Transport Layer

Typically an API embedded in Operating System.
e.g. Berkeley UNIX sockets

First true end-to-end layer.
What defines boundary of Transport and Network layers is that Transport code runs only on user machines, not on routers.
Boundary of Network layer is boundary of what routers need to run.

Quality of service:

Often quality of service provided by Network layer is out of our control - because it involves Network layer on many routers across the Internet, which we do not own (the carriers own them). These have errors, lose packets, crash, etc.
To deal with poor service, we can't change Network layer - We don't own those machines.
Our only option is to put a further layer on top of Network layer to try to improve quality of service.
Transport layer tries to provide reliable service to Application layer.
Transport service can be more reliable than underlying Network service. Unreliability of Network service is hidden from higher layer.
This is what we mean when we said previously that error-recovery can be done in higher (Transport) layer. e.g. TCP.

Transport layer has to manage:

acks (or lack of ack)
timers
re-transmits
and hide this from higher layer

API:

Transport layer provides API for application programmers.
Library calls are independent of the underlying network.
Application-layer code should work on many different networks.
Transport is the real boundary of the layered model, the first one that really hides the network from higher layers.
Layers 1-4 - "transport service provider"
Higher layers - "transport service user"

Transport Protocol Data Unit (TPDU)

What transport layers send to each other. Contains the "real" data of the communication.

Remember discussion of packet and frame.

TPDUs (exchanged by transport layer) are contained in packets (exchanged by network layer) which are contained in frames (exchanged by data link layer).

TCP

Internet protocol stack:
TCP - Transport layer for Internet.
IP is Network layer (does the routing). IP packets. IP is unreliable (may lose packets).

TCP provides reliable, connection-oriented service on top of IP.
Provides:

acks of IP packets
timers and timeouts
re-transmits
re-ordering of packets into correct sequence at receiver before passed to higher layer (have to wait for missing packet before send all up to higher layer - we may not want this wait - e.g. media stream)

Applications that use TCP:

HTTP
FTP
telnet, ssh
SMTP, POP3

UDP

Internet protocol stack:
UDP - alternative Transport layer for Internet.
Unreliable, connectionless.
No acks and re-transmits.
Faster, but may lose packets, or get damaged packet, and packets may arrive out of order.
But much faster.

Applications that use UDP, not TCP:

Streaming media, e.g. RealAudio and RealVideo.
Often, streaming client uses its own error-checking to compensate for lost/damaged data.
RTP can use TCP, but mostly uses UDP.
VoIP
Online multiplayer games
DNS

Usage

As at c.2002, c.95% of all Internet packets were TCP, c.5% UDP, less than 1% other.
UDP use (video, audio streams, VoIP, online games) growing since,
but TCP use (TCP file sharing, TCP torrents, HTTP video streaming, YouTube) also growing since.

2006 article claims UDP is about 20% of packets.
2008 article claims UDP is less than 2% of all traffic.

Sockets

Ports

6.2 Transport protocols

We saw previously algorithms for acks, re-transmits and flow control used on frames in Data Link layer (i.e. across a single physical link).
See here and here.

Similar algorithms may be used on higher-level objects to provide a reliable service in Transport layer (i.e. across entire network).
More difficult because instead of a single line (a) the entire network is now in the way (b):

6.5 TCP

TCP header.
See meaning.

Note port nos.
Uses sliding window protocol.
Initially go back n. More recently selective repeat.
Note seq and ack. These refer to next byte expected. Every byte is numbered 0 .. n (and then repeat) in the TCP byte-stream.
n = 2³²-1 = 4 billion.

TCP checksum (explained here) is quite weak (not CRC).
This is ok since CRC probably also used in Data Link layer (e.g. PPP and Ethernet).
Normal communication sessions will have error-checking in both Data Link and Transport layers.