Other than the protocol (or payload) field SLARP (Serial Line ARP) is the only other major difference. Below is the white paper published some time ago on the Cisco HDLC spec:
cisco Serial Line Encapsulation
-------------------------------
cisco's default encapsulation on synchronous serial lines uses HDLC framing,
with packet contents defined as follows:
The first ("address") octet is set to 0x0F for unicast packets and 0x8F
for broadcast packets. Broadcast just means that the higher-level protocol
thought this was a broadcast packet; cisco doesn't support multidrop
HDLC at this time.
The second ("control") octet is always 0.
The next two octets are a 16-bit protocol code, sent most-significant-first.
These codes are usually Ethernet type codes. cisco has added some codes to
support packet types that don't appear on Ethernets. The current list of codes
is as follows:
TYPE_PUP 0x0200 PUP
TYPE_XNS 0x0600 XNS
TYPE_IP10MB 0x0800 IP
TYPE_CHAOS 0x0804 Chaos
TYPE_IEEE_SPANNING 0x4242 DSAP/SSAP for IEEE bridge spanning prot.
TYPE_DECNET 0x6003 DECnet phase IV
TYPE_BRIDGE 0x6558 Bridged Ethernet/802.3 packet
TYPE_APOLLO 0x8019 Apollo domain
TYPE_REVERSE_ARP 0x8035 cisco SLARP (not real reverse ARP!)
TYPE_DEC_SPANNING 0x8038 DEC bridge spanning tree protocol
TYPE_ETHERTALK 0x809b Apple EtherTalk
TYPE_AARP 0x80f3 Appletalk ARP
TYPE_NOVELL1 0x8137 Novell IPX
TYPE_CLNS 0xFEFE ISO CLNP/ISO ES-IS DSAP/SSAP
This list is shared between serial and Ethernet encapsulations. Not all
these codes will necessarily appear on serial lines. This list will probably
be extended as cisco adds support for more protocols.
Bytes after this are higher-level protocol data. These normally look the
same as they'd look on Ethernet. Bridging packets include Ethernet/802.3
MAC headers; no other packets do.
Packets with type 8035 (reverse ARP) don't contain reverse ARP data as
they would on an Ethernet. Instead, they carry a protocol cisco refers to
as SLARP. SLARP has two functions: dynamic IP address determination and
serial line keepalive.
The serial line model supported by SLARP assumes that each serial line is
a separate IP subnet, and that one end of the line is host number 1, while
the other end is host number 2. The SLARP address resolution protocol allows
system A to request that system B tell system A system B's IP address,
along with the IP netmask to be used on the network. It does this by sending
a SLARP address resolution request packet, to which system B responds with a
SLARP address resolution reply packet. System A then attempts to determine its
own IP address based on the address of system B. If the host portion of system
B's address is 1, system A will use 2 for the host portion of its own IP
address. Conversely, if system B's IP host number is 2, system A will use IP
host number 1. If system B replies with any IP host number other than 1 or 2,
system A assumes that system B is unable to provide it with an address via
SLARP.
For the SLARP keepalive protocol, each system sends the other a keepalive
packet at a user-configurable interval. The default interval is 10 seconds.
Both systems must use the same interval to ensure reliable operation.
Each system assigns sequence numbers to the keepalive packets it sends,
starting with zero, independent of the other system. These sequence numbers
are included in the keepalive packets sent to the other system. Also included
in each keepalive packet is the sequence number of the last keepalive packet
_received_ from the other system, as assigned by the other system. This number
is called the returned sequence number. Each system keeps track of the last
returned sequence number it has received. Immediately before sending a keepalive
packet, it compares the sequence number of the packet it is about to send with
the returned sequence number in the last keepalive packet it has received.
If the two differ by 3 or more, it considers the line to have failed, and
will route no further higher-level data across it until an acceptable keepalive
response is received.
There is interaction between the SLARP address resolution protocol and the
SLARP keepalive protocol. When one end of a serial line receives a SLARP
address resolution request packet, it assumes that the other end has restarted
its serial interface and reset its keepalive sequence numbers. In addition
to responding to the address resolution request, it will act as if the
other end had sent it a keepalive packet with a sequence number of zero,
and a returned sequence number the same as the returned sequence number
of the last real keepalive packet it received from the other end.
The following is a C definition for the SLARP packet. The "long" and "ulong"
types are 32-bit numbers, high octet sent first. The "ushort" type is a 16-bit
number, high octet sent first.
struct slarp {
long code; /* SLARP packet type code */
union sl { /* followed by one of: */
struct { /* Address resolution functions */
ulong address; /* Address of system sending this pkt */
ulong mask; /* IP subnet mask for this line */
ushort unused; /* Unused: contents undefined */
} add; /* -- or -- */
struct { /* Keepalive probing functions */
ulong mysequence; /* Outgoing sequence number */
ulong yoursequence; /* Returned sequence number */
ushort reliability; /* Reserved: set to FFFF */
} chk;
} t;
};
Note that the data storage for t.add is overlayed on the data storage for
t.chk. The whole SLARP packet consists of a 32-bit type code, followed by
two 32-bit quantities and one 16-bit quantity. The overall length of the
SLARP packet is 14 octets. The "code" field is used to identify the packet's
SLARP type. Legal values for the "code" field are as follows:
SLARP_REQUEST 0 Address resolution request
SLARP_REPLY 1 Address resolution reply
SLARP_LINECHECK 2 Line keepalive
For address resolution request packets, the "address" and "mask" fields are
set to zero, and the contents of the "unused" field field are undefined. For
address resolution reply packets, the "address" field contains the IP address
of the _replying_ system, and the "mask" field contains the IP subnet mask
to be used. The contents of the "unused" field are undefined.
For keepalive packets, the "mysequence" field contains the sequence number
of the packet and the "yoursequence" field contains the returned sequence
number, which is the sequence number of the last keepalive packet the sending
system has gotten from the receiving system. The "reliability" field is
reserved for future use, and _must_ be set to FFFF hexadecimal.
