Computer Networks Week 4   Corboy Law 522


Read:
Chapter 1: 1.1-1.3, 1.5
Chapter 2: 2.1-2.6, 2.8.2 (wi-fi)

 

2.6: Ethernet

See also my notes (link on course home page).
 
Ethernet: Read section 2.6 on Ethernet
Logical: point-to-point
Physical: broadcast bus (not counting switching)

Packet format:
destaddr
6 bytes
 srcaddr
6 bytes
 type
2 bytes
 data
min: 42 bytes (data is padded as necessary)  max: 1500 bytes
 crc-32
4 bytes


The Network Interface (NI, or Ethernet card) interrupts CPU if any of the following apply:
That covers RECEIVING; what about SENDING?

Why we need a TYPE field

Similarities to 802.11 wifi packet format: srcaddr/destaddr/type are the same; wifi radio headers have additional fields for the associated access point.
             

     
Ethernet (addresses, collisions, performance)
traditional broadcast-bus; role of hub
True eavesdropping story:
In 1994 I changed the admin password on several remote unix machines, using telnet. I told no one. Within two hours, someone else logged into one of the remote machines, using the new password, from inria.fr (then rife with hackers, as I suppose was Loyola). Two months later was the Kevin Mitnick "Christmas Day Attack", launched from apollo.it.luc.edu.

                 
compare min-packet requirement to max packet size

repeaters

These are simple amplifiers. The original intent was to allow for longer segments, by providing enough amplification that the signal would still be strong enough to allow for collision detection at the remote end.

It was soon realized that multi-port repeaters allow a change in the geometry too. Multiport repeaters are often called hubs. Hubs are slowly being phased out in favor of switches.

Collisions and hubs: simple digital sensing
Collisions and switches: occur only if both ends want to transmit at the same time. This is relatively common during a busy file transfer, as the sender always has more data to send and the receiver has a steady supply of TCP Acks to send. However, the impact on overall throughput is minimal!

Two issues relating to cable length: 
                faintness of signal  (addressed by repeaters)
                window of opportunity for an undetected collision (related to max network diameter)
             
             
             
            scaling to 100Mbps; min packet revisited
            collisions and hubs
            collisions and switches
             
             

Exponential backoff algorithm

Stations transmit immediately when the line is free. This leads to a collision if we were waiting for the line to become free, and someone else was waiting also. This is not considered a problem, however; Ethernet collisions are considered to be a relatively inexpensive way of sorting out who gets to send next. Transmitting with probability 1 as soon as the line is clear is known as 1-persistence.

Ethernet defines the slot time to be 51.2 µsec:
           
After N collisions (including N=1):
  1. choose a random k, 0<=k<2N (choose an N-bit random k)
  2. wait k slot times
  3. try again to transmit. Options: idle/seize_channel, idle/collide, busy
Ethernet can be modeled as an alternating sequence of packet transmissions and contention intervals, where the latter can be subdivided into slot-time subintervals that are each idle or contain a collision. At low utilization, most of the contention interval may be idle, and the division into slots may be unnecessary. The interesting case, however, is when there is always at least one packet ready to send, in which case idle slots exist only because of random variation in the backoff.

In general, if M stations are waiting to transmit, it takes O(M) slot times (and O(log M) collisions) before one station succeeds. My informal simulations suggest that one station usually succeeds after M/2 slot times.

hidden bias against hosts that have been waiting longest:  "unfairness"
             
Timeline of typical exponential backoff
             
Ethernet myths re capacity
     
    

Ethernet v Wireless (wifi)

Both have exponential backoff. Wireless, however, cannot detect collisions in progress. This has to do with the relative signal strength of the remote signal at the local transmitter; along a wire-based Ethernet the remote signal might be as week as 1/100 of the transmitted signal but that 1% variation is still detectable. However, with radio the remote signal might be as week as 1/100,000 of the transmitted signal, and it is simply lost.

Recall that Ethernet uses the lack of a detected collision as evidence the packet was delivered successfully. Wifi can't do this, so it adds link-layer ACK packets (unrelated to the later TCP ACK), at least for unicast transmission. Although wifi cannot do collision detection, it does have a much smaller RTT (~1-2 µsec versus the official 51.2 µsec for Ethernet (even fast)). Wifi takes advantage of this by having the link-layer ACK sent only 10 µsec after the sender stops (802.11b/g). The next regular packet, on the other hand, waits ~50µsec. Because there is only one station that wants to send a link-layer ACK, this ACK will never collide with anything.

Wifi collisions, unlike Ethernet, are expensive. To deal with this, senders wait a full 50 µsec after first sensing the medium to be sure it is idle. If no other traffic is seen in this interval, the station may then transmit immediately. However, if other traffic is sensed (and, most likely, waited for), then the station must do an exponential backoff even for the first packet. Furthermore, the initial backoff is to choose k<25 (Ethernet in effect chooses an initial backoff of k<20 = 1; ie k=0).

Wifi stations optionally also use a request-to-send/clear-to-send (RTS/CTS) protocol. Usually they use this only for larger packets; often, the RTS/CTS "threshold" is set to be the maximum packet size, or is otherwise disabled.

One of the rationales for the RTS/CTS protocol is the "hidden node problem", P&D 139. If every station has a 100-meter range, and stations A and B are each 75 meters from C, and are arranged linearly in space as A---C---B, then A and B cannot hear each others' transmissions at all, not while they are themselves transmitting and not even when they are themselves idle. However, if A and B were to simultaneously transmit to C, then a collision would occur and C would receive nothing.

Ethernet BRIDGING

Why switching avoids collisions, mostly
     
Half-duplex: data flows in one direction at a time
Full-duplex: packets can be sent in opposite directions simultaneously; collision-free! This is usually implemented via two half-duplex lines, each with a dedicated direction.
     
Bridge Learning: first look
             


2.7: FDDI. Omit, except for brief discussion of token idea.
                Tokens
                Fairness, round-robin allocation
                uniform performance under heavy loads
 
 

Basics of Datagram Routing

     
        A--S1-----S2--D
           |       |
           |       |
        B--S3-----S4----S5---F
           |       |
           C       E
           
Basics of Adaptive (Learning) Bridges (Ethernet Switches)
 
Peterson & Davie 3.2:
        Bridges and Adaptive Bridges; cycles; scalability
        Bridges join separate physical ethernets.
        Packets are propagated, but collisions are not.
        Limit to total size: total traffic
                Limits to size: b'cast, table sizes (104 v. 106)
                Cannot use loop topology
                Delay (we don't want packets arriving late)
        bridges & security: other parties cannot listen in.
        
There is lots of debate in the networking community regarding the point at which one should convert from switching (bridging) to IP routing. IP routers are relatively slow, so there is some pressure to switch instead.

3.1.2: virtual circuit switching

The road not taken by IP.

In VC switching, routers know about end-to-end connections. To send a packet, a connection needs to be established first. For that connection, each link is assigned a "connection ID" (traditionally called the VCI, for Virtual Circuit Identifier). To send a packet, the host marks the packet with the VCI assigned to the host--router1 link.

More next week.