1. Transmission Control Protocol (TCP)
Reading: Chapter 24
Fall 2004
FSU CIS 5930 Internet Protocols

2. FSU CIS 5930 Internet Protocols
TCP
A reliable, byte-oriented, connection-oriented transport protocol
Requirements
Reliable transmission of byte streams
In-order delivery (to application layer)
No duplicate delivery (to AL)
Transport data of arbitrary length
Synchronization between sender and receiver
Flow control
Fall 2004
FSU CIS 5930 Internet Protocols

3. View of TCP from applications
Connection-oriented
Peer-to-peer communication
Complete reliability
Full-duplex communication
Byte-stream interface
Reliable connection startup
Graceful connection shutdown
Fall 2004
FSU CIS 5930 Internet Protocols

4. TCP header Fall 2004 FSU CIS 5930 Internet Protocols Data (optional)
Options (optional)
Destination Port
Length
Control Flags
Source Port
Sequence Number
Acknowledgement Number
reserved
Window Size
Checksum
Urgent Pointer
Fall 2004
FSU CIS 5930 Internet Protocols

5. FSU CIS 5930 Internet Protocols
struct tcphdr
struct tcphdr {
__u16 source;
__u16 dest;
__u32 seq;
__u32 ack_seq;
#if defined(__LITTLE_ENDIAN_BITFIELD)
__u16 res1:4,
doff:4,
fin:1,
syn:1,
rst:1,
psh:1,
ack:1,
urg:1,
ece:1,
cwr:1;
#elif defined(__BIG_ENDIAN_BITFIELD)
__u16 doff:4,
res1:4,
cwr:1,
fin:1;
#else
#error "Adjust your <asm/byteorder.h> defines"
#endif
__u16 window;
__u16 check;
__u16 urg_ptr; };
Fall 2004
FSU CIS 5930 Internet Protocols

6. Implementation of TCP Fall 2004 FSU CIS 5930 Internet Protocols TCP
ip_input.c
ip_local_deliver
ip_output.c
ip_queue_xmit
tcp_v4_rcv
tcp_v4_do_rcv
__tcp_v4_lookup()
tcp_rcv_ established
TCP_ESTABLISHED
tcp_rcv_ state_process
tcp_sendmsg
tcp_send_skb
tcp_transmit_skb
tcp_ack_ snd_check
tcp_data_ snd_check
tcp_write_ xmit
tcp_write_ timer
tcp_re - transmit_skb
tcp_send_ (delayed)_ack
tcp_ack
tcp_data
tcp_data _queue
sk->data_ready
Fast Path
Slow Path
send
Pure ACK
Retrans. Timer
TCP
Section 24.3
Fall 2004
FSU CIS 5930 Internet Protocols

7. Handling incoming TCP segments
Ip dispatches packets based on protocol
tcp_v4_rcv()
Some sanity check, drop packet if necessary
Looking for proper sock (tcp_v4_lookup())
If found, continue with tcp_v4_do_rcv()
Else, send RESET segment (tcp_send_reset())
Fall 2004
FSU CIS 5930 Internet Protocols

8. FSU CIS 5930 Internet Protocols
tcp_v4_do_rcv()
Processing segment depending on socket state (connection state)
TCP_ESTABLISHED (tcp_rcv_established())
Other states (tcp_rcv_state_process())
tcp_rcv_established()
To speed up segment processing
Fast path vs. Slow path
Fall 2004
FSU CIS 5930 Internet Protocols

9. FSU CIS 5930 Internet Protocols
Fast path vs. Slow path
FP for “normal” TCP segments
A pure ACK segment
The expected packet (next packet in stream)
SP for other segments
Unexpected TCP flags
Not a packet expected (out-of-order)
Both parties exchange data
Window size is zero
Unexpected TCP options
Fall 2004
FSU CIS 5930 Internet Protocols

10. FSU CIS 5930 Internet Protocols
Fast path
Some sanity check
If ACK segment
Processing with tcp_ack()
Releasing socket buffer
Checking if local data can be sent (tcp_data_snd_check())
If data segment
If can be copied to user process,
copy payload
Update expected packet
Fall 2004
FSU CIS 5930 Internet Protocols

11. FSU CIS 5930 Internet Protocols
Fast path (cont’d)
Otherwise (cannot copied to user process)
Inserting into sock queue
Updating expected packet
Some management processing (tcp_event_data_rcv())
Checking if acknowledgement has to sent
Delayed ACK vs. Quick ACK
Fall 2004
FSU CIS 5930 Internet Protocols

12. FSU CIS 5930 Internet Protocols
Slow path
Many many different processing conditions
Checking code
Some functions for handling incoming packets
tcp_ack()
tcp_event_data_recv()
tcp_data_snd_check()
tcp_rcv_state_process()
Fall 2004
FSU CIS 5930 Internet Protocols

13. FSU CIS 5930 Internet Protocols
Sending TCP segments
tcp_sendmsg()
Copying data from user space to kernel
Checking if connection established
If not, wait_for_tcp_connect()
Checking if data can be sent
tcp_send_skb()
Sending data
__tcp_push_pending_frame()
Fall 2004
FSU CIS 5930 Internet Protocols

14. FSU CIS 5930 Internet Protocols
tcp_send_skb()
Adding data into transmit queue
sk->write_queue
Testing if data can be sent
tcp_snd_test()
If positive, send data
tcp_transmit_skb()
Starting retransmission timer
tcp_reset_xmit_timer()
Fall 2004
FSU CIS 5930 Internet Protocols

15. FSU CIS 5930 Internet Protocols
tcp_transmit_skb()
Some processing depending on if ACK set
Passing packet toIP
tp->af_specific->queue_xmit()
tp_queue_xmit()
Adjusting slow-start threshold
Fall 2004
FSU CIS 5930 Internet Protocols

16. tcp_push_pending_frame()
Checking if we can send data
tcp_snd_test()
Sending data
tcp_write_xmit()
Checking sending conditions (slow-start, congestion windows etc)
Fragment data if necessary
Sending data (tcp_transmit_skb())
Fall 2004
FSU CIS 5930 Internet Protocols