Transport Area Working Group J. Saldana
Internet-Draft University of Zaragoza
Obsoletes: 4170 (if approved) D. Wing
Intended status: Best Current Practice Cisco Systems
Expires: June 15, 2014 J. Fernandez Navajas
University of Zaragoza
Muthu. Perumal
Cisco Systems
F. Pascual Blanco
Telefonica I+D
December 12, 2013
Tunneling Compressed Multiplexed Traffic Flows (TCM-TF) Reference Model
draft-saldana-tsvwg-tcmtf-06
Abstract
Tunneling Compressed and Multiplexed Traffic Flows (TCM-TF) is a
method for improving the bandwidth utilization of network segments
that carry multiple flows in parallel sharing a common path. The
method combines standard protocols for header compression,
multiplexing, and tunneling over a network path for the purpose of
reducing the bandwidth used when multiple flows are carried over that
path. The amount of packets per second can also be reduced.
This document describes the TCM-TF framework and the different
options which can be used for each layer (header compression,
multiplexing and tunneling).
Status of This Memo
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This Internet-Draft will expire on June 15, 2014.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Bandwidth efficiency of flows sending small packets . . . 3
1.2.1. Real-time applications using RTP . . . . . . . . . . 3
1.2.2. Real-time applications not using RTP . . . . . . . . 4
1.2.3. Other applications generating small packets . . . . . 4
1.2.4. Optimization of small-packet flows . . . . . . . . . 4
1.2.5. Energy consumption considerations . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.4. Scenarios of application . . . . . . . . . . . . . . . . 7
1.4.1. Multidomain scenario . . . . . . . . . . . . . . . . 7
1.4.2. Single domain . . . . . . . . . . . . . . . . . . . . 8
1.4.3. Private solutions . . . . . . . . . . . . . . . . . . 9
1.4.4. Mixed scenarios . . . . . . . . . . . . . . . . . . . 11
1.5. Potential beneficiaries of TCM optimization . . . . . . . 11
1.6. Current Standard . . . . . . . . . . . . . . . . . . . . 12
1.7. Improved Standard Proposal . . . . . . . . . . . . . . . 13
2. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 14
2.1. Models of implementation . . . . . . . . . . . . . . . . 14
2.2. Choice of the compressing protocol . . . . . . . . . . . 15
2.2.1. Context Synchronization in ECRTP . . . . . . . . . . 16
2.2.2. Context Synchronization in ROHC . . . . . . . . . . . 17
2.3. Multiplexing . . . . . . . . . . . . . . . . . . . . . . 17
2.4. Tunneling . . . . . . . . . . . . . . . . . . . . . . . . 18
2.4.1. Tunneling schemes over IP: L2TP and GRE . . . . . . . 18
2.4.2. MPLS tunneling . . . . . . . . . . . . . . . . . . . 18
2.5. Encapsulation Formats . . . . . . . . . . . . . . . . . . 18
3. Contributing Authors . . . . . . . . . . . . . . . . . . . . 20
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
6. Security Considerations . . . . . . . . . . . . . . . . . . . 21
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.1. Normative References . . . . . . . . . . . . . . . . . . 22
7.2. Informative References . . . . . . . . . . . . . . . . . 23
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Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
1. Introduction
This document describes a way to combine existing protocols for
header compression, multiplexing and tunneling to save bandwidth for
applications that generate long-term flows of small packets.
1.1. Requirements Language
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 [RFC2119].
1.2. Bandwidth efficiency of flows sending small packets
The interactivity demands of some real-time services (VoIP,
videoconferencing, telemedicine, video vigilance, online gaming,
etc.) require a traffic profile consisting of high rates of small
packets, which are necessary in order to transmit frequent updates
between the two extremes of the communication. These services also
demand low network delays. In addition, some other services also use
small packets, although they are not delay-sensitive (e.g., instant
messaging, M2M packets sending collected data in sensor networks
using wireless or satellite scenarios). For both the delay-sensitive
and delay-insensitive applications, their small data payloads incur
significant overhead.
When a number of flows based on small packets (small-packet flows)
share the same path, bandwidth can be saved by multiplexing packets
belonging to different flows. If a transmission queue has not
already been formed but multiplexing is desired, it is necessary to
add a multiplexing delay, which has to be maintained under some
threshold if the service presents tight delay requirements.
1.2.1. Real-time applications using RTP
The first design of the Internet did not include any mechanism
capable of guaranteeing an upper bound for delivery delay, taking
into account that the first deployed services were e-mail, file
transfer, etc., in which delay is not critical. RTP [RTP] was first
defined in 1996 in order to permit the delivery of real-time
contents. Nowadays, although there are a variety of protocols used
for signaling real-time flows (SIP [SIP], H.323, etc.), RTP has
become the standard par excellence for the delivery of real-time
content.
