One document matched: draft-ietf-avt-hc-mpls-reqs-03.txt
Differences from draft-ietf-avt-hc-mpls-reqs-02.txt
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
Abstract:
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 . . . . . . . . . . . . . . . . . . . . . . . . . . 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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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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_____
| |
|R1/HC| Header Compression (HC) Performed
|_____|
|
| voice/compressed-header/MPLS-labels
V
_____
| |
| R2 |
|_____|
|
| voice/compressed-header/MPLS-labels
V
_____
| |
| R3 |
|_____|
|
| voice/compressed-header/MPLS-labels
V
_____
| |
|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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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",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
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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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
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 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
2001.
[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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[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
1999.
10. Authors' Addresses
Jerry Ash
AT&T
Room MT D5-2A01
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: +1 732-420-4578
Email: gash@att.com
Bur Goode
AT&T
Phone: + 1 203-341-8705
E-mail: bgoode@att.com
Jim Hand
Consultant
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
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
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rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
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
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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