What do Protocol Numbers in IP do?

FrankGuthrie

I'f learned that protocol number 88 is used for EIGRP, and number 89 for OSPF, but what is it function?

NetworkVeteran

If the IP address tells where to deliver a packet (e.g., "0001 Cemetery Lane"), the Protocol number tells who to deliver the packet to (e.g., "Morticia"). On a paper envelope, those are listed on separate lines, too!

The same interface theoretically could be running both EIGRP and OSPF.

FrankGuthrie

NetworkVeteran wrote: »

The same interface theoretically could be running both EIGRP and OSPF.

Yeah I guess the Thing also get mails, on the same address .

But who is the mailman in OSPF and EIGRP then? Because it (IP protocol number) is telling someone who it it destined for.

YFZblu

I know the transport layer gets drilled into our heads over time, but it's important to remember; IP is a connectionless protocol itself and even without TCP or UDP an IP datagram will reach its destination. So IP is the mailman in that situation.

My next question was, then why do we need UDP at all? The thing IP will not do is help with multiplexing and demultiplexing - meaning in order to distinguish between multiple services on, say, a server - we need those layer four port numbers. Of course, TCP provides the added benefit of guaranteed delivery by essentially keeping tabs on the session.

I'm still diving into the nitty-gritty of TCP/IP myself, I hope I understood your question correctly and didn't butcher the answer. Perhaps another poster can confirm my understanding.

Ivanjam

@FrankGuthrie - IP protocol numbers are used in a similar fashion to port numbers in TCP or UDP. Just as port numbers tells TCP/IP to direct datagrams to a specific upper OSI layer application such as HTTP, Skype or WoW, the protocol number tells TCP/IP to send the packet to a specific protocol running on top of a layer 3 protocol or at layer 4, such as ICMP, TCP, EIGRP, etc.

NetworkVeteran

My next question was, then why do we need UDP at all? The thing IP will not do is help with multiplexing and demultiplexing - meaning in order to distinguish between multiple services on, say, a server - we need those layer four port numbers.

"Why does RIP need UDP?" is a good question. The IP Protocol field allows some degree of multiplexing (255 values), but the UDP port field allows far more (65,535). And it's easier to reserve port numbers for multiplexing!

More essentially, UDP provides error detection services via a checksum.

FrankGuthrie

Ivanjam wrote: »

@FrankGuthrie - IP protocol numbers are used in a similar fashion to port numbers in TCP or UDP. Just as port numbers tells TCP/IP to direct datagrams to a specific upper OSI layer application such as HTTP, Skype or WoW, the protocol number tells TCP/IP to send the packet to a specific protocol running on top of a layer 3 protocol or at layer 4, such as ICMP, TCP, EIGRP, etc.

Hmmm interesting that you place EIGRP at Layer 4...

Also I always thought of TCP/UDP/Layer 4 as running on top op IP (Layer 3). This is how I see it in the OSI model and visualize it for myself.

So can I say that OSPF is running on top of Layer 3 as Layer 3,5??

Also if the protocol is used to tell the upper layer how to cope with it, what upper layer are there fro OSPF. EIGRP is using RTP, which is said to be the transport (Layer 4) for EIGRP, but OSPF also is said to also have its own mechanism for transport, what is that mechanism...It's neither TCP/UDP or RTP, so what does it use?

powmia

Do not confuse EIGRP RTP with Real-Time Transport Protocol (used for VVoiP). RTP for EIGRP refers to Reliable Transport Protocol. This is just the name that Cisco gave the mechanism EIGRP speakers use to verify the sending of information (ACKs).

OSPF uses the mechanisms specified in the protocol itself to perform the checks and balances function of EIGRP RTP. For details, search for "OSPF Database Synchronization."

FrankGuthrie wrote: »

Hmmm interesting that you place EIGRP at Layer 4...

Also I always thought of TCP/UDP/Layer 4 as running on top op IP (Layer 3). This is how I see it in the OSI model and visualize it for myself.

So can I say that OSPF is running on top of Layer 3 as Layer 3,5??

Also if the protocol is used to tell the upper layer how to cope with it, what upper layer are there fro OSPF. EIGRP is using RTP, which is said to be the transport (Layer 4) for EIGRP, but OSPF also is said to also have its own mechanism for transport, what is that mechanism...It's neither TCP/UDP or RTP, so what does it use?