The IPv4 Protocol Family¶

Overview¶

The IP protocol is the workhorse protocol of the TCP/IP protocol suite. All UDP, TCP, ICMP packets are encapsulated into IP datagrams and transported by the IP layer. While higher layer protocols transfer data among two communication end-point, the IP layer provides an hop-by-hop, unreliable and connectionless delivery service. IP does not maintain any state information about the individual datagrams, each datagram handled independently.

The nodes that are connected to the Internet can be either a host or a router. The hosts can send and recieve IP datagrams, and their operating system implements the full TCP/IP stack including the transport layer. On the other hand, routers have more than one interface cards and perform packet routing between the connected networks. Routers does not need the transport layer, they work on the IP level only. The division between routers and hosts is not strict, because if a host have several interfaces, they can usually be configured to operate as a router too.

Each node on the Internet has a unique IP address. IP datagrams contain the IP address of the destination. The task of the routers is to find out the IP address of the next hop on the local network, and forward the packet to it. Sometimes the datagram is larger, than the maximum datagram that can be sent on the link (e.g. Ethernet has an 1500 bytes limit.). In this case the datagram is split into fragments and each fragment is transmitted independently. The destination host must collect all fragments, and assemble the datagram, before sending up the data to the transport layer.

The INET framework contains several modules to build the IPv4 network layer of hosts and routers:

Ipv4 is the main module that implements RFC791. This module performs IP encapsulation/decapsulation, fragmentation and assembly, and routing of IP datagrams.
The Ipv4RoutingTable is a helper module that manages the routing table of the node. It is queried by the Ipv4 module for best routes, and updated by the routing daemons implementing RIP, OSPF, Manet, etc. protocols.
The Icmp module can be used to generate ICMP error packets. It also supports ICMP echo applications.
The Arp module performs the dynamic translation of IP addresses to MAC addresses.
The Igmpv2 module to generate and process multicast group membership reports.

These modules are assembled into a complete network layer module called Ipv4NetworkLayer. The Ipv4NetworkLayer module is present e.g. in StandardHost and Router.

The subsequent sections describe the IPv4 modules in detail.

Ipv4¶

The Ipv4 module implements the IPv4 protocol.

Its parameters include:

crcMode TODO: @enum(“declared”, “computed”) = default(“declared”);
procDelay processing time of each incoming datagram.
timeToLive default TTL of unicast datagrams.
multicastTimeToLive default TTL of multicast datagrams.
fragmentTimeout the maximum duration until fragments are kept in the fragment buffer.
limitedBroadcast if true, then link-local broadcast datagrams are sent out through each interface, if the higher layer did not specify the outgoing interface.
useProxyARP TODO: default(true);

Ipv4RoutingTable¶

The Ipv4RoutingTable module represents the IPv4 route table. Hosts and routers normally contain one instance of this module. The Ipv4RoutingTable module does not send or receive messages. Instead, C++ methods are for querying and updating the table, as well as for unicast and multicast routing.

The Ipv4RoutingTable module has the following parameters:

routerId: for routers, the router id using IPv4 address dotted notation; specify “auto” to select the highest interface address; should be left empty for hosts.
forwarding: turns IP forwarding on/off. It is always true in a Router and is false by default in a StandardHost.
multicastForwarding: turns multicast IP forwarding on/off. Default is false, should be set to true in multicast routers.

The preferred method for static initialization of routing tables is to use Ipv4NetworkConfigurator. While Ipv4RoutingTable can read the routes from a routing file, that is considered obsolete. Old routing files should be replaced with the XML configuration of Ipv4NetworkConfigurator. The Network Autoconfiguration chapter describes the format of the new configuration files.

Icmp¶

The Icmp module implements the Internet Control Message Protocol (ICMP). ICMP is the error reporting and diagnostic mechanism of the Internet. It uses the services of IP, so it is a transport layer protocol, but unlike TCP or UDP it is not used to transfer user data. It cannot be separated from IP, because the routing errors are reported by ICMP.

The Icmp module can be used to send error messages and ping request. It can also respond to incoming ICMP messages.

Each ICMP message is encapsulated within an IP datagram, so its delivery is unreliable.

Arp¶

The Arp module implements the Address Resolution Protocol (ARP). The ARP protocol is designed to translate a local protocol address to a hardware address. Although the ARP protocol can be used with several network protocol and hardware addressing schemes, in practice they are almost always IPv4 and 802.3 addresses. The Arp module only supports IPv4-to-MAC address translation, but not the opposite direction, Reverse ARP (RARP).

The address to be resolved can be either an IPv4 broadcast/multicast or a unicast address. The corresponding MAC addresses can be computed for broadcast and multicast addresses (RFC 1122, 6.4); unicast addresses are resolved using the ARP procotol.

If the MAC address is found in the ARP cache, then the packet is transmitted to the addressed interface immediately. Otherwise the packet is queued and an address resolution takes place.

For address resolution, ARP broadcasts a request frame on the network. In the request it publishes its own IP and MAC addresses, so each node in the local subnet can update their mapping. The node whose MAC address was requested will respond with an ARP frame containing its own MAC address directly to the node that sent the request. When the original node receives the ARP response, it updates its ARP cache and sends the delayed IP packet using the learned MAC address.

