ACP-WG I-02/WP-09
International Civil Aviation Organization
23/08/07
WORKING PAPER
AERONAUTICAL COMMUNICATIONS PANEL (ACP)
SECOND MEETING OF THE WORKING GROUP I (IPS)
Montreal, Canada 27 – 31 August 2007
Agenda Item xx: Xxx
TRANSPORT LAYER CONSIDERATIONS IN THE ATN/IPS
(Presented by Frank Schreckenbach, Daniel Medina, Christian Kissling)
SUMMARY
This paper discusses transport layer issues and their implication on network
layer technologies for the air/ground ATN/IPS, as investigated in the
NEWSKY project. The performance degradation of connection-oriented
transport layer protocols such as TCP in wireless heterogeneous environments
motivates the search for an efficient end-to-end protocol architecture.
ACTION
To review the material in this paper and develop further guidance and action,
as required.
1.
INTRODUCTION
In the current ATN/OSI, two transport layer protocols have been specified for use in ATN End Systems
[1]: the Connection Oriented Transport Protocol (COTP) and the Connectionless Transport Protocol
(CLTP). A given End System (ES) may implement one or both of these, depending upon the requirements
of the applications it contains.
COTP provides to its users an end-to-end connection mode service, and is a conformant subset of the
Class 4 Transport Protocol (TP4) specified in ISO/IEC 8073. The tasks of the COTP protocol include the
provision of a reliable connection over unreliable lower layers and the provision of flow control and error
control in case datagrams are lost. The COTP protocol splits the data streams of different applications,
assigns a header to them and hands the data over to the network layer. When receiving network layer
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datagrams, the COTP protocol orders the received datagrams and reconstructs the original data stream. If
packets are lost, the COTP protocol recognizes this and requests these datagrams again. This ensures both
data integrity and data sequence integrity.
CLTP provides a connectionless transport service, where no service guarantees are offered. Each message
is transferred as an event unrelated to the transfer of any other message and there is no guarantee either of
delivery or that a given message may not overtake an earlier message. The CLTP is conformant with
ISO/IEC 8602.
Similarly, in the IP world, the most popular transport layer protocols are TCP (Transmission Control
Protocol) [2] and UDP (User Datagram Protocol) [3]. TCP guarantees reliable and in-order delivery of
data from sender to receiver, while with UDP, datagrams may arrive out of order or go missing without
notice.
While transport layer protocol use in the ATN/IPS is still an open issue, the expectation is that both TCP
and UDP will find their role in the ATN/IPS, depending, as in the current ATN/OSI, on each
application’s requirements.
Although regular versions of TCP work fine in the fixed Internet, their performance degrades significantly
in the wireless Internet, since the assumptions of short delays and very low BER that were made during
its design for the former are no longer valid in the latter.
The properties of wireless networks, especially the satellite link differ significantly from these
assumptions. Some properties of satellite links are:
 long and very long delays
 delays are variable over time, especially for non-geostationary satellites
 large delay  bandwidth product
 higher BER, especially for mobile terminals
 losses and fading
 high asymmetry of data rates in the uplink and downlink
Further considering the example of a satellite link, the following list provides a non-comprehensive
overview of problems arising when using a regular version of the TCP protocol for this link:




Connection establishment with handshaking takes very long.
The TCP slow-start phase takes very long. This means that for short-duration connections the link
is used in a highly inefficient manner. This especially affects application sessions which use
several short TCP connections throughout a single application session.
Flow Control: Due to the credit based Flow Control mechanisms in TCP the throughput is limited
depending on the round trip delay.
Congestion Avoidance: In the terrestrial internet segment losses are usually caused by an
overload of the network. For this reason TCP reduces the congestion window for each lost
segment to reduce the network load (congestion control). However, the TCP protocol does not
distinguish whether segment losses occur due to congestion or link loss phenomena. In the
satellite link, bit errors also cause the loss of segments and lead to a transmitter time-out. The
following slow-start and congestion avoidance phases reduce the throughput on the link
significantly. In addition, the retransmission timer is increased (RTO back off). Due to the long
propagation delays of the satellite link many data segments have to be retransmitted and it takes
long until slow-start and congestion avoidance phases are completed.
