The Transport Layer

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Nixu Ltd.

The Transport Layer

Part 2

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TCP Congestion Control

• RFC 2581

• The basic features are the TCP slow start and congestion control algorithms

• These features are described in the standards

— Note that the TCP protocol format is fixed

— However these features depend on the TCP implementation, which has more liberty in implementing different strategies for managing window and packet sizes

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Nixu Ltd. 3/26 The Transport Layer

TCP Slow-Start

• TCP escalates the amount of traffic it sends rapidly, to achieve a reasonable data transmission rate

• The TCP header window size field is used to manage the advertised window

— Initially set to maximum size segment

— Each received ACK increases the window size by one element (expo- nential growth)

— Window growth stops at the congestion treshold or when a packet is lost

— Packet loss is detected using a round trip timer

— At packet loss the size of the window is halved

— Actually max (data in transit / 2, 2*MSS)

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TCP Timeout

• How to know when a packet is lost?

• An average round trip time (RTT) can be estimated from session initiation and revised with new samples

— RTT = (A * RTT_old) + ((1 - A) * RTT_sample), 0 < A < 1

— A typical value for A is 0.875

• Then the timeout can be set to be B * RTT, where B is for example 2

— Or timeout = RTT + 4 * Deviation

• Setting the values for A and B has been a subjet of research and several methods exist, where the values are dynamic

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The Basic TCP Congestion Control Algorithm

• Congestion is overload in a switching point (router) that delays packets or causes them to be dropped

• At packet loss TCP halves the sending window (congestion window) and doubles the retransmission timer

• After congestion is detected TCP enters the congestion avoidance phase and the sending window is increased linearily, not exponentially

• There are several versions of this algorithm

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TCP Congestion Control Algorithms

• TCP Tahoe, the original algorithm

• TCP Reno

— Faster window increase in congestion avoidance

— Fast recovery algorithm optimized for single packet loss

• TCP Vegas

— More complex analysis of the network condition

— The sending window size is continously adjusted based on the meas- ured RTT

• Reno and Vegas are compared much in literature

— Vegas is more efficient in ideal conditions, but suffers when it co- exists with Reno

— Vegas can be hurt by the Random Early Detection (RED) in routers, which drops packets

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Some problems with TCP

• Silly window syndrome

— When the sending window is full and the first packet has been one byte, the sending buffer will be emptied one byte at a time

— Now corrected with heuristics

• Sending window size oscillation

• TCP and UDP checksums are calculated using a pseudo header

— Source and destination IP addresses are added to the header

— Makes life difficult for Network Address Translation

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Alternatives to TCP

• TCP is far from perfect

— Slow start and slow restart assume that packets are dropped because of congestion

— Only service is a reliable byte stream

• Other alternatives exist

— Mowgli

— XTP

— WAP

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Mowgli

• Mobile Office Workstations using GSM Links

• Developed at Helsinki University

• Attempts to optimize the capacity of radio links

• A research project

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Mowgli Architecture

• The Mowgli Agent provides the BSD socket interface to the wireless device

— TCP/IP services available

— Additional Mowgli services

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The Mowgli Proxy

• Delivers the data sent by the agent and transferred by the Mowgli Data Transfer Service to the actual TCP/IP layer in a Mobile Connection Host connected to the Internet

• Can perform operations to the application data (like image compression)

• The IP address that is used to send and recieve traffic in the network is shared by the proxy and the wireless workstation

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XTP, Express Transport Protocol

• A large and complex protocol, that attempts to provide

— Reliable and unreliable streams and datagrams

— Transactions

— Reliable multicast

— Error control, flow control, rate control

— And other features

• XTP has not gained wide suport outside the research community

• It has many interesting features and ideas

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XTP Implicit Connection Open with Acceptance

• The connection is set up and transmission started by the sender

• Acceptance is delivered at leisure

Client Packets in transit Server

FIRST

DATA active open

DATA passive open

CNTL

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XTP Connection Refusal

• The reciever (server) refuses the connection with the DIAG packet

• All data sent by the sender is lost

Client Packets in transit Server

FIRST

DATA active open

DATA passive open

DIAG

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Wireless Application Protocol

• An industry-wide standard to develop applications for wireless environments

• Ericsson, Motorola, Nokia...

