Computer Networks
Transport Layer
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Transport Layer Objectives
• Goals
– Deals with end host pacing, congestion control
– Provides internal addresses on host (ports)
• Upward multiplexing
– Reliable, end-to-end data delivery
• Challenges
– Network reliability – lost, delayed, duplicated,
out of order packets
– Network delay variation (RTT estimation)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Transport Layer Protocols
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UDP (User Datagram Protocol)
TCP (Transmission Control Protocol)
SCTP (Stream Control Transmission Prot)
DCCP (Datagram Congestion Ctl Prot)
RTP (Realtime Transport Protocol)
ATP (AppleTalk Transaction Protocol)
NetBEUI (NetBIOS Extended User Interf.)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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User Datagram Protocol
• “Passthrough” for IP
– Upward multiplexing
– No reliability guarantees at all
– No congestion control
• Fields (8 bytes!)
– Src & Dest Port (16 bits each)
– Optional checksum (16 bits)
– 16-bit length (including header)
• See IETF RFC 768
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Transmission Control Protocol
• Much more complex
– Upward multiplexing
– Reliable, in-order byte stream delivery
– Congestion control
– No timing guarantees/QoS
• Session Semantics
– Setup: 3-way handshake (SYN, SYN-ACK,ACK)
– Use: sequence #s, ACKs
– Teardown: FIN, FIN-ACK
• See RFC 793, RFC2581, RFC 3168
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Stream Control Transmission
Protocol
• Multiple Stream Service
– Upward multiplexing
– Reliable, in-order delivery
– Congestion control
– Message streams
– Multihoming support
• See RFC 2960 (protocol), RFC 3286 (text)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Datagram Congestion Control
Protocol
• Intermediate between TCP and UDP
– Upward multiplexing
– Order, reliability not guaranteed
– Congestion control
– No timing guarantees/QoS
• Session semantics (setup, use, teardown)
• See RFC 4340
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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RTP
• For realtime traffic
– Has ports for upward multiplexing
– Marker bit
– Payload type
– 32-bit Timestamp for delivery jitter
– No congestion control
– 16-bit Sequence numbers, but no order
guarantee
– Runs on top of UDP
– See RFC 3550
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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AppleTalk Transaction Protocol
• Part of AppleTalk self-configuration
– AppleTalk addr resolution protocol (AARP)
hosts automatically generate own NW addrs
– Name Binding Protocol (NBP) like DNS
• Proprietary 
• Request/Response/ACK exchanges
– 1 req pkt could garner up to 8 resp pkts
– Requestor sent bitmap selective ACK
• At-least-once and exactly-once options
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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AppleTalk Data Stream Protocol
• Late addition to AppleTalk suite
• Proprietary 
• TCP-like, except:
– Host can reject a connection
– No half-open connections
• See http://en.wikipedia.org/wiki/AppleTalk
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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NetBIOS Extended User Interface
• Unrouted network-and-transport layer
– Can only be used in a broadcast domain
• A.k.a. NetBIOS Frame (NBF)
• Uses IEEE 802.2 type 1 for name svc,
datagram
• Uses IEEE 802.2 type 2 for session svc.
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Back to TCP
• Fields (20 bytes!)
– Src & Dest Port (16 bits each)
– 32-bit sequence #, ACK # (byte stream)
– 4-bit Header Length (in 32-bit words)
– Reserved (6 bits)
– Flags (6 bits)
– 16-bit Window size (bytes willing to Rx)
– Checksum (16 bits)
– Urgent pointer
– Options (0 or more 32-bit words)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP Flags
• URG
– Urgent pointer valid (offset in current segment)
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ACK – ACK field is valid
PSH – deliver immediately (don’t buffer)
RST – Reset connection (abort)
SYN – Synchronize (3-way handshake)
FIN – Finish (normal close connection)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP Session Initiation
Host A
Host B
Syn, X, -
Syn, ACK, Y, X+1
ACK, X+1, Y+1
Normal
Three-way Handshake
-Client makes connection req
-Server responds (half-open
Connection now)
-Client confirms
-Data transfer begins
-Syn Flooding Attack…
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP Initiation Mishaps
Host B
Host A
RST
Call Collision
Unknown/Late
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP States
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CLOSED – no active/pending connection
LISTEN – server waits for incoming call
SYN RCVD – connection req received, wait for ACK
SYN SENT – connection requested, wait for SYN-ACK
ESTAB – normal data transfer state
FIN WAIT 1 – application has said it’s done (active)
FIN WAIT 2 – other side has agreed to release (active)
TIMED WAIT – wait for all packets to die off (active)
CLOSING – both sides closed simultaneously (active)
CLOSE WAIT – other side initiated a release (passive)
LAST ACK – wait for all packets to die off (passive)
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP State Machine - Open
Normal Server
Connect/SYN
Normal Client
CLOSED
Unusual
Close/-
Listen/-
SYN/SYN-ACK
SYN RCVD
Close/LISTEN
Send/SYN
RST/-
SYN SENT
SYN/SYN-ACK
Close/FIN
ACK/ESTABLISHED
FIN WAIT 1
SYN-ACK/ACK
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP State Machine - Close
SYN RCVD
Close/FIN
Close/FIN
ESTABLISHED
FIN/ACK
[Active Close]
FIN WAIT 1
FIN/ACK
CLOSING
ACK/-
FIN WAIT 2
[Passive Close]
CLOSE WAIT
ACK/FIN/ACK
TIMED WAIT
Close/FIN
LAST ACK
(Timeout)
ACK/-
CLOSED
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP Transmission Policy
• Sender not required to send as soon as
application delivers data
• Windows not tied directly to ACKs, but also
to receive window size
• Receiver may ACK when segt received,
when receive window changes, etc.
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP Transmission Example
Receiver’s 4K buffer
0
4K
empty
Appl writes
2 K bytes
Sender may
send up to
2 K bytes
2K
Sender blocked
Appl writes
3 K bytes
Full
Appl reads
2 K bytes
2K
1K
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
2K
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TCP Telnet Example
• Interactive editor – reacts on every keystroke
• Sender sends each character on arrival
– 41 bytes (1 char + 20 TCP + 20 IP)
• Receiver sends 40-byte ACK
• Editor reads byte, Rx updates WIN (40 more)
• Editor processes character, echoes it
– 41 more bytes!
• Total of 162 bytes in 4 datagrams per char!!
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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TCP Solution
• Delayed ACKs can alleviate this
• Wait 500 ms before sending ACK
• Gives time for read, echo, so only 1 DG sent
– Ack timer needed, reverse flow trigger
– Also trigger ACK on second segment arrival
• Still inefficient (82 bytes in 2 DGs)
• Nagle’s Algorithm helps further!
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Nagle’s Algorithm
• Sender side solution
• When data arrives one byte at a time,
send first byte, then buffer rest until ACKed
• When ACK arrives, or when MSS can be
filled, or if half WIN filled, send all chars in
buffer in a single segment
• Huge bandwidth savings! Widely used.
• Disable if using mouse, avoid jerkiness
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Silly Window Syndrome
Receiver’s buffer
Full
Sender
sends
1 byte
Appl reads 1 byte
40 bytes ACK
room for 1 byte
40 bytes hdrs
1 byte of data
Full
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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Clark’s Solution
• Prevent Rx from sending window update for
one byte
• Rx forced to wait until WIN >= MSS, or
buffer is half empty, whichever is smaller
• Sender should not send tiny segments
• Wait until full MSS or half receiver’s buffer
size, whichever smaller
CEN 5501C - Computer Networks - Spring 2007 - UF/CISE - Newman
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