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RTP was designed to work over UDP datagrams. This implies that an
IPv4 packet carrying real-time information has to include 40 bytes of
headers: 20 for IPv4 header, 8 for UDP, and 12 for RTP. This
overhead is significant, taking into account that many real-time
services send very small payloads. It becomes even more significant
with IPv6 packets, as the basic IPv6 header is twice the size of the
IPv4 header. Table 1 illustrates the overhead problem of VoIP for
two different codecs.
+---------------------------------+---------------------------------+
| IPv4 | IPv6 |
+---------------------------------+---------------------------------+
| IPv4+UDP+RTP: 40 bytes header | IPv6+UDP+RTP: 60 bytes header |
| G.711 at 20 ms packetization: | G.711 at 20 ms packetization: |
| 25% header overhead | 37.5% header overhead |
| G.729 at 20 ms packetization: | G.729 at 20 ms packetization: |
| 200% header overhead | 300% header overhead |
+---------------------------------+---------------------------------+
Table 1: Efficiency of different voice codecs
1.2.2. Real-time applications not using RTP
At the same time, there are many real-time applications that do not
use RTP. Some of them send UDP (but not RTP) packets, e.g., First
Person Shooter (FPS) online games [First-person], for which latency
is very critical. The quickness and the movements of the players are
important, and can decide if they win or lose a fight.
1.2.3. Other applications generating small packets
Other applications without delay constraints are also becoming
popular (e.g., instant messaging, M2M packets sending collected data
in sensor networks using wireless or satellite scenarios). The
number of wireless M2M (machine-to-machine) connections is steady
growing since a few years, and a share of these is being used for
delay-intolerant applications, e.g., industrial SCADA (Supervisory
Control And Data Acquisition), power plant monitoring, smart grids,
asset tracking.
1.2.4. Optimization of small-packet flows
In the moments or places where network capacity gets scarce,
allocating more bandwidth is a possible solution, but it implies a
recurring cost. However, including optimization techniques between a
pair of network nodes (reducing bandwidth and packets per second)
when/where required is a one-time investment.
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Thus, in scenarios including a bottleneck with a single Layer-3 hop,
header compression standard algorithms [cRTP], [ECRTP], [IPHC],
[ROHC] can be used for reducing the overhead of each flow, at the
cost of additional processing.
However, if header compression is to be deployed in a network path
including several Layer-3 hops, tunneling can be used in order to
allow the header-compressed packets to travel end-to-end, thus
avoiding the need to compress and decompress at each intermediate
node. In these cases, compressed packets belonging to different
flows can be multiplexed together, in order to share the tunnel
overhead. In this case, a small multiplexing delay will be necessary
as a counterpart, in order to join a number of packets to be sent
together. This delay has to be maintained under a threshold in order
to grant the delay requirements.
A demultiplexer is necessary at the end of the common path, so as to
rebuild the packets as they were originally sent, making multiplexing
a transparent process for the extremes of the flow.
If only one stream is tunneled and compressed, then little bandwidth
savings will be obtained. In contrast, multiplexing is helpful to
amortize the overhead of the tunnel header over many payloads. The
obtained savings grow with the number of flows optimized together
[VoIP_opt], [FPS_opt].
1.2.5. Energy consumption considerations
As an additional benefit, the reduction of the sent information, and
especially the reduction of the amount of packets per second to be
managed by the intermediate routers, can be translated into a
reduction of the overall energy consumption of network equipment.
According to [Efficiency] internal packet processing engines and
switching fabric require 60% and 18% of the power consumption of
high-end routers respectively. Thus, reducing the number of packets
to be managed and switched will reduce the overall energy
consumption. The measurements deployed in [Power] on commercial
routers corroborate this: a study using different packet sizes was
presented, and the tests with big packets made the energy consumption
get reduced, since a certain amount of energy is associated to header
processing tasks, and not only to the sending of the packet itself.
All in all, a tradeoff appears: on the one hand, energy consumption
is increased in the two extremes due to header compression
processing; on the other hand, energy consumption is reduced in the
intermediate nodes because of the reduction in the number of packets
transmitted. Thi tradeoff should be explored more deeply.
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1.3. Terminology
This document uses a number of terms to refer to the roles played by
the entities using TCM-TF.
o native packet
A packet sent by an application, belonging to a flow that can be
optimized by means of TCM-TF.
o native flow
A flow of native packets. It can be considered a "small-packet flow"
when the vast majority of the generated packets present a low
payload-to-header ratio.
o TCM packet
A packet including a number of multiplexed and header-compressed
native ones, and also a tunneling header.
o TCM flow
A flow of TCM packets, including a number of optimized native flows.
o TCM optimizer
The host where TCM optimization is deployed. It corresponds to both
the ingress and the egress of the tunnel transporting the compressed
and multiplexed packets.
If the optimizer compresses headers, multiplexes packets and creates
the tunnel, it behaves as a "TCM-ingress optimizer", or "TCM-IO". It
takes native packets or flows and "optimizes" them.
