1. Transport Layer: UDP, TCP

2. Internet Transport Protocols
TCP
Connection-oriented
setup required between client and server
Reliable transport
Flow control
Won’t overwhelm receiver
Congestion control
Won’t overwhelm network
Does not provide
Timing and throughput guarantees, security
UDP
Unreliable data transfer
Does not provide
Connection setup, reliability, flow control, congestion control, timing and throughput guarantee, or security
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UDP Services
User Datagram Protocol Service
OSI Transport Layer
Provides a thin layer over IP
16-bit port space (distinct from TCP ports) allows multiple recipients on a single host
Copyright ©: Nahrstedt, Angrave, Abdelzaher, Kravets, Gupta, Caccamo

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UDP Services
Unit of Transfer
Datagram (variable length packet)
Packet boundaries are preserved when receiving data (a read operation yields packet payload as it was originally sent.
Unreliable
No guaranteed delivery
Drops packets silently
Unordered
No guarantee of maintained order of delivery
Unlimited Transmission
No flow control
Postal Mail
Unreliable
Not necessarily in-order
Letters sent independently
Must address each reply
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UDP Packet structure
Quiz: by locking at UDP packet structure, can you guess what services are provided by UDP?
bits
0-15
16-31 32 64
Source port
Destination port
Length
Checksum
Data
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UDP Packet structure
UDP provides
application multiplexing (via port numbers)
integrity verification (via checksum) of the header and payload
packets are sent individually and error checked only if they arrive
if any kind of transmission reliability is needed, it must be implemented at application level
bits
0-15
16-31 32 64
Source port
Destination port
Length
Checksum
Data
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When should I use UDP?
Why should I use UDP? After all, TCP is a “better” algorithm!
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When should I use UDP?
Why should I use UDP? After all, TCP is a “better” algorithm!
Reliability of TCP comes with some costs…
TCP needs 3 packets to set up a connection
UDP header has 8 bytes instead of 20 (like TCP header)
UDP is a better choice for interactive (soft real-time) voice and video traffic (like Skype).
During real-time streaming, it is better to occasionally lose few packets (slight QoS degradation) rather than introducing large delays if lost packets are retransmitted.
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TCP Connections
Transmission Control Protocol (TCP) Service
TCP is a protocol running on OSI Transport Layer
Service Model
Byte stream (interpreted by application); packet boundaries cannot be distinguished by application
16-bit port space allows multiple connections on a single host
Connection-oriented
Set up connection before communicating
Tear down connection when done
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TCP Service
Reliable Data Transfer
Guaranteed delivery
Exactly once
Sequenced Data Transfer
In-order delivery
Regulated Data Flow
Monitors network and adjusts transmission appropriately
Data Transmission
Full-Duplex byte stream
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11. TCP Connection Establishment
3-Way Handshake
Sequence Numbers
J,K
Message Types
Synchronize (SYN)
Acknowledge (ACK)
Passive Open
Server listens for connection from client
Active Open
Client initiates connection to server
Passive open
Active open
Client
Server
Time flows down
listen
Synchronize SYN J
acknowledge ACK J+1
SYN K,
ACK K+1
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12. TCP Connection Termination
Active close
Either client or server can initiate connection teardown (4-way handshake)
Message Types
Finished (FIN)
Acknowledge (ACK)
3-way handshake is also possible. The sequence is FIN J, ACK J+1 & FIN K (as single packet), ACK K+1
Client
Server
Time flows down
Passive close
Finished FIN J
ACK J+1
FIN K
ACK K+1
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13. TCP Reliable transmission
TCP uses a sequence number to identify each byte of data.
When establishing a connection, the 3-way handshake allows remote hosts to exchange and ACK an initial sequence number (ISN).
Exploiting sequence numbers, data can be reconstructed in order, regardless of fragmentation, lack of ordering, or packet loss
Sequence number is incremented for each payload byte that is sent.
Receiver uses cumulative acknowledgment
For example, if host A sends 10 bytes with sequence numbers 100 to 109, then the receiver transmits ACK 110 since next expected byte should have sequence number 110
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14. Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher
TCP Segment Format
TCP segments have a 20 byte header with >= 0 bytes of data.
Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher

15. Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher
TCP header fields
Port number
A port number identifies the endpoint of a connection
A pair <IP address, port number> identifies one endpoint of a connection
Two pairs <client IP address, client port number> and <server IP address, server port number> identify a TCP connection
Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher

16. Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher
TCP header fields
Sequence Number (SeqNo):
Sequence number is 32 bits long.
Each sequence number identifies a byte in the byte stream
Initial Sequence Number (ISN) of a connection is set during connection establishment
Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher

17. Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher
TCP header fields
Acknowledgement Number (AckNo):
Acknowledgements are piggybacked
A host uses the AckNo field to send acknowledgements. (If AckNo has a valid value, then the “ACK flag” is set)
The AckNo contains the next SeqNo that a host (receiver) expects to receive Example: The acknowledgement for a segment with sequence numbers is AckNo=1501
Copyright Jorg Liebeherr 98, Modified with permission, Abdelzaher