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The ATN/IPS is by definition a wireless internet, and any potential transport layer protocol should take
this into account. In addition, in the ATN/IPS, mobile nodes (aircraft) can communicate with
correspondent nodes on the ground (e.g. ATC Centers, Airline Operations Center, Public Internet…) via
multiple wireless links (depending on their location and equipage) such as Gatelink, WiMAX, VDL Mode
2, B-AMC, P34, Inmarsat SwiftBroadband, DVB-S2/RCS, to name a few candidates. This scenario is
shown in a simplified fashion in Fig. 1. This makes the problem even more complicated, since each
potential wireless access technology has different characteristics, affecting the performance of TCP in a
different way.
To summarize, we are confronted with the problem of enhancing TCP performance in a heterogeneous
wireless environment, where each link has its particular idiosyncrasies.
These transport layer issues have to be tackled at the very start of the network development phase since
they strongly influence most network layer issues, including, mobility, routing, and security.
AIRCRAFT
ES
ES
ES
Airborne
Router
ES
ES
ES
A/G
Router
G/G
Router
ES
A/G
Router
G/G
Router
ES
Airborne
Router
AIRCRAFT
Fig. 1 ATN/IPS End-to-End Architecture
2.
ENHANCING TCP PERFORMANCE IN A
HETEROGENEOUS WIRELESS ENVIRONMENT
This is a complex task, and may be decomposed intuitively into two subtasks:
a. TCP needs information about the characteristics of the link used to transmit the packets, so that it
can personalize its behaviour for that specific link.
b. Once it is aware of the specific characteristics of the link in use, how can TCP personalize its
behaviour to function optimally over that specific link? In other words, how can TCP performance
be enhanced over that link?
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Keeping TCP informed of link layer events
Such a heterogeneous wireless environment is not only found in aviation, but also in the context of
modern mobile devices, such as those considered in the IEEE 802.21 standard (“Media Independent
Handover”) [4]. These devices are equipped with a variety of access technologies, such as 802.11
(WLAN), Bluetooth, 3GPP, 3GPP2, WiMAX, etc. The scope of IEEE 802.21 is to develop a standard that
provides link layer intelligence and other related network information to upper layers (e.g. IP, Mobile IP,
SIP, TCP, UDP, application layer) to optimize handovers between heterogeneous media.
To illustrate how this standard relates to the problem at hand, consider these examples:
1.
The Link_Up.indication primitive notifies upper layers when a layer 2 connection is
established for the specified link interface. When higher layer connections are being
established over this link, the transport layer may adjust transport (TCP) related parameters
based on link properties.
2.
The MIH_Link_Handover_Complete.indication primitive notifies upper layers when a link
layer inter- or intra-technology handover is completed. Transport layers (e.g., TCP) may make
use of this primitive to fine tune their flow control and flow congestion mechanisms.
A solution analogous to IEEE 802.21 is expected to be realized in aviation to optimize handovers between
heterogeneous media such as satellite and terrestrial radio links. TCP parameter adaptation could benefit
from the presence of this functionality.
However, the functionality provided by 802.21 only solves subtask a above. It keeps TCP (among others)
well informed about what is going on at the link level. A solution is also needed for subtask b, indeed for
each particular link in turn.
2.2
Enhancing TCP performance over wireless links
Several approaches have been proposed to enhance the performance of TCP over wireless links
Split connection approaches like I-TCP [6] and MTCP [7] propose to split the TCP connection between
the fixed host (FH) and mobile host (MH) at the intermediate base station (BS). The split connection
approaches propose to have a protocol optimized for wireless links to enhance performance of TCP. This
mechanism is typically implemented by a so-called Performance Enhancing Proxy (PEP) located in the
base station and mobile host.
RFC 3135 [5] is a survey of existing PEP solutions that include different approaches for improving TCP
performance and their implications.