If it extracts packets from the tunnel, demultiplexes packets and
decompresses headers, it behaves as a "TCM-egress optimizer", or
"TCM-EO". The TCM-egress optimizer takes a TCM flow and "rebuilds"
the native packets as they were originally sent.
o TCM-TF session
The relationship between a pair of TCM optimizers exchanging TCM
packets.
o policy manager
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A network entity which makes the decisions about TCM-TF parameters:
multiplexing period to be used, flows to be optimized together,
depending on their IP addresses, ports, etc. It is connected with a
number of TCM-TF optimizers, and orchestrates the optimization that
takes place between them.
1.4. Scenarios of application
Different scenarios of application can be considered for the
tunneling, compressing and multiplexing solution. They can be
classified according to the domains involved in the optimization:
1.4.1. Multidomain scenario
In this scenario, the TCMT-TF tunnel goes all the way from one
network edge (the place where users are attached to the ISP) to
another, and therefore it can cross several domains. As shown in
Figure 1, the optimization is performed before the packets leave the
domain of an ISP; the traffic crosses the Internet tunnelized, and
the packets are rebuilt in the second domain.
_ _ _ _ _ _
( ` ) _ _ _ ( ` )_ _
( +------+ )`) ( ` )_ ( +------+ `)
-->(_ -|TCM-IO|--- _) ---> ( ) `) ----->(_-|TCM-EO|--_)-->
( +------+ _) (_ (_ . _) _) ( +------+ _)
(_ _ _ _) (_ _ ( _) _)
ISP 1 Internet ISP 2
<------------------TCM-TF-------------------->
Figure 1
Note that this is not from border to border (where ISPs connect to
the Internet, which could be covered with specialized links) but from
an ISP to another (e.g. managing all traffic from individual users
arriving at a Game Provider, regardless users' location).
Some examples of this could be:
o An ISP could place a TCM optimizer in its aggregation network, in
order to tunnel all the packets of a service, sending them to the
application provider, who would rebuild the packets before
forwarding them to the application server. This would result in
savings for both actors.
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o A service provider (e.g., an online gaming company) could be
allowed to place a TCM optimizer in the aggregation network of an
ISP, being able to optimize all the flows of a game or service.
Another TCM optimizer would rebuild these packets once they arrive
to the network of the provider.
1.4.2. Single domain
TCM-TF is only activated inside an ISP, from the edge to border,
inside the network operator. The geographical scope and network
depth of TCM-TF activation could be on demand, according to traffic
conditions.
If we consider the residential users of a real-time interactive
application (e.g., VoIP, an online game generating small packets) in
a town or a district, a TCM optimizing module can be included in
network devices, in order to group packets with the same destination.
As shown in Figure 2, depending on the number of users of the
application, the packets could be grouped at different levels in DSL
fixed network scenarios, at gateway level in LTE mobile network
scenarios or even in other ISP edge routers. TCM-TF may also be
applied for fiber residential accesses, and in 2G/3G mobile networks.
This would reduce bandwidth requirements in the provider aggregation
network
+------+
N users -|TCM-IO|\
+------+ \
\ _ _ _ _
+------+ \--> ( ` )_ +------+ ( ` )_
M users -|TCM-IO|------> ( ) `) --|TCM-EO|--> ( ) `)
+------+ / ->(_ (_ . _) _) +------+ (_ (_ . _) _)
/
+------+ / ISP Internet
P users -|TCM-IO|/
+------+
<------------TCM-TF-------------->
Figure 2
At the same time, the ISP would implement TCM-TF capabilities within
its own MPLS network in order to optimize internal network resources:
optimizing modules could be embedded in the Label Edge Routers of the
network. In that scenario MPLS would be the "tunneling" layer, being
the tunnels the paths defined by the MPLS labels and avoiding the use
of other tunneling protocols.
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Finally, some networks use cRTP [cRTP] in order to obtain bandwidth
savings on the access link, but as a counterpart it consumes
considerable CPU resources on the aggregation router. In these
cases, by means of TCM, instead of only saving bandwidth on the
access link, it could also be saved across the ISP network, without
the CPU impact on the aggregation router.
1.4.3. Private solutions
End users can also optimize traffic end-to-end from network borders.
TCM-TF is used to connect private networks geographically apart (e.g.
corporation headquarters and subsidiaries), without the ISP being
aware (or having to manage) those flows, as shown in Figure 3, where
two different locations are connected through a tunnel traversing the
Internet or another network.
