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CSCI 5273 Computer Networks Stevens, Chapter 2 Link Layer

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Encapsulation of an IP Datagram in different link layers ... RFC 893 describes 'trailer encapsulation' The IP and TCP headers move to the end of the frame ... – PowerPoint PPT presentation

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Title: CSCI 5273 Computer Networks Stevens, Chapter 2 Link Layer


1
CSCI 5273Computer NetworksStevens, Chapter
2Link Layer
  • Dirk GrunwaldAssoc. ProfessorDept. of Computer
    ScienceUniversity of Colorado, Boulder

2
Whats Covered
  • Encapsulation of an IP Datagram in different link
    layers
  • Describes link layer for Ethernet 802.3
  • Describes link layer for serial protocols
    (SLIP/PPP)
  • Describes link layer for Loopback
  • Structure in BSD O/S and implications
  • Describes the Maximum Transmission Unit and how
    MTUs are set.

3
Standards
  • Ethernet defined by Digital, Xerox and Intel
  • Later, the IEEE published a different set of
    standards
  • http//grouper.ieee.org/groups/802/
  • 802 defines a logical link control common to
    all 802 nets
  • 802.3 covers many CSMA/CD networks
  • 802.4 covers token bus networks
  • 802.5 covers token ring networks
  • 802.11 covers wireless ethernet

4
Standards
  • In the IP world,
  • RFC 894 defines IP-in-ethernet
  • RFC 1042 defines IP-in-802
  • The host requirements RFC says that all hosts
    connected to 10-Mbit Ethernet cable should
  • Be able to send/receive using RFC 894
  • Be able to send/receive a mix of RFC 1042 and 894
    packets
  • May be able to send packets using RFC 1042. If
    either can be sent, you must default to 894
    packets

5
Ethernet 802.3 Encapsulation
  • Destination MAC or hardware address
  • Each NIC has a unique hardware address
  • Source MAC or hardware address
  • Protocol type to allow sharing the same physical
    media with several different protocols
  • Type fields are defined by RFC 1700, which makes
    RFC 1340 obsolete
  • Some data
  • A checksum

6
Ethernet Encapsulation (RFC 894)
6
6
2
46-1500 bytes
4
...
DestAddr.
SrcAddr.
Type
CRC
Payload
0800
IP Datagram
46-1500 bytes
0806
ARP request/reply
PAD
28 bytes
18 bytes
8035
RARP request/reply
PAD
7
Variations
  • Observation
  • Ethernet MAC information is fixed and can be
    pre-computed
  • Data is typically fixed size
  • Other fields (IP and TCP headers) can vary in
    size and also have CRC fields for end-to-end IP
    checksums
  • RFC 893 describes trailer encapsulation
  • The IP and TCP headers move to the end of the
    frame
  • Helps in computing IP checksum
  • Allows more efficient use of scatter/gather DMA
    hardware

8
802.3 Encapsulation
  • Explicit length - number of bytes up to but not
    including the CRC
  • 802.2 LLC - link layer control common to all 802
    networks and needed for e.g. wireless
    communication
  • DSAP - desination service access point (0xaa)
  • SSAP - source service access point (0xaa)
  • Control field is set to 3
  • 802.2 SNAP - sub-network access protocol
  • Fixed origin code (0)
  • Type field, as in the Ethernet type field

9
802.3 Encapsulation
802.2LLC
802.2SNAP
802.3 MAC
DSAP AA
Control
SSAP AA
Payload has same format as Ethernet encapsulation
10
SLIP - Serial Line IP
  • Specified in RFC 1055
  • IP datagram is terminated by the special END
    (0xc0) character. Most implementations transmit
    END at the start as well.
  • If a byte in the IP datagram contains END, the 2
    byte sequence 0xdb, 0xdc is transmitted (byte
    stuffing).
  • 0xdb is the SLIP escape (ESC) character.
  • If a byte in the IP datagram equals the SLIP ESC,
    the 2 byte sequence 0xdb, 0xdd is transmitted

11
SLIP Encapsulation w/Byte Stuffing
IP Datagram
C0
DB
DB
DC
DB
DD
C0
12
Problems with SLIP
  • Each endpoint must know the IP address of the
    other endpoint.
  • Theres no TYPE field -- thus, SLIP only supports
    a single protocol
  • Theres no checksum - thus, all retransmissions
    are initiated by end-to-end re-transmissions

13
CSLIP - Compressed SLIP
  • Delivering one byte of data requires
  • 20 byte IP header
  • 20 byte TCP header
  • CSLIP RFC 1144 reduces overhead to 3 or 5 bytes
  • Maintains state of up to 16 TCP connections at
    each end
  • shadows some of the fields in each connection,
    and only updates components when they change

14
CSLIP Compression (stylized)
  • SLIP just sends IP packets
  • But, we tend to talk to a small number of
    connections at any one time...
  • Remember redundant information for each
    connection
  • Both sender and receiver agree on the meaning of
    each connection
  • Add a message type header. E.g.
  • 0/N/data setup compressed IP information for
    connection N
  • 1/N/data Send data to connection N

15
CSLIP example (stylized)
Host AMap connection 1 toSRC128.138.240.10D
ST128.138.241.78Version4, TOS0x10,
...Send ltip1gt to connection 1decode ltip2gt
Host BOkMap connection 2
toSRC128.138.241.78DST 128.138.240.10Version
4, TOS0x10ltdecode ip1gtSend ltip2gt to
connection 2
16
PPP - Point-to-Point Protocol
  • Encapsulate IP datagrams on a serial link
  • A Link Control Protocol (LCP) to establish,
    configure and test the data-link connection.
  • This allows connection feature negotiation
  • A family of Network Control Protocols specific to
    different network layer protocols
  • IP
  • OSI networks (X.25)
  • DECnet
  • AppleTalk

17
PPP Protocol
FLAG
Addr
Cntl
Proto
Payload
CRC
FLAG
  • Byte stuffing as in SLIP/CSLIP protocol
  • Bytes with values less than 0x20 are also escaped
    to avoid problems with flow-control
  • Most implementations can negotiate to eliminate
    ADDR and CNTL fields, reducing overhead to 1 byte.

18
MTU
  • Most link layers have a limit to the size of an
    IP datagram, or Message Transmission Unit (MTU)
  • If an IP datagram gt MTU, then it is fragmented
    (Chap 11.5)

Network MTU (bytes)Hyperchannel 6553516Mb token
ring 179144MB token ring 4464FDDI 4352Ethernet
1500IEEE 802.3 1492X.25 576PPP 296
19
Path MTU
  • Messages traverse a route or path through a
    network.
  • The smallest MTU along that path is called the
    Path MTU.
  • Not always constant, since the route between two
    nodes in the network can vary
  • Also, routing isnt a bijective relationship, and
    thus the A-gtB MTU may differ from the B-gtA MTU
  • RFC 1191 defines path MTU discovery.
  • Well discuss in 11.6 11.7
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