ECRTP is therefore a candidate
   method to make HC over MPLS more tolerant of packet loss and to guard
   against frequent resynchronizations.  ECRTP may need some
   implementation adaptations to address the reordering requirement in
   Section 3 (requirement e), since a default implementation will
   probably not meet the requirement.  ECRTP protocol extensions may be
   required to identify FULL_HEADER, CONTEXT_STATE, and compressed
   packet types.  [cRTP-ENCAP] specifies a separate link-layer packet

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

   type defined for HC. Using a separate link-layer packet type avoids
   the need to add extra bits to the compression header to identify the
   packet type. However, this approach does not extend well to MPLS
   encapsulation conventions [MPLS-ENCAP], in which a separate
   link-layer packet type translates into a separate LSP for each packet
   type. In order to extend ECRTP to HC over MPLS, each packet type
   defined in [ECRTP] would need to be identified in an appended packet
   type field in the ECRTP header.

   [ROHC] is also very tolerant of packet loss, and therefore is a
   candidate method to guard against frequent resynchronizations.  ROHC
   also achieves a somewhat better level of compression as compared to
   ECRTP.  ROHC may need some implementation adaptations to address the
   reordering requirement in Section 3 (requirement e), since a default
   implementation will probably not meet the requirement.  ROHC already
   has the capability to identify the packet type in the compression
   header, so no further extension is needed to identify packet type.

   Extensions to MPLS signaling may be needed to identify the LSP from
   HC to HD egress point, negotiate the HC algorithm used and protocol
   parameters, and negotiate the session context IDs (SCIDs) space
   between the ingress and egress routers on the MPLS LSP.  For example,
   new objects may need to be defined for [RSVP-TE] to signal the SCID
   spaces between the ingress and egress routers, and the HC algorithm
   used to determine the context; these HC packets then contain the SCID
   identified by using the RSVP-TE objects.  It is also desirable to
   signal HC over MPLS tunnels with the label distribution protocol
   [LDP], since many RFC2547 VPN [MPLS-VPN] implementations use LDP as
   the underlying LSP signaling mechanism, and LDP is very scalable.
   However, extensions to LDP may be needed to signal SCIDs between
   ingress and egress routers on HC over MPLS LSPs.  For example,
   'targeted LDP sessions' might be established for signaling SCIDs,
   or perhaps methods described in [LDP-PWE3] and [GVPLS] to signal
   pseudo-wires and multipoint-to-point LSPs might be extended to
   support signaling of SCIDs for HC over MPLS LSPs. These MPLS
   signaling protocol extensions need coordination with other working
   groups (e.g., MPLS).

5. Example Scenario

   As illustrated in Figure 2, many VoIP flows are originated from
   customer sites, which are served by routers R1, R2 and R3, and
   terminated at several large customer call centers, which are served
   by R5, R6 and R7.  R4 is a service-provider router, and all VoIP
   flows traverse R4.  It is essential that the R4-R5, R4-R6, and R4-R7
   low-speed links all use HC to allow a maximum number of simultaneous
   VoIP flows.  To allow processing at R4 to handle the volume of
   simultaneous VoIP flows, it is desired to use HC over MPLS for these
   flows.  With HC over MPLS, R4 does not need to do HC/HD for the flows
   to the call centers, enabling more scalability of the number of
   simultaneous VoIP flows with HC at R4.

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

     voice/C-HDR/MPLS-labels ______ voice/C-HDR/MPLS-labels
R1/HC---------------------->|      |-----------------------> R5/HD
                            |      |
     voice/C-HDR/MPLS-labels|      |voice/C-HDR/MPLS-labels
R2/HC---------------------->|  R4  |-----------------------> R6/HD
                            |      |
     voice/C-HDR/MPLS-labels|      |voice/C-HDR/MPLS-labels
R3/HC---------------------->|______|-----------------------> R7/HD

[Note: HC = header compression; C-HDR = compressed header; HD =
header decompression]

     Figure 2. Example Scenario for Application of HC over MPLS

6. Security Considerations

   The high processing load of HC makes HC a target for
   denial-of-service attacks.  For example, an attacker could send a
   high bandwidth data stream through a network, with the headers in the
   data stream marked appropriately to cause HC to be applied.  This
   would use large amounts of processing resources on the routers
   performing compression and decompression, and these processing
   resources might then be unavailable for other important functions on
   the router. This threat is not a new threat for HC, but is addressed
   and mitigated by HC over MPLS.  That is, by reducing the need for
   performing compression and decompression cycles, as proposed in this
   draft, the risk of this type of denial-of-service attack is reduced.

7. IANA Considerations

   No IANA actions are required.

8. Normative References

   [cRTP] Casner, S., Jacobsen, V., "Compressing IP/UDP/RTP Headers for
   Low-Speed Serial Links", RFC 2508, February 1999.

   [cRTP-ENCAP] Engan, M., Casner, S., Bormann, C., "IP Header
   Compression over PPP", RFC 2509, February 1999.

   [ECRTP] Koren, T., et. al., "Compressing IP/UDP/RTP Headers on Links
   with High Delay, Packet Loss, and Reordering," RFC 3545, July 2003.

   [KEY] Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Levels", RFC 2119, March 1997.

   [LDP] Andersson, L., et. al., "LDP Specification", RFC 3036, January

   [MPLS-ARCH] Rosen, E., et. al., "Multiprotocol Label Switching
   Architecture," RFC 3031, January 2001.

   [ROHC] Bormann, C., et. al., "Robust Header Compression (ROHC)," RFC
   3091, July 2001.

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

   [RSVP] Braden, R. et al., "Resource ReSerVation Protocol (RSVP) --
   Version 1, Functional Specification", RFC 2205, September 1997.

   [RSVP-TE] Awduche, D., et. al., "RSVP-TE: Extensions to RSVP for LSP
   Tunnels", RFC 3209, December 2001.

9. Informative References

   [ECRTP-MPLS-PROTO] Ash, G., Goode, B., Hand, J., "Protocol Extensions
   for Header Compression over MPLS", work in progress.

   [GVPLS] Radoaca, V., et. al., "GVPLS/LPE - Generalized VPLS Solution
   based on LPE Framework," work in progress.

   [LDP-PWE3] Martini, L., et. al., "Pseudowire Setup and Maintenance
   using LDP", work in progress.

   [MPLS-ENCAP] Rosen, E., et. al., "MPLS Label Stack Encoding", RFC
   3032, January 2001.

   [MPLS-VPN] Rosen, E., Rekhter, Y., "BGP/MPLS VPNs", RFC 2547, March

10. Authors' Addresses

   Jerry Ash
   Room MT D5-2A01
   200 Laurel Avenue
   Middletown, NJ 07748, USA
   Phone: +1 732-420-4578
   Email: gash@att.com

   Bur Goode
   Phone: + 1 203-341-8705
   E-mail: bgoode@att.com

   Jim Hand
   E-mail: hand17@earthlink.net

   Raymond Zhang
   Infonet Services Corporation
   2160 E. Grand Ave. El Segundo, CA 90025 USA
   Email: zhangr@sa.infonet.com

11. Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights. Information
   on the procedures with respect to rights in RFC documents can be

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Internet Draft  Requirements for Header Compression over MPLS   June 2004

   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard. Please address the information to the IETF at

12. Full Copyright Statement

   Copyright (C) The Internet Society (2004). This document is subject
   to the rights, licenses and restrictions contained in BCP 78 and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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