_ _ _ _ _ _
( ` )_ +------+ ( ` )_ +------+ ( ` )_
( ) `) --|TCM-IO|-->( ) `) --|TCM-EO|-->( ) `)
(_ (_ . _) _) +------+ (_ (_ . _) _) +------+ (_ (_ . _)_)
Location 1 ISP/Internet Location 2
<-----------TCM-TF---------->
Figure 3
Some examples of these scenarios:
o The case of an enterprise with a number of distributed central
offices, in which an appliance could be placed next to the access
router, being able to optimize traffic flows with a shared origin
and destination. Thus, a number of remote desktop sessions to the
same server could be optimized, or a number of VoIP calls between
two offices could also require less bandwidth and fewer packets
per second. In some cases the tunnel is already included for
security reasons, so the additional overhead of TCM-TF is lower.
o An Internet cafe, which is suitable of having many users of the
same application (e.g., VoIP, a game) sharing the same access
link. Internet cafes are very popular in countries with
relatively low access speeds in households, where home computer
penetration is usually low as well. In many of these countries,
bandwidth can become a serious limitation for this kind of
business, so TCM-TF savings may become interesting for their
viability.
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o Community networks [topology_CNs] (typically deployed in rural
areas or in developing countries), in which a number of people in
the same geographical place share their connections in a
cooperative way, and a number of wireless hops are required in
order to reach a router connected to the Internet.
o Satellite communication links that often manage the bandwidth by
limiting the transmission rate, measured in packets per second
(pps), to and from the satellite. Applications like VoIP that
generate a large number of small packets can easily fill the
maximum number of pps slots, limiting the throughput across such
links. As an example, a G.729a voice call generates 50 pps at 20
ms packetization time. If the satellite transmission allows 1,500
pps, the number of simultaneous voice calls is limited to 30.
This results in poor utilization of the satellite link's bandwidth
as well as places a low bound on the number of voice calls that
can utilize the link simultaneously. TCM optimization of small
packets into one packet for transmission would improve the
efficiency.
o In a M2M/SCADA (Supervisory Control And Data Acquisition) context,
TCM optimization can be applied when a satellite link is used for
collecting the data of a number of sensors. M2M terminals are
normally equipped with sensing devices which can interface to
proximity sensor networks through wireless connections. The
terminal can send the collected sensing data using a satellite
link connecting to a satellite gateway, which in turn will forward
the M2M/SCADA data to the to the processing and control center
through Internet. The size of typical M2M application transaction
depends on the specific service and it may vary from a minimum of
20 bytes (e.g., tracking and metering in private security) to
about 1,000 bytes (e.g., video-surveillance). In this context,
TCM-TF concepts can be also applied to allow a more efficient use
of the available satellite link capacity, matching the
requirements demanded by some M2M services. If the case of large
sensor deployments is considered, where proximity sensor networks
transmit data through different satellite terminals, the use of
compression algorithms already available in current satellite
systems to reduce the overhead introduced by UDP and IPv6
protocols is certainly desirable. In addition to this, tunneling
and multiplexing functions available from TCM-TF allows extending
compression functionality throughout the rest the network, to
eventually reach the processing and control centers.
o Desktop or application sharing where the traffic from the server
to the client typically consists of the delta of screen updates.
Also, the standard for remote desktop sharing emerging for WebRTC
in the RTCWEB Working Group is: {something}/SCTP/UDP (Stream
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Control Transmission Protocol [SCTP]). In this scenario, SCTP/UDP
could be used in other cases: chatting, file sharing and
applications related to WebRTC peers. There could be hundreds of
clients at a site talking to a server located at a datacenter over
a WAN. Compressing, multiplexing and tunneling this traffic could
save WAN bandwidth and potentially improve latency.
1.4.4. Mixed scenarios
Different combinations of the previous scenarios can be considered.
Agreements between different companies can be established in order to
save bandwidth and to reduce packets per second. As an example,
Figure 4 shows a game provider that wants to TCM-optimize its
connections by establishing associations between different TCM-IO/EOs
placed in the game server and several TCM-IO/EOs placed in the
networks of different ISPs (agreements between the game provider and
each ISP would be necessary). In every ISP, the TCM-IO/EO would be
placed in the most adequate point (actually several TCM-IO/EOs could
exist per ISP) in order to aggregate enough number of users.
_ _
N users ( ` )_
+---+ ( ) `)
|TCM|->(_ (_ . _)
+---+ ISP 1 \
_ _ \ _ _ _ _ _
M users ( ` )_ \ ( ` ) ( ` ) ( ` )
+---+ ( ) `) \ ( ) `) ( ) `) +---+ ( ) `)
|TCM|->(_ (_ ._)---- (_ (_ . _) ->(_ (_ . _)->|TCM|->(_ (_ . _)
+---+ ISP 2 / Internet ISP 4 +---+ Game Provider
_ _ / ^
O users ( ` )_ / |
+---+ ( ) `) / +---+
|TCM|->(_ (_ ._) P users->|TCM|
+---+ ISP 3 +---+
Figure 4
1.5. Potential beneficiaries of TCM optimization
In conclusion, a standard able to compress headers, multiplex a
number of packets and send them together using a tunnel, can benefit
various stakeholders:
o network operators can compress traffic flows sharing a common
network segment;
o ISPs;
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o developers of VoIP systems can include this option in their
solutions;
o service providers, who can achieve bandwidth savings in their
supporting infrastructures;
o users of Community Networks, who may be able to save significant
bandwidth amounts, and to reduce the number of packets per second
in their networks.