ARP resolution is initiated with a C++ call.

The module parameters of Arp are:

retryTimeout: number of seconds ARP waits between retries to resolve an IPv4 address (default is 1s)
retryCount: number of times ARP will attempt to resolve an IPv4 address (default is 3)
cacheTimeout: number of seconds unused entries in the cache will time out (default is 120s)
proxyARP: enables proxy ARP mode (default is true)
globalARP: use global ARP cache (default is false)

Igmp¶

The Igmpv3 module implements the Internet Group Management Protocol (IGMP). IGMP is a communications protocol used by hosts and adjacent routers on IPv4 networks to establish multicast group memberships. IGMP is an integral part of IP multicast.

IGMP is responsible for distributing the information of multicast group memberships from hosts to routers. When an interface of a host joins to a multicast group, it will send an IGMP report on that interface to routers. It can also send reports when the interface leaves the multicast group, so it does not want to receive those multicast datagrams. The IGMP module of multicast routers processes these IGMP reports: it updates the list of groups, that has members on the link of the incoming message.

The IIgmp module interface defines the connections of IGMP modules. IGMP reports are transmitted by IP, so the module contains gates to be connected to the IP module (ipIn/ipOut). The IP module delivers packets with protocol number 2 to the IGMP module. However some multicast routing protocols (like DVMRP) also exchange routing information by sending IGMP messages, so they should be connected to the routerIn/routerOut gates of the IGMP module. The IGMP module delivers the IGMP messages not processed by itself to the connected routing module.

The Igmpv2 module implements version 2 of the IGMP protocol (RFC 2236). Next we describe its behaviour in host and routers in details. Note that multicast routers behaves as hosts too, i.e. they are sending reports to other routers when joining or leaving a multicast group.

Host behaviour¶

When an interface joins to a multicast group, the host will send a Membership Report immediately to the group address. This report is repeated after unsolicitedReportInterval to cover the possibility of the first report being lost.

When a host’s interface leaves a multicast group, and it was the last host that sent a Membership Report for that group, it will send a Leave Group message to the all-routers multicast group (224.0.0.2).

This module also responds to IGMP Queries. When the host receives a Group-Specific Query on an interface that belongs to that group, then it will set a timer to a random value between 0 and Max Response Time of the Query. If the timer expires before the host observe a Membership Report sent by other hosts, then the host sends an IGMPv2 Membership Report. When the host receives a General Query on an interface, a timer is initialized and a report is sent for each group membership of the interface.

Router behaviour¶

Multicast routers maintains a list for each interface containing the multicast groups that have listeners on that interface. This list is updated when IGMP Membership Reports and Leave Group messages arrive, or when a timer expires since the last Query.

When multiple routers are connected to the same link, the one with the smallest IP address will be the Querier. When other routers observe that they are Non-Queriers (by receiving an IGMP Query with a lower source address), they stop sending IGMP Queries until otherQuerierPresentInterval elapsed since the last received query.

Routers periodically (queryInterval) send a General Query on each attached network for which this router is a Querier. On startup the router sends startupQueryCount queries separated by startupQueryInterval. A General Query has unspecified Group Address field, a Max Response Time field set to queryResponseInterval, and is sent to the all-systems multicast address (224.0.0.1).

When a router receives a Membership Report, it will add the reported group to the list of multicast group memberships. At the same time it will set a timer for the membership to groupMembershipInterval. Repeated reports restart the timer. If the timer expires, the router assumes that the group has no local members, and multicast traffic is no more forwarded to that interface.

When a Querier receives a Leave Group message for a group, it sends a Group-Specific Query to the group being left. It repeats the Query lastMemberQueryCount times in separated by lastMemberQueryInterval until a Membership Report is received. If no Report received, then the router assumes that the group has no local members.

Parameters¶

The following parameters have effects in both hosts and routers:

enabled if false then the IGMP module never sends any message and discards incoming messages. Default is true.

The following parameters are only used in hosts:

unsolicitedReportInterval the time between repetitions of a host’s initial report of membership in a group. Default is 10s.

Router timeouts are configured by these parameters:

robustnessVariable the IGMP is robust to robustnessVariable-1 packet losses. Default is 2.
queryInterval the interval between General Queries sent by a Querier. Default is 125s.
queryResponseInterval the Max Response Time inserted into General Queries
groupMembershipInterval the amount of time that must pass before a multicast router decides there are no more members of a group on a network. Fixed to robustnessVariable * queryInterval + queryResponseInterval.
otherQuerierPresentInterval the length of time that must pass before a multicast router decides that there is no longer another multicast router which should be the querier. Fixed to robustnessVariable * queryInterval + queryResponseInterval / 2.
startupQueryInterval the interval between General Queries sent by a Querier on startup. Default is queryInterval / 4.
startupQueryCount the number of Queries sent out on startup, separated by the startupQueryInterval. Default is robustnessVariable.
lastMemberQueryInterval the Max Response Time inserted into Group-Specific Queries sent in response to Leave Group messages, and is also the amount of time between Group-Specific Query messages. Default is 1s.
lastMemberQueryCount the number of Group-Specific Queries sent before the router assumes there are no local members. Default is robustnessVariable.