• Based on WWW technologies and philosophies

• See http://www.wapforum.org/

• Several parts (version 1.0):

— WAE - Wireless application environment

— WAP - Wireless application protocol

— WSP - Wireless session protocol

— WTP - Wireless transport protocol

— WTA* - Wireless telephony application...

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Goals

• To design a protocol family suitable for building interactive applications for devices with

— limited capabilities:

— battery power

— memory

— screen size

— limited bandwidth

— possibly long latencies

— different user interfaces

• Global and international usability

• Security and access control

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The WAP 1.0 Protocol Stack

WML

Common services Session Layer (WSP) Security Layer

Transport Layer (WTP)

Bearer systems (GSM data, SMS, GPRS etc)

Other Services and applications

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The Transport Protocol (WAP 1.0)

• Connection oriented WTP/C

— Optimized for low bandwidth wireless bearers

— Efficient for request-reply interaction

— Realiability

• Datagram oriented WTP/D

— Datagrams are moved transparently on the bearer network

• Port based abstract interface to upper layers.

• Low memory and CPU speed requirements.

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The Session Protocols

• Mechanisms for exchanging typed data between WAP applications in a secure manner

• In WAP Architecture specification coupled with the Security Layer

Application layer Session Layer Security Layer Transport Layer Bearers

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The Session Protocols (cont.)

• Optimized for asymmetric client–server functionality.

Includes the server initiated “push” model.

• Several possible session protocols

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Multicast

• Unicast - one sender, one receiver

• Broadcast - one sender, delivery to all entities in a certain area

• Multicast - one sender, many receivers

— Many, not all

• Not necessarily interactive

— one way broadcast only

• May cause bandwidth problems if in full effect

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MBONE basics

• MBONE - Multicast Backbone

• First tests in 1994

• A virtual network

— layered on top of portions of the physical Internet to support routing of IP multicast packets

— commercial routers don’t always implement multicast routing

• Consists of multicast capable networks joined with a multicast tunnel

• Multicast UDP packets tunneled in normal unicast IP packets

• Usually special event broadcasts (IETF etc)

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Multicast technology

• Groups

— hosts tell their routers, which multicast groups they are interested in

— routers tell their up-stream routers they want to receive that traffic

— IGMP – Internet Group Management Protocol, based on ICMP

— v1 in RFC 1112 most widely used, v2 RFC 2236 emerging

• Reliable protocols

— no single multicast transport protocol is general and good enough for all types of applications

— RTP - Real Time Transport Protocol, MFTP - Multicast FTP etc

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MBONE requirements

• Anyone can join MBONE

• ISP’s must provide at least one IP multicast router to tunnel traffic

— a real router (MOSFP - Multicast OSPF routing protocol) or

— a Unix host running the free mrouted software (DVMRP)

• Regional (USA, EU etc) tunnel coordinating done to ensure best connections

• Client software

— several audio or videoconferencing programs for Unixes

— liveGate for Windows, can do multicast over non-multicast dial-in connections

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Group Addresses

• IPv4: class D

— addresses 224.0.0.0 - 239.255.255.255

— addresses 244.0.0.0 - 244.255.255.255 are reserved for routing etc.

• IPv6:

— flags: fourth bit tells whether the route is permanent

— scope: tells how wide the group is

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The Future of the Transport Layer

• TCP and UDP are good enough and will continue to be used for a long time

• WAP WTP is likely to make a comeback with GPRS

— Or maybe not?

• The ruling factor for technology adaptation is not the quality or performance of a particular technology but the choice of technology by commercial actors

• Next years’ focus is on wireless

• Then maybe satellite and multicast