Other fact that has to be taken into account is that the technique
not only saves bandwidth but also reduces the number of packets per
second, which sometimes can be a bottleneck for a satellite link or
even for a network router.
1.6. Current Standard
The current standard [TCRTP] defines a way to reduce bandwidth and
pps of RTP traffic, by combining three different standard protocols:
o Regarding compression, [ECRTP] is the selected option.
o Multiplexing is accomplished using PPP Multiplexing [PPP-MUX]
o Tunneling is accomplished by using L2TP (Layer 2 Tunneling
Protocol [L2TPv3]).
The three layers are combined as shown in the Figure 5:
RTP/UDP/IP
|
| ----------------------------
|
ECRTP compressing layer
|
| ----------------------------
|
PPPMUX multiplexing layer
|
| ----------------------------
|
L2TP tunneling layer
|
| ----------------------------
|
IP
Figure 5
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1.7. Improved Standard Proposal
In contrast to the current standard [TCRTP], TCM-TF allows other
header compression protocols in addition to RTP/UDP, since services
based on small packets also use by bare UDP, as shown in Figure 6:
UDP/IP RTP/UDP/IP
\ /
\ / ------------------------------
\ /
Nothing or ROHC or ECRTP or IPHC header compressing layer
|
| ------------------------------
|
PPPMUX or other mux protocols multiplexing layer
|
/ \ ------------------------------
/ \
/ \
GRE or L2TP \ tunneling layer
| MPLS
| ------------------------------
IP
Figure 6
Each of the three layers is considered as independent of the other
two, i.e. different combinations of protocols can be implemented
according to the new proposal:
o Regarding compression, a number of options can be considered: as
different standards are able to compress different headers
([cRTP], [ECRTP], [IPHC], [ROHC]). The one to be used could be
selected depending on the traffic to compress and the concrete
scenario (packet loss percentage, delay, etc.). It also exists
the possibility of having a null header compression, in the case
of wanting to avoid traffic compression, taking into account the
need of storing a context for every flow and the problems of
context desynchronization in certain scenarios. Although non
shown in Figure 6, ESP (Encapsulating Security Payload [ESP])
headers can also be compressed.
o Multiplexing is also accomplished using PPP Multiplexing
[PPP-MUX]. Nevertheless, other multiplexing protocols can also be
considered.
o Tunneling is accomplished by using L2TP (Layer 2 Tunneling
Protocol [L2TPv3]) over IP, GRE (Generic Routing Encapsulation
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[GRE]) over IP, or MPLS (Multiprotocol Label Switching
Architecture [MPLS]).
It can be observed that TCRTP [TCRTP] is included as an option in
TCM-TF, combining [ECRTP], [PPP-MUX] and [L2TPv3].
If a single link is being optimized a tunnel is unnecessary. In that
case, both optimizers can perform header compression between both of
them. Multiplexing may still be useful, since it reduces packets per
second which is interesting in some environments (e.g., satellite).
Another reason for that is the desire of reducing energy consumption.
Although no tunnel is employed, this can still be considered as TCM-
TF optimization, so TCM-TF signaling protocols can be employed here
in order to negotiate the compression and multiplexing parameters to
be employed.
Payload compression schemes could also be used, but they are not the
aim of this document.
2. Protocol Operation
This section describes how to combine protocols belonging to trhee
layers (compressing, multiplexing, and tunneling), in order to save
bandwidth for the considered flows.
2.1. Models of implementation
TCM-TF can be implemented in different ways. The most
straightforward is to implement it in the devices terminating the
flows (these devices can be e.g., voice gateways, or proxies grouping
a number of flows):
[ending device]---[ending device]
^
|
TCM-TF over IP
Figure 7
Another way TCM-TF can be implemented is with an external optimizer.
This device could be placed at strategic places in the network and
could dynamically create and destroy TCM-TF sessions without the
participation of the endpoints that generate the flows (Figure 8).
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[ending device]\ /[ending device]
[ending device]----[optimizer]-----[optimizer]------[ending device]
[ending device]/ \[ending device]
^ ^ ^
| | |
Native IP TCM-TF over IP Native IP
Figure 8
A number of already compressed flows can also be merged in a tunnel
using an optimizer in order to increase the number of flows in a
tunnel (Figure 9):
[ending device]\ /[ending device]
[ending device]----[optimizer]------[optimizer]-----[ending device]
[ending device]/ \[ending device]
^ ^ ^
| | |
Compressed TCM-TF over IP Compressed
Figure 9
2.2. Choice of the compressing protocol
There are different protocols that can be used for compressing IP
flows:
o IPHC (IP Header Compression [IPHC]) permits the compression of UDP
/IP and ESP/IP headers. It has a low implementation complexity.
