tcp fundamentals - core concepts and service model

learning objectives

after this guide, you will understand:

• tcp’s service model and core guarantees
• the tcp header structure and key fields
• how tcp differs from udp
• the byte-stream abstraction
• sequence and acknowledgment number semantics

why tcp exists

the internet protocol (ip) provides best-effort packet delivery - packets can be:

• lost (dropped by congested routers)
• duplicated (retransmission ambiguities)
• reordered (different paths through network)
• corrupted (bit errors)

many applications need reliable, ordered data delivery. tcp solves this by adding:

• sequencing - data bytes are numbered
• acknowledgments - receiver confirms receipt
• retransmission - lost data is resent
• flow control - receiver controls sender rate
• congestion control - adapts to network conditions

tcp service model

guarantees provided

what tcp does not provide

• message boundaries - application must add framing
• timing guarantees - no real-time delivery promises
• minimum throughput - best-effort bandwidth
• security - no encryption or authentication (use tls)

tcp vs udp comparison

tcp header structure

the tcp header is minimum 20 bytes, with up to 40 bytes of options:

0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

header fields explained

identification fields:

• source port (16 bits) - sender’s port number
• destination port (16 bits) - receiver’s port number

sequence control:

• sequence number (32 bits) - first byte’s position in stream
• acknowledgment number (32 bits) - next expected byte

control fields:

• data offset (4 bits) - header length in 32-bit words
• reserved (3 bits) - must be zero
• flags (9 bits) - control bits (see below)
• window (16 bits) - receive window size

error detection:

• checksum (16 bits) - covers header + data + pseudo-header
• urgent pointer (16 bits) - offset to urgent data

tcp flags

byte stream abstraction

tcp provides a byte stream service, not a message service:

what this means

1. no message boundaries - tcp doesn’t preserve application message boundaries
2. byte numbering - every byte has a sequence number
3. segmentation - tcp breaks stream into segments for transmission
4. reassembly - receiver reassembles segments into continuous stream

example

# application writes
send("Hello ")  # 6 bytes
send("World")   # 5 bytes

# tcp might transmit as:
# Segment 1: "Hello Wo"  (8 bytes)
# Segment 2: "rld"       (3 bytes)

# receiver gets continuous stream
recv(1024) -> "Hello World"  # all 11 bytes

implications for applications

• must add own message framing if needed
• can’t assume write boundaries match read boundaries
• may need to buffer partial messages

sequence and acknowledgment numbers

sequence numbers

• initial sequence number (isn) - randomly chosen at connection start
• byte position - each byte gets sequential number
• wrapping - 32-bit field wraps around at 2^32

acknowledgment semantics

• cumulative acks - ack number = next expected byte
• acknowledges all prior bytes - ack of 1000 means bytes 0-999 received
• piggybacking - acks carried with data segments

example exchange

Client                          Server
------                          ------
SEQ=1000, DATA="Hello" (5 bytes)  -->
                                   <-- ACK=1005
SEQ=1005, DATA="World" (5 bytes)  -->
                                   <-- ACK=1010

maximum segment size (mss)

what is mss

• maximum tcp payload in single segment
• negotiated during connection establishment
• typically derived from mtu minus headers

calculation

MTU (Maximum Transmission Unit)     = 1500 bytes (ethernet)
- IP header                          = 20 bytes
- TCP header                         = 20 bytes
----------------------------------------
MSS (Maximum Segment Size)          = 1460 bytes

path mtu discovery

• determines smallest mtu along path
• avoids ip fragmentation
• uses icmp “fragmentation needed” messages

tcp timers

tcp uses several timers to ensure reliability and performance:

typical tcp applications

well-suited for tcp

better with udp

practical implications

for developers

1. don’t assume message boundaries - add framing if needed
2. handle partial reads/writes - tcp is stream-oriented
3. tune socket buffers - affects throughput
4. consider nagle’s algorithm - may delay small writes
5. understand time-wait - affects rapid connection cycling

for operators

1. monitor retransmission rates - indicates network problems
2. check window sizes - may limit throughput
3. watch for syn floods - dos attack vector
4. tune kernel parameters - optimize for workload
5. understand connection states - aids troubleshooting

key takeaways

• tcp provides reliable, ordered byte-stream delivery over unreliable ip
• adds significant complexity for reliability guarantees
• header overhead and control mechanisms trade efficiency for reliability
• applications must understand byte-stream semantics
• choice between tcp and udp depends on application requirements

next steps

• explore connection lifecycle to understand establishment and termination
• study sequence and acknowledgment numbers for retransmission and acknowledgments
• examine sliding window protocol for receiver-paced transmission