The Data Link Layer

Data Link Layer Design Issues

• Services Provided to the Network Layer

• Framing

• Error Control

• Flow Control

Functions of the Data Link Layer

• Provide service interface to the network layer

• Dealing with transmission errors

• Regulating data flow• Slow receivers not swamped by fast senders

Functions of the Data Link Layer (2)

Relationship between packets and frames.

Services Provided to Network Layer

(a) Virtual communication.(b) Actual communication.

Services Provided to Network Layer (2)

Placement of the data link protocol.

Framing

A character stream. (a) Without errors. (b) With one error.

Framing (2)

(a) A frame delimited by flag bytes.(b) Four examples of byte sequences before and after stuffing.

Framing (3)

Bit stuffing

(a) The original data.

(b) The data as they appear on the line.

(c) The data as they are stored in receiver’s memory after destuffing.

Sliding Window Protocols

• A One-Bit Sliding Window Protocol

• A Protocol Using Go Back N

• A Protocol Using Selective Repeat

Sliding Window Protocols (2)

A sliding window of size 1, with a 3-bit sequence number.

(a) Initially.

(b) After the first frame has been sent.

(c) After the first frame has been received.

(d) After the first acknowledgement has been received.

A Protocol Using Go Back N

Pipelining and error recovery. Effect on an error when

(a) Receiver’s window size is 1.

(b) Receiver’s window size is large.

Go-Back-N

Sender:a) k-bit seq # in pkt header

b) “window” of up to N, consecutive unack’ed pkts allowed

a) ACK(n): ACKs all pkts up to, including seq # n - “cumulative ACK”– may deceive duplicate ACKs (see receiver)

b) timer for each in-flight pkt

c) timeout(n): retransmit pkt n and all higher seq # pkts in window

GBN: sender extended FSM

Wait start_timerudt_send(sndpkt[base])udt_send(sndpkt[base+1])…udt_send(sndpkt[nextseqnum-1])

timeout

rdt_send(data)

if (nextseqnum < base+N) { sndpkt[nextseqnum] = make_pkt(nextseqnum,data,chksum) udt_send(sndpkt[nextseqnum]) if (base == nextseqnum) start_timer nextseqnum++ }else refuse_data(data)

base = getacknum(rcvpkt)+1If (base == nextseqnum) stop_timer else start_timer

rdt_rcv(rcvpkt) && notcorrupt(rcvpkt)

base=1nextseqnum=1

rdt_rcv(rcvpkt) && corrupt(rcvpkt)

GBN: receiver extended FSM

ACK-only: always send ACK for correctly-received pkt with highest in-order seq #– may generate duplicate ACKs

– need only remember expectedseqnum

a) out-of-order pkt: – discard (don’t buffer) -> no receiver buffering!

– Re-ACK pkt with highest in-order seq #

Wait

udt_send(sndpkt)

default

rdt_rcv(rcvpkt) && notcurrupt(rcvpkt) && hasseqnum(rcvpkt,expectedseqnum)

extract(rcvpkt,data)deliver_data(data)sndpkt = make_pkt(expectedseqnum,ACK,chksum)udt_send(sndpkt)expectedseqnum++

expectedseqnum=1sndpkt = make_pkt(expectedseqnum,ACK,chksum)

GBN inaction

Selective Repeat

a) receiver individually acknowledges all correctly received pkts– buffers pkts, as needed, for eventual in-order delivery to upper layer

b) sender only resends pkts for which ACK not received– sender timer for each unACKed pkt

c) sender window– N consecutive seq #’s

– again limits seq #s of sent, unACKed pkts

Selective repeat: sender, receiver windows

Selective repeat in action

Selective repeat: dilemma

Example: a) seq #’s: 0, 1, 2, 3

b) window size=3

c) receiver sees no difference in two scenarios!

d) incorrectly passes duplicate data as new in (a)

Q: what relationship between seq # size and window size?

Example Data Link Protocols

• HDLC – High-Level Data Link Control

• The Data Link Layer in the Internet

High-Level Data Link Control

Frame format for bit-oriented protocols.

High-Level Data Link Control (2)

Control field of

(a) An information frame.

(b) A supervisory frame.

(c) An unnumbered frame.

The Data Link Layer in the Internet

A home personal computer acting as an internet host.

PPP – Point to Point Protocol

The PPP full frame format for unnumbered mode operation.

PPP Design Requirements [RFC 1557]

a) packet framing: encapsulation of network-layer datagram in data link frame

– carry network layer data of any network layer protocol (not just IP) at same time

– ability to demultiplex upwards

b) bit transparency: must carry any bit pattern in the data field

c) error detection (no correction)

d) connection liveness: detect, signal link failure to network layer

e) network layer address negotiation: endpoint can learn/configure each other’s network address

PPP non-requirements

a) no error correction/recovery

b) no flow control

c) out of order delivery OK

d) no need to support multipoint links (e.g., polling)

Error recovery, flow control, data re-ordering all relegated to higher layers!

PPP Data Frame

a) Flag: delimiter (framing)

b) Address: does nothing (only one option)

c) Control: does nothing; in the future possible multiple control fields

d) Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

PPP Data Frame

a) info: upper layer data being carried

b) check: cyclic redundancy check for error detection

Byte Stuffing

a) “data transparency” requirement: data field must be allowed to include flag pattern <01111110>

– Q: is received <01111110> data or flag?

b) Sender: adds (“stuffs”) extra < 01111110> byte after each < 01111110> data byte

c) Receiver:

– two 01111110 bytes in a row: discard first byte, continue data reception

– single 01111110: flag byte

Byte Stuffing

flag bytepatternin datato send

flag byte pattern plusstuffed byte in transmitted data

Other Topics

a) Error recovery. Parity checking. Hamming distance.

b) Maximal channel utilization rate.

c) HDLC protocol (in more details).