On the other hand, the resynchronization of the context can be
slow over long RTT links. It should be used in scenarios
presenting very low packet loss percentage.
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o cRTP (compressed RTP [cRTP]) works the same way as IPHC, but is
also able to compress RTP headers. The link layer transport is
not specified, but typically PPP is used. For cRTP to compress
headers, it must be implemented on each PPP link. A lot of
context is required to successfully run cRTP, and memory and
processing requirements are high, especially if multiple hops must
implement cRTP to save bandwidth on each of the hops. At higher
line rates, cRTP's processor consumption becomes prohibitively
expensive. cRTP is not suitable over long-delay WAN links commonly
used when tunneling, as proposed by this document. To avoid the
per-hop expense of cRTP, a simplistic solution is to use cRTP with
L2TP to achieve end-to-end cRTP. However, cRTP is only suitable
for links with low delay and low loss. Thus, if multiple router
hops are involved, cRTP's expectation of low delay and low loss
can no longer be met. Furthermore, packets can arrive out of
order.
o ECRTP (Enhanced Compressed RTP [ECRTP]) is an extension of cRTP
[cRTP] that provides tolerance to packet loss and packet
reordering between compressor and decompressor. Thus, ECRTP
should be used instead of cRTP when possible (e.g., the two TCM
optimizers implementing ECRTP).
o ROHC (RObust Header Compression [ROHC]) is able to compress UDP/
IP, ESP/IP and RTP/UDP/IP headers. It is a robust scheme
developed for header compression over links with high bit error
rate, such as wireless ones. It incorporates mechanisms for quick
resynchronization of the context. It includes an improved
encoding scheme for compressing the header fields that change
dynamically. Its main drawback is that it requires significantly
more processing and memory resources than the ones necessary for
IPHC or ECRTP.
This standard does not determine which of the existing protocols has
to be used for the compressing layer. The decision will depend on
the scenario, and will mainly be determined by the packet loss
probability, RTT, and the availability of memory and processing
resources. The standard is also suitable to include other
compressing schemes that may be further developed.
2.2.1. Context Synchronization in ECRTP
When the compressor receives an RTP packet that has an unpredicted
change in the RTP header, the compressor should send a COMPRESSED_UDP
packet (described in [ECRTP]) to synchronize the ECRTP decompressor
state. The COMPRESSED_UDP packet updates the RTP context in the
decompressor.
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To ensure delivery of updates of context variables, COMPRESSED_UDP
packets should be delivered using the robust operation described in
[ECRTP].
Because the "twice" algorithm described in [ECRTP] relies on UDP
checksums, the IP stack on the RTP transmitter should transmit UDP
checksums. If UDP checksums are not used, the ECRTP compressor
should use the cRTP Header checksum described in [ECRTP].
2.2.2. Context Synchronization in ROHC
ROHC [ROHC] includes a more complex mechanism in order to maintain
context synchronization. It has different operation modes and
defines compressor states which change depending on link behavior.
2.3. Multiplexing
Header compressing algorithms require a layer two protocol that
allows identifying different protocols. PPP [PPP] is suited for
this, although other multiplexing protocols can also be used for this
layer of TCM-TF.
When header compression is used inside a tunnel, it reduces the size
of the headers of the IP packets carried in the tunnel. However, the
tunnel itself has overhead due to its IP header and the tunnel header
(the information necessary to identify the tunneled payload).
By multiplexing multiple small payloads in a single tunneled packet,
reasonable bandwidth efficiency can be achieved, since the tunnel
overhead is shared by multiple packets belonging to the flows active
between the source and destination of an L2TP tunnel. The packet
size of the flows has to be small in order to permit good bandwidth
savings.
If the source and destination of the tunnel are the same as the
source and destination of the compressing protocol sessions, then the
source and destination must have multiple active small-packet flows
to get any benefit from multiplexing.
Because of this, TCM-TF is mostly useful for applications where many
small-packet flows run between a pair of hosts. The number of
simultaneous sessions required to reduce the header overhead to the
desired level depends on the average payload size, and also on the
size of the tunnel header. A smaller tunnel header will result in
fewer simultaneous sessions being required to produce adequate
bandwidth efficiencies.
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2.4. Tunneling
Different tunneling schemes can be used for sending end to end the
compressed payloads.
2.4.1. Tunneling schemes over IP: L2TP and GRE
L2TP tunnels should be used to tunnel the compressed payloads end to
end. L2TP includes methods for tunneling messages used in PPP
session establishment, such as NCP (Network Control Protocol). This
allows [IPCP-HC] to negotiate ECRTP compression/decompression
parameters.
Other tunneling schemes, such as GRE [GRE] may also be used to
implement the tunneling layer of TCM-TF.
