IPv4 Header of Datagram

Notes:

• IP checksum is computed with only the IP header, not the whole datagram.
• TTL refers to the number of hops to destination.
• The payload is a Transport layer TCP segment or UDP datagram.

Flag: 1 if there’s a next segment, otherwise 0

Lenght: length of entire IP fragment (header + payload)

Offset: offset from start of original datagram in 8-bytes (sets of 8 bytes)

MTU (Max Transfer Unit): The maximum amount of data (bytes) a link-level frame can carry.

Checksum: same checksum as in transport layer

Why fragment?

Different link layers have different physically determined Maximum Transfer Units (MTUs) – the max amount a link level frame can carry.

From 1 IP header + 1 payload $\rightarrow$ 3 $\times$ (IP header + $\frac{1}{3}$ payload).

• IP datagram = IP header + payload.
• Fragment offset: in units of 8-bytes (since flags take up 3 bytes).
• Frag Flag: 1 if next fragment from same segment, 0 otherwise.

Network Address Translation

Public IP addresses are globally unique and routable.

Private IP addresses are routable within an organization/home etc, but not globally.

• 10.0.0.0/8
• 172.16.0.0/16
• 192.168.0.0/16

Serving a packet from a Private IP address to the internet will fail (ISP will never).

All $n$ local area network (LAN) addresses $\rightarrow$ NAT $\rightarrow$ 1 public address to the wide area network (WAN).

• Outgoing datagram: Map (source ip, port num) to (NAT ip, new port num).
• Caching this mapping in NAT translation table
• Incoming datagram: Map (NAT ip, port num) to (source ip, orig port num)

Problems:

• p2p applications will not work directly on two local connections.
  • The two NAT devices cannot find each other when they only know each other’s local addresses.

IPv6

Same as IPv4 in header structure, except source, destination addresses are now 128-bit. Header is 40-byte.

Routing

Routing between Autonomous Systems (ASs) in the internet (network of networks) is done hierarchically.

AS: Mini internet under one organization’s control.

Intra-AS

• Single admin
• No policy decisions
• Performance priority
• Ignore hosts; view only routers and the links between them.
• Least cost path between source and destination routers.
  • Shortest path? Congestion level? Money?

Inter-AS routing.

• Multiple admins
• Policy priority

Routing algos

Link state algorithms: all routers know the whole network topology (graph) and link cost (edge weight).

• Can use Dijkstra’s Algorithm.
• Routers periodically broadcast links to each other
• A lot of broadcasting global information between each other, so not used cos too costly.

Distance vector algorithms:

• routers know immediate neighbours only.
• Exchange of “local views” and update its own “local views”.
• Iterative
  • Swap local view with neighbours
  • Update local view
  • Repeat until no change in local view.

Bellman-Ford

Let $d_x(y)$ be the least cost path from $x$ to $y$.

Let $c(x,y)$ be the cost of the direct link between nodes $x$ and $y$. $c(x,y) = \infty$ if $x, y$ are not neighbours.

n = V.length
for i in range(0, n):
  for edge in graph:
    relax(edge)

Key property: If P is the shortest path from S to D, and node X is on P, then P must be the shortest path from S to X $\cup$ shortest path from X to D.

Distributed: Each node receive info from neighbours, recompute and distributes again

Iterative: Continues until two iterations are the same (no more information)

Asynchronous: Nodes do not have to compute in a certain order.

Routing Information Protocol (RIP)

• Hop count as metric, using the distance-vector (DV) protocol with BF algorithm.
• Entries in routing table: subnet masks
• Exchange routing table updates every 30s
• If no update <3min, assume neighbour failed

Internet Control Message Protocol (ICMP)

ICMP are carriedin IP datagrams.

Contained after IP header (IP header - ICMP header - data).

Header: Type + Code (for specific status) + Checksum

Examples

• Echo request reply (ping).
• Use of ping and TTL to create traceroute: