ATM

ATM Background

1990s, Asynchronous Transfer Mode (as opposed to SONET)
designed for telephony: real-time plus data, short- or long-distance, high reliability
53-byte cells
ATM Forum: telcos + networking companies
UNI (User-Network Interface) between hosts and switches
NNI (Network-Network Interface) between switches (P-NNI within a private network)
in-order delivery, low loss

53 bytes = 424 bits
cells sent when ready (Asynchronous Transfer Mode)
5 byte header + 48 byte payload
Header (40 bits):
- GFC (4 bits; UNI only),
- VPI (8 bits/UNI, 12 bits/NNI),
- VCI (16),
- Payload Type (3),
- Cell Loss Priority (1),
- Header Error Correction (CRC, 1)
No sequence numbers (in-order delivery), payload CRC, addresses, or payload length

Adaptation Layer:
- AAL 1: constant-bit-rate (CBR) traffic
- AAL 3/4: data (full service)
- AAL 5: data (simpler)
AAL Structure:
1. Segmentation and Reassembly (cell-level services)
2. Convergence (message-level services)
Service Data Unit (SDU) is a packet
Only done by end-systems, not switches

Segmentation:
- SDU from the higher layer
- add 4-byte header (not used), 0-47 byte pad, length and CRC
- break it into 48-byte payloads
- add 5-byte ATM headers
- send
Reassembly:
- receive 53-byte cells in order
- check HEC and strip header
- reassemble
- when SDU complete (specified by bit in header), check length/CRC, then deliver

like a telephone connection, but more efficient: statistical multiplexing
A transmission link may carry a number of virtual paths (VPs), each identified by a VPI
A virtual path may carry a number of virtual channels, each identified by a VCI
The combination of (link, VPI, VCI) uniquely identifies a virtual circuit

cell incoming on a link l has VPI and VCI
Switch has table from incoming (l, VPI, VCI) to outgoing (l', VPI', VCI')
to route cells, change VPI to VPI' and VCI to VCI', and places the cell on l' (for path, leave VCI unchanged)
multicast is easy
in-class exercise: given this table, forward cell from link 5, VPI=33, VCI=7

Quality of Service: what the network provides to the connection
Traffic Descriptor: what the host provides to the network (traffic contract)
Service classes:
- Constant bit-rate (CBR)
- Variable bit-rate (VBR): Real-Time (RT), Non-Real-Time(NRT)
- Available bit-rate (ABR)
- Unspecified bit-rate (UBR)
QoS parameters include maximum Cell Delay Variation (CDV) and Cell Loss Ratio (CLR)

Dual Leaky Bucket, in parallel (any overflow is tagged by Cell Loss Priority)
Each Leaky Bucket has:
- maximum rate R (cells/s)
- capacity C (cells)
Peak rate leaky bucket: PCR (Peak Cell Rate), CDVT (Cell Delay Variation Tolerance)
Mean rate leaky bucket: SCR (Sustained Cell Rate), BT (Burst Tolerance)

20-byte ATM addresses (telephone number superset, hierarchical)
once connected, transfer (including multicast) is easy
how to find an ATM address given a higher-level address?
- broadcast every message to everyone in the Virtual LAN (VPN) -- this is LAN emulation, LANE
- configure end systems with the ATM address of an address server. The server records my addresses when I connect, and provides translations of others' addresses. Subsequent transfers occur over a regular ATM connection -- CLassical IP over ATM, CLIP

a computer with multiple interfaces that is programmed to move packets from one interface to another is a packet switch
some interfaces are connected to computers, some are connected to long-distance networks (Wide-Area Network, WAN)
the same packet switch (hardware/software combination) can generally be used for many different specific networks, usually with some configuration required
the packet switch stores each packet in its memory, then forwards it on the appropriate interface: store-and-forward
packets are saved in queues until the outgoing line becomes available

a switch forwarding a packet needs to know where to send it:
- a switch does not have a map of the network, or even if it does, it would take too long to search for the destination
- a switch keeps a table of destinations and the interface that can be used to reach that destination
- this table is the routing table, and this switch is a router
every possible destination must be in every routing table: universal routing
the routing table should show the shortest path to each destination: optimal routing
independent of source address

if addresses are hierarchically assigned based on a computer's packet switch, the routing table only needs to store the addresses of packet switches (and directly-connected computers)
if a switch (or host) sends most of its traffic to a particular router, perhaps because it is the only router it is connected to, that router can be the default router, requiring only one routing table entry to cover all destinations not otherwise listed

networks can be represented by graphs where each node of the graph is a switch (or host) in the network, and each edge of the graph is a physical connection in the network
the path taken by a packet in the network is a path (that is, an ordered set of edges) in the graph
if my route for destination D sends packets to S, and S's route to D sends packets to me, I have a routing loop because my routing table is inconsistent with S's
routing tables are constructed:
- statically (configuration)
- dynamically (routing algorithm)

for all n in N' set d(n) = oo
the set of nodes with known distances is K: initially, K = {}.
loop: compute the distance for each node n in N' - K: d(n) = min(d(n), min_{n' in N", l in (n' * n)}(d(n')+m(l)) the set N" is: N" = N' - K - n
find the node n in (N' - K) with minimum distance d(n), add the node to K
if N' = K, we are done, otherwise return to step "loop"

node distances (d(n)) can only decrease, never increase
if a path to n goes through n', then at the end d(n) > d(n'), since for all l, m(l) > 0
if a node n is chosen for K, d(n) <= the distance of all nodes still in N - K
therefore, the distance of all nodes in K is less than or equal to the distance of each node in N - K
therefore, the distance of all nodes in K can never decrease beyond the current value of d(n)
therefore, the distance of all nodes in K is the shortest path distance