2.4.2. MPLS tunneling
In some scenarios, mainly in operator's core networks, the use of
MPLS is widely deployed as data transport method. The adoption of
MPLS as tunneling layer in this proposal intends to natively adapt
TCM-TF to those transport networks.
In the same way that layer 3 tunnels, MPLS paths, identified by MPLS
labels, established between Label Edge Routers (LSRs), could be used
to transport the compressed payloads within an MPLS network. This
way, multiplexing layer must be placed over MPLS layer. Note that,
in this case, layer 3 tunnel headers do not have to be used, with the
consequent data efficiency improvement.
2.5. Encapsulation Formats
The packet format for a packet compressed is:
+------------+-----------------------+
| | |
| Compr | |
| Header | Data |
| | |
| | |
+------------+-----------------------+
Figure 10
The packet format of a multiplexed PPP packet as defined by [PPP-MUX]
is:
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+-------+---+------+-------+-----+ +---+------+-------+-----+
| Mux |P L| | | | |P L| | | |
| PPP |F X|Len1 | PPP | | |F X|LenN | PPP | |
| Prot. |F T| | Prot. |Info1| ~ |F T| | Prot. |InfoN|
| Field | | Field1| | | |FieldN | |
| (1) |1-2 octets| (0-2) | | |1-2 octets| (0-2) | |
+-------+----------+-------+-----+ +----------+-------+-----+
Figure 11
The combined format used for TCM-TF with a single payload is all of
the above packets concatenated. Here is an example with one payload,
using L2TP or GRE tunneling:
+------+------+-------+----------+-------+--------+----+
| IP |Tunnel| Mux |P L| | | | |
|header|header| PPP |F X|Len1 | PPP | Compr | |
| (20) | | Proto |F T| | Proto | header |Data|
| | | Field | | Field1| | |
| | | (1) |1-2 octets| (0-2) | | |
+------+------+-------+----------+-------+--------+----+
|<------------- IP payload -------------------->|
|<-------- Mux payload --------->|
Figure 12
If the tunneling technology is MPLS, then the scheme would be:
+------+-------+----------+-------+--------+----+
|MPLS | Mux |P L| | | | |
|header| PPP |F X|Len1 | PPP | Compr | |
| | Proto |F T| | Proto | header |Data|
| | Field | | Field1| | |
| | (1) |1-2 octets| (0-2) | | |
-+------+-------+----------+-------+--------+----+
|<---------- MPLS payload -------------->|
|<-------- Mux payload --------->|
Figure 13
If the tunnel contains multiplexed traffic, multiple "PPPMux
payload"s are transmitted in one IP packet.
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3. Contributing Authors
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
Email: Gonzalo.Camarillo@ericsson.com
Michael A. Ramalho
Cisco Systems, Inc.
8000 Hawkins Road
Sarasota, FL 34241-9300
US
Phone: +1.732.832.9723
Email: mramalho@cisco.com
Jose Ruiz Mas
University of Zaragoza
Dpt. IEC Ada Byron Building
50018 Zaragoza
Spain
Phone: +34 976762158
Email: jruiz@unizar.es
Diego Lopez Garcia
Telefonica I+D
Ramon de la cruz 84
28006 Madrid
Spain
Phone: +34 913129041
Email: diego@tid.es
David Florez Rodriguez
Telefonica I+D
Ramon de la cruz 84
28006 Madrid
Spain
Phone: +34 91312884
Email: dflorez@tid.es
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Manuel Nunez Sanz
Telefonica I+D
Ramon de la cruz 84
28006 Madrid
Spain
Phone: +34 913128821
Email: mns@tid.es
Juan Antonio Castell Lucia
Telefonica I+D
Ramon de la cruz 84
28006 Madrid
Spain
Phone: +34 913129157
Email: jacl@tid.es
Mirko Suznjevic
University of Zagreb
Faculty of Electrical Engineering and Computing, Unska 3
10000 Zagreb
Croatia
Phone: +385 1 6129 755
Email: mirko.suznjevic@fer.hr
4. Acknowledgements
5. IANA Considerations
This memo includes no request to IANA.
6. Security Considerations
The most straightforward option for securing a number of non-secured
flows sharing a path is by the use of IPsec [IPsec], when TCM using
an IP tunnel is employed. Instead of adding a security header to the
packets of each native flow, and then compressing and multiplexing
them, a single IPsec tunnel can be used in order to secure all the
flows together, thus achieving a higher efficiency. This use of
IPsec protects the packets only within the transport network between
tunnel ingress and egress and therefore does not provide end-to-end
authentication or encryption.
When a number of already secured flows including ESP [ESP] headers
are optimized by means of TCM, and the addition of further security
is not necessary, their ESP/IP headers can still be compressed using
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suitable algorithms [RFC5225], in order to improve the efficiency.
This header compression does not change the end-to-end security
model.
