Add discussion about MTU & fragmentation

[nicorusti]

Feedback Harald Alvestrand The description of SCION refers to using UDP/IP as the internal substrate inside an AS, and as a framing protocol on inter-AS links. This means that the Internet MTU of 1500 bytes (1200 for IPv6) will be in effect. Path hops take 12 bytes + a couple of “segment headers” of 16 bytes - which means that a 10-hop, 3-segment path will take 168 bytes for the routing information. This is enough to reduce the MTU significantly, and the MTU will be unpredictable and maybe variable, since the path for a given packet may change dynamically. (The packet format, like UDP’s, permits 68K-byte packets. However, fragmentation is A Thing.)

Related issues:

• CP draft: https://github.com/scionassociation/scion-cp_I-D/issues/57
• Implementations: https://github.com/scionproto/scion/issues/2608

Also:

• [ ] Add discussion about fragmentation
• [ ] Adapt this text in the Common Header paragraphThe value of the traffic class bits in a received packet or fragment might differ from the value sent by the packet's source.

[jiceatscion]

Here's what I think we could say:

Fragmentation:

• At the UDP/SCION layer: no fragmentation ever. Even less than IPV6 (i.e. even the sending host can't fragment); the application must comply with the paths MTU.
• In the underlay network. SCION is agnostic to datagram fragmentation by the underlay UDP layer. The open source implementation does not change the system defaults regarding fragmentation and MTU discovery (under Linux MTU discovery is on, UDP fragmentation is off). For inter-AS links, changing this default is the joint decision (not recommended) of the administrators of the two ASes involved. The decision to change this default for intra-AS interfaces is the decision of that AS' administrator.

MTU:

• SCION assumes that its layer 2 has a minimum MTU of 1232 (1280 - 48, assuming UDP/IPV6 encapsulation as the worst case). SCMP relies on this minimum. UDP/SCION takes advantage on any larger MTU configured.
• The MTU of an entire path is defined as the MIN of the MTUs of the links traversed by that path. The control plane makes those numbers available in segment records.
• SCION assumes that the MTU of a path segment remains correct for the life time of that segment.
• The MTU of each link may be discovered or administratively configured (current practice is for it to be configured). It must be less than or equal to the MTU of the underlying link-layer in either direction.
• Intra-AS network MTUs are a result of the network configuration of each AS and therefore predictable.
• Inter-AS links MTU are normally under the joint control of the administrators of the two ASes involved and therefore equally predictable.
• Although that isn't a normal use case, SCION allows inter-AS links to be routed through multiple IP routers, not necessarily under the control of either AS' administrator. In that case, the link's MTU MUST be configured statically to a conservative value. 1280 is a safe value. The same approach applies to all cases where MTUs cannot be assumed to be stable.