| Transport Area Working Group | J. Saldana |
| Internet-Draft | University of Zaragoza |
| Intended status: Best Current Practice | D. Wing |
| Expires: June 14, 2015 | Cisco Systems |
| J. Fernandez Navajas | |
| University of Zaragoza | |
| M. Perumal | |
| Ericsson | |
| F. Pascual Blanco | |
| Telefonica I+D | |
| December 11, 2014 |
Tunneling Compressing and Multiplexing (TCM) Traffic Flows. Reference Model
draft-saldana-tsvwg-tcmtf-08
Tunneling, Compressing and Multiplexing (TCM) is a method for improving the bandwidth utilization of network segments that carry multiple small-packet flows in parallel sharing a common path. The method combines different protocols for header compression, multiplexing, and tunneling over a network path for the purpose of reducing the bandwidth. The amount of packets per second can also be reduced.
This document describes the TCM framework and the different options which can be used for each layer (header compression, multiplexing and tunneling).
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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.
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].
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 or IoT scenarios 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, the traffic can be optimized by multiplexing packets belonging to different flows. As a consequence, bandwidth can be saved and the amount of packets per second can be reduced. 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. It is a believed fact that this delay and jitter can be of the same order of magnitude or less than other common sources of delay and jitter currently present on the Internet without causing harm to flows that employ congestion control based on delay.
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 [H.323], etc.), RTP has become the standard par excellence for the delivery of real-time content.
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: 25% header overhead | G.711 at 20 ms packetization: 37.5% header overhead |
| G.729 at 20 ms packetization: 200% header overhead | G.729 at 20 ms packetization: 300% header overhead |
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. In addition to latency, these applications may be sensitive to jitter and, to a lesser extent, to packet loss [Gamers], since they implement mechanisms for packet loss concealment.
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). IoT traffic generated in Constrained RESTful Environments, where UDP packets are employed [RFC7252]. 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.
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 (able to reduce bandwidth and packets per second) when/where required is a one-time investment.
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 at the same time 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 series of recommendations about delay limits have been summarized in [I-D.suznjevic-tsvwg-mtd-tcmtf], in order to maintain this additional delay and jitter in the same order of magnitude than other sources of jitter currently present on the Internet.
A demultiplexer and a decompressor are necessary at the end of the common path, so as to rebuild the packets as they were originally sent, making traffic optimization a transparent process for the origin and destination 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].
All in all, the combined use of header compression and multipexing provides a trade-off: bandwidth can be exchanged by processing capacity (mainly required for header compression and decompression) and a small additional delay (required for gathering a number of packets to be multiplexed together).
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 showed a reduction of the energy consumption, 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 of the number of packets transmitted. Thi tradeoff should be explored more deeply.
This document uses a number of terms to refer to the roles played by the entities using TCM.
A packet sent by an application, belonging to a flow that can be optimized by means of TCM.
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.
A packet including a number of multiplexed and header-compressed native ones, and also a tunneling header.
A flow of TCM packets, each one including a number of multiplexed header-compressed packets.
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.
The relationship between a pair of TCM optimizers exchanging TCM packets.
A network entity which makes the decisions about TCM optimization parameters (e.g., 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 optimizers, and orchestrates the optimization that takes place between them.
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:
In this scenario, the TCM 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--------------------->
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:
In this case, TCM is only activated inside an ISP, from the edge to border, inside the network operator. The geographical scope and network depth of TCM activation could be on demand, according to traffic conditions.
If we consider the residential users of real-time interactive applications (e.g., VoIP, online games generating small packets) in a town or a district, a TCM optimizing module can be included in some 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 can 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 may also be applied for fiber residential accesses, and in 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--------------->
Figure 2
At the same time, the ISP may implement TCM capabilities within its own MPLS network in order to optimize internal network resources: optimizing modules can be embedded in the Label Edge Routers of the network. In that scenario MPLS will act as the “tunneling” layer, being the tunnels the paths defined by the MPLS labels and avoiding the use of additional tunneling protocols.
Finally, some networks use cRTP [cRTP] in order to obtain bandwidth savings on the access link, but as a counterpart considerable CPU resources are required 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, thus avoiding the impact on the CPU of the aggregation router.
End users can also optimize traffic end-to-end from network borders. TCM 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----------->
Figure 3
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 near the game server and several TCM-IO/EOs placed in the networks of different ISPs (agreements between the game provider and each ISP will 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
In conclusion, a standard able to compress headers, multiplex a number of packets and send them together using a tunnel, can benefit various stakeholders:
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 [Online].
The current standard [TCRTP] defines a way to reduce bandwidth and pps of RTP traffic, by combining three different standard protocols:
The three layers are combined as shown in the
RTP/UDP/IP | | ---------------------------- | ECRTP compressing layer | | ---------------------------- | PPPMUX multiplexing layer | | ---------------------------- | L2TP tunneling layer | | ---------------------------- | IP
Figure 5
In contrast to the current standard [TCRTP], TCM 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
|
| ------------------------------
|
PPPMus 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:
It can be observed that TCRTP [TCRTP] is included as an option in TCM, combining [ECRTP], [PPP-MUX] and [L2TPv3], so backwards compatibility with TCRTP is provided. If a TCM optimizer implements ECRTP, PPPMux and L2TPv3, compatibility with RFC4170 MUST be granted.
If a single link is being optimized a tunnel is unnecessary. In that case, both optimizers MAY perform header compression between 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 optimization, so TCM signaling protocols will be employed here in order to negotiate the compression and multiplexing parameters to be employed.
Payload compression schemes may also be used, but they are not the aim of this document.
This section describes how to combine protocols belonging to trhee layers (compressing, multiplexing, and tunneling), in order to save bandwidth for the considered flows.
TCM 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 over IP
Figure 7
Another way TCM can be implemented is with an external optimizer. This device can be placed at strategic places in the network and can dynamically create and destroy TCM sessions without the participation of the endpoints that generate the flows (Figure 8).
[ending device]\ /[ending device]
\ /
[ending device]----[optimizer]-----[optimizer]-----[ending device]
/ \
[ending device]/ \[ending device]
^ ^ ^
| | |
Native IP TCM 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 over IP Compressed
Figure 9
There are different protocols that can be used for compressing IP flows:
The present document does not determine which of the existing protocols has to be used for the compressing layer. The decision will depend on the scenarioand the service being optimized. It will also be determined by the packet loss probability, RTT, jitter, and the availability of memory and processing resources. The standard is also suitable to include other compressing schemes that may be further developed.
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.
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].
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.
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.
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 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.
Different tunneling schemes can be used for sending end to end the compressed payloads.
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.
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 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.
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:
+-------+---+------+-------+-----+ +---+------+-------+-----+
| 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 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.
Gonzalo Camarillo
Ericsson
Advanced Signalling Research Lab.
FIN-02420 Jorvas
Finland
Email: Gonzalo.Camarillo@ericsson.com
Michael A. Ramalho Cisco Systems, Inc. 6310 Watercrest Way, Unit 203 Lakewood Ranch, FL 34202 USA 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
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
This memo includes no request to IANA.
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 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 to denial of service, and the use of TCM to deny service to other parts of the network infrastructure, is for future study.