The resilience of TCM-TF to denial of service, and the use of TCM-TF
to deny service to other parts of the network infrastructure, is for
future study.
7. References
7.1. Normative References
[ECRTP] Koren, T., Casner, S., Geevarghese, J., Thompson, B., and
P. Ruddy, "Enhanced Compressed RTP (CRTP) for Links with
High Delay, Packet Loss and Reordering", RFC 3545, 2003.
[ESP] Kent, S., "IP Encapsulating Security Payload", RFC 4303,
2005.
[GRE] Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
2000.
[IPCP-HC] Engan, M., Casner, S., Bormann, C., and T. Koren, "IP
Header Compression over PPP", RFC 3544, 2003.
[IPHC] Degermark, M., Nordgren, B., and S. Pink, "IP Header
Compression", RFC 2580, 1999.
[IPsec] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[L2TPv3] Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
Protocol - Version 3 (L2TPv3)", RFC 3931, 2005.
[MPLS] Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
Label Switching Architecture", RFC 3031, January 2001.
[PPP-MUX] Pazhyannur, R., Ali, I., and C. Fox, "PPP Multiplexing",
RFC 3153, 2001.
[PPP] Simpson, W., "The Point-to-Point Protocol (PPP)", RFC
1661, 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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[RFC5225] Pelletier, G. and K. Sandlund, "RObust Header Compression
Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
UDP-Lite", RFC 5225, April 2008.
[ROHC] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795, 2010.
[RTP] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", RFC 3550, 2003.
[SCTP] Stewart, Ed., R., "Stream Control Transmission Protocol",
RFC 4960, 2007.
[SIP] Rosenberg, J., Schulzrinne, H., Camarillo, G., and et.
al., "SIP: Session Initiation Protocol", RFC 3261, 2005.
[TCRTP] Thomson, B., Koren, T., and D. Wing, "Tunneling
Multiplexed Compressed RTP (TCRTP)", RFC 4170, 2005.
[cRTP] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508, 1999.
7.2. Informative References
[Efficiency]
Bolla, R., Bruschi, R., Davoli, F., and F. Cucchietti,
"Energy Efficiency in the Future Internet: A Survey of
Existing Approaches and Trends in Energy-Aware Fixed
Network Infrastructures", IEEE Communications Surveys and
Tutorials vol.13, no.2, pp.223,244, 2011.
[FPS_opt] Saldana, J., Fernandez-Navajas, J., Ruiz-Mas, J., Aznar,
J., Viruete, E., and L. Casadesus, "First Person Shooters:
Can a Smarter Network Save Bandwidth without Annoying the
Players?", IEEE Communications Magazine vol. 49, no.11,
pp. 190-198, 2011.
[First-person]
Ratti, S., Hariri, B., and S. Shirmohammadi, "A Survey of
First-Person Shooter Gaming Traffic on the Internet", IEEE
Internet Computing vol 14, no. 5, pp. 60-69, 2010.
[Power] Chabarek, J., Sommers, J., Barford, P., Estan, C., Tsiang,
D., and S. Wright, "Power Awareness in Network Design and
Routing", INFOCOM 2008. The 27th Conference on Computer
Communications. IEEE pp.457,465, 2008.
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[VoIP_opt]
Saldana, J., Fernandez-Navajas, J., Ruiz-Mas, J., Murillo,
J., Viruete, E., and J. Aznar, "Evaluating the Influence
of Multiplexing Schemes and Buffer Implementation on
Perceived VoIP Conversation Quality", Computer Networks
(Elsevier) Volume 6, Issue 11, pp 2920 - 2939. Nov. 30,
2012.
[topology_CNs]
Vega, D., Cerda-Alabern, L., Navarro, L., and R. Meseguer,
"Topology patterns of a community network: Guifi. net.",
Proceedings Wireless and Mobile Computing, Networking and
Communications (WiMob), 2012 IEEE 8th International
Conference on (pp. 612-619) , 2012.
Authors' Addresses
Jose Saldana
University of Zaragoza
Dpt. IEC Ada Byron Building
Zaragoza 50018
Spain
Phone: +34 976 762 698
Email: jsaldana@unizar.es
Dan Wing
Cisco Systems
771 Alder Drive
San Jose, CA 95035
US
Phone: +44 7889 488 335
Email: dwing@cisco.com
Julian Fernandez Navajas
University of Zaragoza
Dpt. IEC Ada Byron Building
Zaragoza 50018
Spain
Phone: +34 976 761 963
Email: navajas@unizar.es
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Muthu Arul Mozhi Perumal
Cisco Systems
Cessna Business Park
Sarjapur-Marathahalli Outer Ring Road
Bangalore, Karnataka 560103
India
Phone: +91 9449288768
Email: mperumal@cisco.com
Fernando Pascual Blanco
Telefonica I+D
Ramon de la Cruz 84
Madrid 28006
Spain
Phone: +34 913128779
Email: fpb@tid.es
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