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

                                                         Raymond Zhang
                                          Infonet Services Corporation

                                                           April, 2004

            Requirements for Header Compression over MPLS

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Copyright (C) The Internet Society (2004). All Rights Reserved.

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Internet Draft  Requirements for Header Compression over MPLS   April 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 scenario.

Table of Contents:

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

Specification of Requirements

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

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

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Internet Draft  Requirements for Header Compression over MPLS   April 2004

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.
                   |     |
                   |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,

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Internet Draft  Requirements for Header Compression over MPLS   April 2004

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

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

3. Goals & Requirements

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

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   April 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]).

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 solution.

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

[ECRTP] minimizes feedback based error recovery using CONTEXT_STATE
packets to make cRTP more tolerant of packet loss, and minimize the need
to use the cRTP error recovery mechanism. ECRTP is therefore a candidate
method to make HC over MPLS more tolerant of packet loss and to guard
against frequent resynchronizations.  ECRTP also is able to accommodate
out-of-sequence packets.  ECRTP protocol extensions may be required to
identify FULL_HEADER, CONTEXT_STATE, and compressed packet types.
[cRTP-ENCAP] specifies a separate link-layer packet 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.

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Internet Draft  Requirements for Header Compression over MPLS   April 2004

[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.  However, ROHC currently does not accommodate packet reordering
needed to protect against out-of-sequence packets that can occur on MPLS
LSPs, and would need to be extended to do so.  On the other hand, ROHC
already has the capability to identify the packet type in the
compression header, so no further extension is needed to identify packet

Extensions to MPLS signaling are 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 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 would 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.

     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

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Internet Draft  Requirements for Header Compression over MPLS   April 2004

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.

[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.

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Internet Draft  Requirements for Header Compression over MPLS   April 2004

[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

Bur Goode
Phone: + 1 203-341-8705

Jim Hand

Raymond Zhang
Infonet Services Corporation
2160 E. Grand Ave. El Segundo, CA 90025 USA

11. 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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