Other approaches to improve performance of TCP like SNOOP [8], M-TCP [9], WTCP [10], ELN [11]
preserve end-to-end semantics of TCP. However, all these approaches require base station mediation and
are based on the assumption that TCP headers are readable by the base station.
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Coexistence of IPsec end-to-end security and TCP performance enhancements
The above mentioned approaches supplement TCP either by explicitly providing a mechanism for
detecting the nature of loss or by shielding the wireless losses from the TCP sender. However, the
mechanisms are tightly coupled with the transport layer, which inhibit the use of security mechanisms at
network layer. Achieving improved TCP performance together with ensuring end-to-end security
necessitates the co-existence of security mechanisms like IPsec and performance enhancing solutions.
However, IP security and TCP performance have been traditionally dealt with in a mutually exclusive
manner.
IPsec provides security services at the IP layer and can be used to protect one or more paths between a
pair of hosts, a pair of security gateways or between a security gateway and a host [12]. The security at
the IP layer provides protection to both the application data or payload and the transport layer headers
(TCP or UDP headers). IPsec uses two protocols to provide the security services, namely Authentication
Header (AH) [13] and Encapsulating Security Payload (ESP) [14]. AH may be used to provide data
integrity, origin authentication and anti-replay protection. ESP provides the entire authentication services
provided by AH and along with it, it also provides confidentiality or encryption.
In the ATN/IPS, IPsec plays a vital role, guaranteeing end-to-end security between aircraft and their
correspondent nodes (as well as between different organizations within the ground internet). Therefore, a
solution must be found to enhance TCP performance over the various access technologies without the
respective base station or satellite gateway requiring access to the TCP headers of packets exchanged with
the aircraft, since these will be encrypted by IPsec end-to-end. This is currently a hot topic in the Internet
research community [15].
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3.
REFERENCES
[1]
“Comprehensive ATN Manual, Part IV Communication Services”, January 1999.
[2]
IETF RFC793 “Transmission Control Protocol (TCP)”, September 1981.
[3]
IETF RFC768 “User Datagram Protocol (UDP)”, August 1980.
[4]
IEEE P802.21/D05.00 “Draft Standard for Local and Metropolitan Area Networks: Media
Independent Handover Services”, April 2007.
[5]
IETF RFC3135 “Performance Enhancing Proxies Intended to Mitigate Link-Related
Degradations”, June 2001.
[6]
A. Bakre, B.R. Badrinath, “I-TCP: Indirect TCP for Mobile Hosts”, in Proceedings of 15th
International Conference on Distributed Computing Systems, May 1995.
[7]
R. Yavatkar, N. Bhagawat, “Improving end-to-end Performance of TCP over Mobile
Internetworks”, in Proceedings of IEEE Workshop on Mobile Computing Systems and
Applications, December 1994.
[8]
H. Balakrishnan, S. Seshan, E. Amir, R.H. Katz, “Improving TCP/IP performance over wireless
networks”, in Proceedings of ACM Mobicom, November 1995.
[9]
K. Brown, S. Singh, “M-TCP: TCP for mobile cellular networks”, ACM Computer
Communications Review (CCR) 27 (1997) 5.
[10]
K. Ratnam, I. Matra, “WTCP: an efficient mechanism for improving TCP performance over
wireless links”, in IEEE Symposium on Computers and Communications, June 1998.
[11]
H. Balakrishnan, R.H. Katz, “Explicit loss notification and wireless Web performance”, in
Proceedings of IEEE Globecom Internet Mini-Conference, November 1998.
[12]
IETF RFC4301 “Security Architecture for the Internet Protocol”, December 2005
[13]
IETF RFC4302 “IP Authentication Header (AH)”, December 2005
[14]
IETF RFC4303 “IP Encapsulating Security Payload (ESP)”, December 2005
[15]
V. Obanaik, L. Jacob, A.L. Ananda, “Secure performance enhancing proxy: To ensure end-toend security and enhance TCP performance over IPv6 wireless networks”, September 2005.